Maharashtra Board Class 8 Science Chapter 5 Inside the Atom Solutions

Class 8 Science Chapter 5 Inside The Atom Question Answer Maharashtra Board

1. Answer The Following.

Question a. What is the difference in the atomic models of Thomson and Rutherford?

Thomson's atomic model	Rutherford's atomic model
1. According to Thomson's atomic model, the negatively charged electrons are embedded in a gel of positive charge.	1. According to Rutherford's atomic model the negatively charged electrons revolve around the nucleus.
2. Atom is homogenous sphere of positive charge.	2. The positive charge is in the nucleus of the atom.

In simple words: Thomson's model suggests electrons are stuck in a uniform positive gel, while Rutherford's model describes electrons orbiting a dense, positively charged nucleus.

🎯 Exam Tip: Focus on the distribution of positive charge and the location/movement of electrons as key differentiating points between the models.

 

Question b. What is meant by valency of an element? What is the relationship between the number of valence electron and valency?
Answer: Valency: The capacity of an element to combine with another element is known as valency.
Valence electrons: The electrons in the outermost shell of an atom of an element are called valence electrons.
Helium and neon, atoms of both these gaseous element do not combine with any other atom. These elements are chemically inert, i.e. their valency is zero.
Helium atom contains two electrons, indicates that the outermost shell of helium has an electron duplet. The valence shell of neon is completely filled, i.e. neon has an electron octet. Similarly argon contains eight electrons in the valence shell, i.e. argon has an electron octet. It is confirmed that the valency is zero when electron octet (or duplet) is complete.
Atoms of all the elements except inert gases have tendency to combine with other atoms, i.e. they have a non-zero valency. The molecules formed by combination with hydrogen (E.g. \(H_2\), \(HCl\)) that valency of hydrogen is one. The electronic configuration of hydrogen shows that there is one electron less than the complete duplet state. This number 'one' matches with the valency of hydrogen which is also one.
It means that there is relationship between the valency of an element and the number of electrons in its valence shell.
In simple words: Valency is an element's combining power, determined by its valence electrons (outermost electrons). Elements with less than or equal to 4 valence electrons often have valency equal to these electrons, while those with more than 4 may have valency equal to 8 minus the valence electrons, aiming for a stable octet or duplet.

🎯 Exam Tip: Understanding valency and valence electrons is crucial for predicting chemical bonding and reactivity. Remember the octet rule or duplet rule for stability.

 

Question c. What is meant by atomic mass number? Explain how the atomic number and mass number of carbon are 6 and 12 respectively.
Answer: The total number of protons and neutrons in the nucleus of the atom is called the atomic mass number. The atomic number, i.e. the proton number of carbon is 6 and the mass number is total number of protons and neutrons in the carbon, i.e. 6 protons + 6 neutrons = 12. Therefore, the atomic number and mass number of carbon are 6 and 12 respectively.
In simple words: Atomic mass number is the sum of protons and neutrons in an atom's nucleus. For carbon, its atomic number (protons) is 6, and with 6 neutrons, its atomic mass number becomes 12.

🎯 Exam Tip: Clearly distinguish between atomic number (number of protons) and atomic mass number (protons + neutrons). These are fundamental to an element's identity and mass.

 

Question d. What is meant by subatomic particle? Give brief information of three subatomic particles with reference to electrical charge, mass and location.
Answer: A particle which is a constituent of an atom hence smaller than the atom is called subatomic particle.
An atom is formed from the nucleus and the extranuclear part. These contain three types of subatomic particles.
The nucleus contains two types of subatomic particles together called nucleons.
Protons and neutrons are the two types of nucleons or subatomic particles and electrons are subatomic particles in the extra nuclear part.
1. Proton (p): Proton is a positively charged subatomic particle in the atomic nucleus. The positive charge on the nucleus is due to the proton in it. A proton is represented by the symbol 'p'. Each proton carries a positive charge of +1e. (\(1e = 1.6 \times 10^{-19}\) coulomb). When total positive charge on the nucleus is expressed in the unit 'e', its magnitude is equal to the number of proton in the nucleus.
The mass of one proton is approximately 1u (1 Dalton).
(\(1u = 1.66 \times 10^{-27}g\)) (The mass of one hydrogen atom is also approximately 1u.)
2. Neutron (n): Neutron is an electrically neutral subatomic particle and is denoted by the symbol 'n'. The number of neutron in the nucleus is denoted by the symbol 'N' Atomic nuclei of all the elements except hydrogen with atomic mass 1u, contain neutrons. The mass of a neutron is approximately 1u, which is almost equal to that of a proton.
3. Electron (e\(^-\)): Electron is a negatively charged subatomic particle and is denoted by the symbol 'e-'. Each electron carries one unit of negative charge (-1e). Mass of an electron is 1800 times less than that of a hydrogen atom. Therefore the mass of an electron can be treated as negligible. Electron in the extranuclear part revolve in the discrete orbits around the nucleus. The energy of an electron is determined by the shell in which it is present.
In simple words: Subatomic particles are the tiny components of an atom: protons (positive, in nucleus), neutrons (neutral, in nucleus), and electrons (negative, orbiting nucleus). Protons and neutrons have similar masses (approx. 1u), while electrons have negligible mass.

🎯 Exam Tip: Memorize the charge, approximate mass (relative to each other), and location of protons, neutrons, and electrons. This forms the basis of atomic structure.

2. Give Scientific Reasons:

 

Question a. All the mass of an atom is concentrated in the nucleus.
Answer:
1. The nucleus of an atom contains protons and neutrons.
2. The electrons revolve around the nucleus.
3. The mass of an electron is negligible compared to that of a proton or a neutron.
4. Hence, the mass of an atom depends mainly on the number of protons and neutrons. Therefore, practically all the mass of an atom is concentrated in the nucleus.
In simple words: The nucleus holds almost all of an atom's mass because it contains the much heavier protons and neutrons, while the orbiting electrons have a comparatively tiny mass.

🎯 Exam Tip: When explaining this, emphasize the comparative masses of subatomic particles. The sum of proton and neutron masses significantly outweighs the electron masses.

 

Question b. Atom is electrically neutral.
Answer:
1. An atom is made of two parts, viz. the nucleus and the extranuclear part.
2. The nucleus is positively charged. The positive charge on the nucleus is due to protons.
3. The extranuclear part of an atom is made of negatively charged electrons.
4. In an atom, the number of protons is equal to the number of extranuclear electrons.
5. The magnitude of the positive charge on the nucleus equals the magnitude of the negative charge on the electrons. As the opposite charges are balanced, the atom is electrically neutral.
In simple words: An atom is electrically neutral because the number of positively charged protons in its nucleus exactly equals the number of negatively charged electrons orbiting it, causing their charges to cancel out.

🎯 Exam Tip: The balance between positive protons and negative electrons is key. Ensure you state that the *number* of protons equals the *number* of electrons in a neutral atom.

 

Question c. Atomic mass number is a whole number.
Answer:
1. The total number of protons and neutrons in the nucleus of the atom is called the atomic mass number (A).
2. As protons and neutrons are whole numbers, the atomic mass number is also a whole number.
In simple words: The atomic mass number is always a whole number because it's calculated by adding the number of protons and neutrons, and these are discrete, whole particles.

🎯 Exam Tip: This is a straightforward concept; focus on the discrete nature of protons and neutrons as individual particles.

 

Question d. Atoms are stable though negatively charged electron are revolving within it.
Answer:
1. The entire mass of the atom is concentrated in the nucleus and the positively charged nucleus at centre of an atom.
2. The negatively charged electrons revolve around the nucleus.
3. The total negative charge on all the electron is equal to positive charge on the nucleus. As the opposite charges are balanced, the atom is stable.
In simple words: Atoms are stable because the attractive force between the positively charged nucleus and the negatively charged orbiting electrons perfectly balances the repulsive forces, holding the atom together in a stable state.

🎯 Exam Tip: Highlight the balance of electrostatic forces between the positive nucleus and negative electrons. This balance prevents the electrons from spiraling into the nucleus.

3. Define The Following Terms.

 

Question a. Atom:
Answer: An atom is the smallest particle of an element which retains its chemical identity in all physical and chemical changes.
In simple words: An atom is the smallest unit of an element that still has the chemical properties of that element.

🎯 Exam Tip: Emphasize "retains its chemical identity" as a key part of the definition. It distinguishes an atom from subatomic particles.

 

Question b. Isotope
Answer: Atoms of the same element having the same atomic number, but different atomic mass numbers are called isotopes.
In simple words: Isotopes are different versions of the same element, meaning they have the same number of protons but a different number of neutrons, leading to different atomic masses.

🎯 Exam Tip: The core of isotopes is "same atomic number (protons)" and "different mass number (neutrons)". This ensures they are the same element but have different masses.

 

Question c. Atomic number
Answer: The number of electrons or protons in an atom is called the atomic number. It is denoted by Z.
In simple words: Atomic number (Z) is the count of protons in an atom's nucleus, which also equals the number of electrons in a neutral atom, defining the element's identity.

🎯 Exam Tip: Crucially, the atomic number defines the element. Always link it to the number of protons.

 

Question d. Atomic mass number
Answer: The total number of protons and neutrons in the nucleus of the atom is called the atomic mass number. It is denoted by A.
In simple words: The atomic mass number (A) is the sum of an atom's protons and neutrons in its nucleus.

🎯 Exam Tip: Remember it's the sum of protons AND neutrons. Don't confuse it with atomic weight which is an average.

 

Question e. Moderator in nuclear reactor
Answer: The substance which reduces the speed of fast-moving neutrons produced in a fission is called a moderator.
In simple words: A moderator is a material used in nuclear reactors to slow down fast neutrons, making them more likely to cause further fission reactions.

🎯 Exam Tip: The key function of a moderator is to "slow down" neutrons, which is essential for sustaining a nuclear chain reaction effectively.

4. Draw A Neat Labelled Diagram.

 

Question a. Explain Rutherford's scattering experiment.
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र रदरफोर्ड के अल्फा-कण प्रकीर्णन प्रयोग को दर्शाता है। इसमें एक अल्फा-कण उत्सर्जक से निकलने वाले अल्फा कणों को एक पतली सोने की पन्नी (गोल्ड फॉइल) पर बमबारी की जाती है। इन कणों के मार्ग को फ्लोरोसेंट स्क्रीन द्वारा ट्रैक किया जाता है, जिससे यह पता चलता है कि अधिकांश कण सीधे निकल जाते हैं, कुछ छोटे कोणों पर विक्षेपित होते हैं, और बहुत कम कण बड़े कोणों पर या पीछे की ओर उछल जाते हैं। Alpha particles emitted by radioactive element bear a positive charge. Rutherford bombarded alpha particles through a very thin gold foil. He observed the path of \(\alpha\)-particles by means of a fluorescent screen around the gold foil. It was expected that
1. Most of the alpha particles passed through the gold foil without any deviation.
2. Some alpha particles were deflected from their path through small angles.
3. A few alpha particles were scattered at large angles.
4. A still smaller number of same sign particles get deflected through a larger angle and one \(\alpha\)-particle out of 20000 bounced back in the direction opposite to the original path.
In simple words: Rutherford's experiment involved firing positive alpha particles at a gold foil. Most passed through, some deflected slightly, and a very few bounced back, indicating a small, dense, positively charged nucleus within atoms.

🎯 Exam Tip: For this question, describing the setup, observations, and conclusions (especially the existence of a small, dense, positive nucleus) are crucial. The diagram explanation helps visualize the experiment.

 

Question b. Thomson's atomic model.
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र थॉमसन के 'प्लम पुडिंग' परमाणु मॉडल को दर्शाता है। इसमें परमाणु को एक धनावेशित गोले के रूप में चित्रित किया गया है, जिसके अंदर ऋणावेशित इलेक्ट्रॉन (छोटे वृत्त) ऐसे धँसे हुए हैं जैसे पुडिंग में बेर या तरबूज में बीज। यह मॉडल परमाणु की समग्र विद्युत उदासीनता को भी दर्शाता है। 1. According to Thomson's model the positive charge is distributed throughout the atom and the negatively charged electron: are embedded in a gel of positive charge (a plum pudding model).
2. The distributed positive charge is balanced by the negative charge on the electrons. Therefore the atom becomes electrically neutral.
In simple words: Thomson's atomic model, known as the 'plum pudding model', proposed that an atom is a uniform sphere of positive charge with negatively charged electrons embedded within it, like plums in a pudding, making the atom electrically neutral.

🎯 Exam Tip: Remember the "plum pudding" analogy for Thomson's model. Key features are uniform positive charge distribution and embedded, stationary electrons, leading to overall neutrality.

 

Question c. Diagrammatic sketch of electronic configuration of magnesium (Atomic number 12).
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र मैग्नीशियम (परमाणु संख्या 12) के इलेक्ट्रॉनिक विन्यास का एक आरेखीय स्केच है। इसमें केंद्रीय नाभिक में 12 प्रोटॉन और 12 न्यूट्रॉन दिखाए गए हैं। इलेक्ट्रॉनों को नाभिक के चारों ओर विभिन्न कक्षाओं में वितरित किया गया है: पहली K-शेल में 2 इलेक्ट्रॉन, दूसरी L-शेल में 8 इलेक्ट्रॉन, और तीसरी M-शेल में 2 इलेक्ट्रॉन।
Magnesium (Z=12)
Atomic number of magnesium \(12 = \begin{bmatrix} \text{12 protons} \\ \text{12 electrons} \end{bmatrix}\)
Electronic configuration of magnesium
In simple words: For magnesium (atomic number 12), the electronic configuration diagram shows 12 protons and 12 neutrons in the nucleus, with electrons arranged as 2 in the K-shell, 8 in the L-shell, and 2 in the M-shell.

🎯 Exam Tip: When drawing electronic configurations, ensure the correct number of protons and neutrons in the nucleus, and fill electron shells according to the 2n\(^2\) rule (2, 8, 18, 32...).

 

Question d. Diagrammatic sketch of electronic configuration of argon (Atomic number 18).
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र आर्गन (परमाणु संख्या 18) के इलेक्ट्रॉनिक विन्यास का एक आरेखीय स्केच है। केंद्रीय नाभिक में 18 प्रोटॉन और 22 न्यूट्रॉन दिखाए गए हैं। इलेक्ट्रॉनों को नाभिक के चारों ओर विभिन्न कक्षाओं में वितरित किया गया है: पहली K-शेल में 2 इलेक्ट्रॉन, दूसरी L-शेल में 8 इलेक्ट्रॉन, और तीसरी M-शेल में 8 इलेक्ट्रॉन, जो एक पूर्ण बाहरी शेल को दर्शाता है।
Argon (Z=18)
Atomic number of argon \(18 = \begin{bmatrix} \text{18 protons} \\ \text{18 electrons} \end{bmatrix}\)
Electronic configuration of argon
In simple words: The electronic configuration for argon (atomic number 18) illustrates 18 protons and 22 neutrons in its nucleus, with electrons filling its shells as 2 in K, 8 in L, and 8 in M, making its outermost shell complete.

🎯 Exam Tip: For noble gases like Argon, the electronic configuration should show a fully filled outermost shell, which explains their chemical inertness. Clearly label the nucleus with protons and neutrons, and distribute electrons correctly in shells.

5. Fill In The Blanks.

 

Question 1. Electron, proton, neutron are the types of .............. in an atom.
Answer: Electron, proton, neutron are the types of subatomic particles in an atom.
In simple words: Electrons, protons, and neutrons are the fundamental subatomic particles that make up an atom.

🎯 Exam Tip: Remember these three as the primary constituents of an atom. "Subatomic particles" is the precise term.

 

Question 2. An electron carries a .............. charge.
Answer: An electron carries a negative charge.
In simple words: An electron always carries a negative electrical charge.

🎯 Exam Tip: It's fundamental knowledge: electrons are negative, protons are positive, and neutrons are neutral.

 

Question 3. The electron shell .............. is nearest to the nucleus.
Answer: The electron shell K is nearest to the nucleus.
In simple words: The K-shell is the innermost electron shell, located closest to the atom's nucleus.

🎯 Exam Tip: Know the order of electron shells (K, L, M, N...) and their proximity to the nucleus, as this affects electron energy levels.

 

Question 4. The electronic configuration of magnesium is 2, 8, 2. From this it is understood that the valence shell of Magnesium is ..............
Answer: The electronic configuration of magnesium is 2, 8, 2. From this it is understood that the valence shell of Magnesium is M.
In simple words: Given magnesium's electronic configuration of 2, 8, 2, its outermost shell, which is the M-shell, contains 2 valence electrons.

🎯 Exam Tip: The valence shell is always the outermost shell with electrons. Identify it by its letter designation (K, L, M, N) based on the electron distribution.

 

Question 5. The valency of hydrogen is one as per the molecular formula H2O. Therefore valency of 'Fe' turns out to be .............. as per the formula Fe2O3.
Answer: The valency of hydrogen is 'one' as per the molecular formula H2O. Therefore valency of 'Fe' turns out to be 3 as per the formula Fe2O3.
In simple words: If hydrogen has a valency of 1 (as seen in H2O), then in the compound Fe2O3, iron (Fe) must have a valency of 3 to balance with oxygen's valency of 2.

🎯 Exam Tip: To determine valency from a formula, use known valencies (e.g., O=2) and the criss-cross method to deduce the unknown valency. The total valency of one element must balance that of the other in a compound.

6. Match The Pairs.

 

Question 1.

Column I	Column II
1. Proton	a. Negatively charged
2. Electron	b. Neutral
3. Neutron	c. Positively charged

Answer:

Column I	Column II
1. Proton	c. Positively charged
2. Electron	a. Negatively charged
3. Neutron	b. Neutral

In simple words: Protons are positively charged, electrons are negatively charged, and neutrons have no charge, making them neutral.

🎯 Exam Tip: This is a basic recall question. Ensure you accurately associate each subatomic particle with its correct electrical charge.

7. Deduce From The Datum Provided.

 

Question 1.

Datum	To deduce
\({^{23}_{11}Na}\)	Neutron number
\({^{14}_{6}C}\)	Mass number
\({^{37}_{17}Cl}\)	Proton number

Answer:
1. There are 12 neutrons in the sodium (\({^{23}_{11}Na}\)).
(N = A - Z) 23 - 11 = 12
2. Atomic mass number of \({^{14}_{6}C}\) is 14.
3. There are 17 protons in chlorine (\({^{37}_{17}Cl}\))
In simple words: Using the given notation \({^{A}_{Z}X}\) where A is mass number and Z is atomic number, we deduce neutrons (A-Z) for sodium, mass number (A) for carbon, and proton number (Z) for chlorine.

🎯 Exam Tip: Understand the notation \({^{A}_{Z}X}\) where A = mass number (protons + neutrons) and Z = atomic number (protons). From this, you can always find the number of neutrons (A - Z).

Class 8 Science Chapter 5 Inside The Atom Additional Important Questions And Answers

Rewrite The Sentences After Filling The Blanks:

 

Question 1. An atom is electrically ..............
Answer: An atom is electrically neutral.
In simple words: An atom typically has no net electrical charge, meaning it is neutral.

🎯 Exam Tip: This is a foundational concept. Remember that in its natural state, an atom's positive and negative charges balance out.

 

Question 2. Except hydrogen, the nuclei of all atoms contain ..............
Answer: Except hydrogen, the nuclei of all atoms contain neutrons.
In simple words: All atomic nuclei, except for the most common form of hydrogen, contain neutrons alongside protons.

🎯 Exam Tip: Hydrogen-1 (protium) is unique in having only a proton in its nucleus. All other isotopes and elements have neutrons.

 

Question 3. \({^{12}_{6}C}\), .............. and .............. are isotopes of carbon.
Answer: \({^{12}_{6}C}\), \({^{13}_{6}C}\) and \({^{14}_{6}C}\) are isotopes of carbon.
In simple words: The three main isotopes of carbon are Carbon-12, Carbon-13, and Carbon-14, all having 6 protons but differing in their neutron count.

🎯 Exam Tip: When identifying isotopes, the atomic number (subscript) must be the same, and the mass number (superscript) must be different.

 

Question 4. An atom has 11 protons and .............. neutrons and hence its atomic mass number is 23.
Answer: An atom has 11 protons and 12 neutrons and hence its atomic mass number is 23.
In simple words: If an atom has 11 protons and an atomic mass number of 23, it must have 12 neutrons (23 - 11 = 12).

🎯 Exam Tip: Remember the formula: Atomic Mass Number = Number of Protons + Number of Neutrons. Use this to find any missing value if the other two are given.

 

Question 5. The element .............. has two electrons in the K shell, but it is a noble gas.
Answer: The element helium has two electrons in the K shell, but it is a noble gas.
In simple words: Helium is the noble gas with two electrons, which completely fills its K-shell, making it stable.

🎯 Exam Tip: Helium is unique as a noble gas with a duplet (2 electrons) in its outermost shell, unlike most other noble gases that achieve an octet (8 electrons).

 

Question 6. Isotopes of the element have the same .............. properties.
Answer: Isotopes of the element have the same chemical properties.
In simple words: Since isotopes of an element have the same number of electrons, their chemical reactions and properties are identical.

🎯 Exam Tip: Chemical properties are determined by electron configuration, which is identical for isotopes. Physical properties, however, can differ due to mass variations.

 

Question 7. Electrons must absorb .............. to transit between orbits.
Answer: Electrons must absorb energy to transit between orbits.
In simple words: Electrons absorb specific amounts of energy to jump to higher energy orbits.

🎯 Exam Tip: This relates to Bohr's model: electrons absorb energy to move to higher shells and release energy to move to lower shells, in discrete packets (quanta).

 

Question 8. .............. discovered the electron.
Answer: J. J. Thomson discovered the electron.
In simple words: J. J. Thomson is credited with the discovery of the electron using cathode ray experiments.

🎯 Exam Tip: Remember key scientists and their discoveries: Thomson (electron), Rutherford (nucleus), Chadwick (neutron), Bohr (electron shells).

 

Question 9. \(\alpha\)-particles have .............. charge.
Answer: \(\alpha\)-particles have positive charge.
In simple words: Alpha particles carry a positive charge, as they are essentially helium nuclei.

🎯 Exam Tip: Knowing the charge of alpha particles (+2) is crucial for understanding experiments like Rutherford's, where electrostatic repulsion plays a role.

 

Question 10. Electrons revolve around the .............. in certain discrete orbits.
Answer: Electrons revolve around the nucleus in certain discrete orbits.
In simple words: Electrons orbit the central nucleus in specific, fixed paths called discrete orbits or shells.

🎯 Exam Tip: This describes the planetary model of the atom (Rutherford-Bohr), where electrons are not randomly scattered but follow defined paths.

 

Question 11. The shell-wise distribution of electrons is called the ...............
Answer: The shell-wise distribution of electrons is called the electronic configuration.
In simple words: Electronic configuration is how electrons are arranged in different energy shells around an atom's nucleus.

🎯 Exam Tip: Electronic configuration is vital for understanding an element's chemical properties and its position in the periodic table.

 

Question 12. Democritus termed the smallest particles of matter as ...............
Answer: Democritus termed the smallest particles of matter as atoms.
In simple words: The ancient Greek philosopher Democritus proposed that all matter is composed of indivisible, fundamental units he called "atomos," from which the word "atom" is derived.

🎯 Exam Tip: Connect Democritus to the initial philosophical concept of the atom, predating experimental evidence.

 

Question 13. .............. discovered neutron.
Answer: James Chadwick discovered neutron.
In simple words: James Chadwick discovered the neutron, a neutral subatomic particle, in 1932.

🎯 Exam Tip: Chadwick's discovery was critical for explaining isotopes and the mass of atomic nuclei that couldn't be accounted for by protons alone.

 

Question 14. Electrons revolve around the nucleus in paths called ..............
Answer: Electrons revolve around the nucleus in paths called orbits.
In simple words: Electrons follow specific, well-defined paths, or orbits, around the central nucleus of an atom.

🎯 Exam Tip: While more advanced models use "orbitals," for introductory chemistry, "orbits" (as per Bohr's model) is an acceptable term for the paths electrons take.

 

Question 15. The second shell has the capacity of ...............
Answer: The second shell has the capacity of 8 electrons.
In simple words: The second electron shell, known as the L-shell, can hold a maximum of 8 electrons.

🎯 Exam Tip: Remember the 2n\(^2\) rule for shell capacity: K (n=1) = 2, L (n=2) = 8, M (n=3) = 18, N (n=4) = 32.

 

Question 16. Electrons in the .............. shell have minimum energy.
Answer: Electrons in the K or first shell have minimum energy.
In simple words: Electrons located in the innermost K-shell are closest to the nucleus and possess the lowest amount of energy.

🎯 Exam Tip: Electron energy increases as the shell number increases (i.e., further from the nucleus). The K-shell is always the lowest energy level.

 

Question 17. Electrons in an atom occupy shells in increasing order of ..............
Answer: Electrons in an atom occupy shells in increasing order of energy.
In simple words: Electrons fill the energy shells around an atom starting from the lowest energy levels closest to the nucleus and moving outwards.

🎯 Exam Tip: This is a fundamental principle of electron configuration: electrons prefer to occupy the lowest available energy states first (Aufbau principle).

 

Question 18. Atomic masses are measured in a unit called ..............
Answer: Atomic masses are measured in a unit called dalton.
In simple words: The standard unit for measuring atomic masses is the dalton, also known as the atomic mass unit (amu or u).

🎯 Exam Tip: Be precise with units. Atomic mass is measured in daltons (u), not grams, as it's a relative scale.

 

Question 19. According to the atomic model of ............... electrons are embedded in a gel of positive charge.
Answer: According to the atomic model of Thomson, electrons are embedded in a gel of positive charge.
In simple words: Thomson's atomic model suggested electrons were dispersed within a larger, diffuse sphere of positive charge.

🎯 Exam Tip: This refers specifically to Thomson's "plum pudding" model. Differentiate this from Rutherford's nuclear model where electrons orbit.

 

Question 20. All atoms, except .............. contain neutrons in their nuclei.
Answer: All atoms, except normal hydrogen contain neutrons in their nuclei.
In simple words: All elements except protium (the most common isotope of hydrogen) have neutrons in their atomic nuclei.

🎯 Exam Tip: Normal hydrogen (\({^{1}_{1}H}\)) contains only one proton and no neutrons, making it an exception.

 

Question 21. The maximum capacity of the M shell is .............. electrons.
Answer: The maximum capacity of the M shell is 18 electrons.
In simple words: The M-shell (third shell) can hold a maximum of 18 electrons according to the 2n\(^2\) rule.

🎯 Exam Tip: Apply the 2n\(^2\) formula (where n=3 for the M-shell) to accurately determine its electron capacity. \(2 \times 3^2 = 18\).

 

Question 22. The atom of the element .............. has eight electrons in the outermost shell.
Answer: The atom of the element neon has eight electrons in the outermost shell.
In simple words: Neon, a noble gas, has a stable configuration with eight electrons in its outermost shell (octet).

🎯 Exam Tip: Elements with eight electrons in their outermost shell (octet) are typically noble gases and are chemically stable. Helium is an exception with a duplet.

Rewrite The Following Statements Selecting The Correct Options:

Question 1. The symbol A is used to denote the ............
(a) atomic number
(b) atomic radius
(c) atomic mass number
(d) atomic mass
Answer: (c) atomic mass number
In simple words: The symbol 'A' represents the atomic mass number, which is the total count of protons and neutrons in an atom's nucleus.

🎯 Exam Tip: Remember that 'A' stands for Atomic Mass Number (protons + neutrons), and 'Z' stands for Atomic Number (number of protons).

Question 2. The existence of isotopes is due to the presence of different number of ............
(a) electrons
(b) protons
(c) neutrons
(d) positrons
Answer: (c) neutrons
In simple words: Isotopes are atoms of the same element that have the same number of protons but differ in the number of neutrons.

🎯 Exam Tip: Isotopes have identical chemical properties because the number of electrons (which determines chemical behavior) is the same, but their physical properties may vary due to different masses.

Question 3. In the nucleus of a sodium atom (\({}^{23}_{11}\mathrm{Na}\)), there are ............ neutrons.
(a) 11
(b) 12
(c) 10
(d) 9
Answer: (b) 12
In simple words: For sodium atom \({}^{23}_{11}\mathrm{Na}\), the atomic mass number (A) is 23 and the atomic number (Z) is 11. The number of neutrons is calculated as A - Z, which is 23 - 11 = 12.

🎯 Exam Tip: Always remember that the number of neutrons is found by subtracting the atomic number (number of protons) from the mass number (total protons and neutrons).

Question 4. Isotopes of an element have the same number of ............
(a) neutrons
(b) nucleons
(c) electrons
(d) atoms
Answer: (c) electrons
In simple words: Isotopes of an element have the same atomic number (number of protons), and in a neutral atom, the number of electrons equals the number of protons.

🎯 Exam Tip: The number of electrons in a neutral atom dictates its chemical identity and behavior; since isotopes are of the same element, they share the same number of electrons.

Question 5. The great Indian philosopher ............ proposed that matter is made up of invisible tiny particles.
(a) Aryabhatta
(b) Kanad
(c) Bhaskaracharya
(d) Chanakya
Answer: (b) Kanad
In simple words: The ancient Indian philosopher Kanad put forth the idea that all matter is composed of minute, indivisible particles called 'anu', similar to the concept of atoms.

🎯 Exam Tip: Recognize key historical figures and their contributions to the development of atomic theory, as these often appear in general knowledge or introductory science questions.

Question 6. The maximum capacity of the M shell is ............ electrons.
(a) 2
(b) 8
(c) 18
(d) 32
Answer: (c) 18
In simple words: The maximum number of electrons that can be held in any electron shell is given by the formula \(2n^2\), where 'n' is the shell number. For the M shell, n=3, so \(2 \times 3^2 = 18\) electrons.

🎯 Exam Tip: Memorize the \(2n^2\) rule for electron shell capacities (K=2, L=8, M=18, N=32) as it's fundamental for understanding electronic configuration.

Question 7. The mass of the electron is ............ times less than that of a hydrogen atom.
(a) 1800
(b) 8100
(c) 1550
(d) 1600
Answer: (a) 1800
In simple words: An electron is significantly lighter than a hydrogen atom, with its mass being approximately 1/1800th that of a hydrogen atom.

🎯 Exam Tip: Understand the relative masses of subatomic particles; electrons have negligible mass compared to protons and neutrons, which contribute most to an atom's mass.

Question 8. The L shell is the valence shell in ............
(a) hydrogen
(b) chlorine
(c) oxygen
(d) sodium
Answer: (c) oxygen
In simple words: Oxygen has an electronic configuration of 2, 6. The K shell has 2 electrons, and the L shell (second shell) has 6 electrons, making the L shell its outermost (valence) shell.

🎯 Exam Tip: To identify the valence shell, write out the electron configuration and locate the outermost filled or partially filled shell. Valence electrons determine an element's reactivity.

Question 9. The M shell is the valence shell in ............
(a) fluorine
(b) neon
(c) carbon
(d) chlorine
Answer: (d) chlorine
In simple words: Chlorine has an electronic configuration of 2, 8, 7. The M shell (third shell) contains 7 electrons and is its outermost shell.

🎯 Exam Tip: Practice writing electron configurations for various elements to quickly identify their valence shells and number of valence electrons, which helps in predicting chemical bonding.

Question 10. The N shell is the valence shell in ............
(a) fluorine
(b) chlorine
(c) bromine
(d) helium
Answer: (c) bromine
In simple words: Bromine has an electronic configuration of 2, 8, 18, 7. The N shell (fourth shell) contains 7 electrons and acts as its valence shell.

🎯 Exam Tip: For larger atoms, applying the \(2n^2\) rule helps determine maximum capacities, but remember that outer shells can start filling before inner shells are completely full for elements beyond the third period.

Question 11. The maximum capacity of the N shell is ............ electrons.
(a) 2
(b) 32
(c) 18
(d) 8
Answer: (b) 32
In simple words: Using the formula \(2n^2\), where n=4 for the N shell, the maximum capacity is \(2 \times 4^2 = 2 \times 16 = 32\) electrons.

🎯 Exam Tip: Ensure you correctly identify the shell number ('n') before applying the \(2n^2\) formula to calculate the maximum electron capacity for any given shell.

Question 12. The maximum number of electrons that can be accommodated in the third orbit is ............
(a) 3
(b) 8
(c) 32
(d) 18
Answer: (d) 18
In simple words: The third orbit is the M shell. Applying the \(2n^2\) rule where n=3, the maximum capacity is \(2 \times 3^2 = 18\) electrons.

🎯 Exam Tip: Distinguish between the capacity of a shell (always \(2n^2\)) and the number of electrons typically found in the outermost shell for stable configurations (octet rule, usually 8).

Question 13. Rutherford alpha-particle scattering experiment was responsible for the discovery of the ............
(a) atomic nucleus
(b) proton
(c) electron
(d) atomic mass
Answer: (a) atomic nucleus
In simple words: Rutherford's gold foil experiment showed that most alpha particles passed through, but some were deflected at large angles, indicating a tiny, dense, positively charged center in the atom, which he called the nucleus.

🎯 Exam Tip: Understand the experimental setup, observations, and key conclusions of Rutherford's experiment, as it fundamentally changed the atomic model.

Question 14. Isotopes of element have ............
(a) different atomic numbers and different atomic mass
(b) different atomic numbers but the same atomic mass number
(c) the same atomic number but different atomic mass numbers
(d) the same atomic number and the same atomic mass number
Answer: (c) the same atomic number but different atomic mass numbers
In simple words: Isotopes are variations of a chemical element that have the same number of protons (atomic number) but different numbers of neutrons, leading to different atomic mass numbers.

🎯 Exam Tip: A clear understanding of atomic number, mass number, protons, and neutrons is essential to correctly define and differentiate isotopes.

Question 15. The nucleus of an atom contains 19 protons and 21 neutrons. The atomic mass number of the element is ............
(a) 19
(b) 21
(c) 40
(d) 39
Answer: (c) 40
In simple words: The atomic mass number is the sum of protons and neutrons in the nucleus. Here, 19 protons + 21 neutrons = 40.

🎯 Exam Tip: Practice calculating atomic mass numbers and neutron counts from given proton and mass numbers, as these are common basic atomic structure questions.

Question 16. The nucleus of an atom contains 18 protons and 22 neutrons. The atomic number of the element is ............
(a) 18
(b) 40
(c) 22
(d) 4
Answer: (a) 18
In simple words: The atomic number of an element is defined by the number of protons in its nucleus. In this case, it is 18.

🎯 Exam Tip: Remember that the atomic number is solely determined by the number of protons and defines the element's identity.

Question 17. When writing the symbol of its ............ and ............ are written.
(a) atomic number, atomic mass number
(b) protons, electrons
(c) protons, neutrons
(d) atomic number, electrons
Answer: (a) atomic number, atomic mass number
In simple words: An element's symbol is often written with the atomic mass number (A) as a superscript and the atomic number (Z) as a subscript.

🎯 Exam Tip: Familiarize yourself with the standard notation for isotopes, where the mass number is at the top and the atomic number at the bottom left of the element symbol.

Question 18. The particles in the atomic nucleus are ............
(a) protons and electrons
(b) electrons
(c) electrons and neutrons
(d) protons and neutrons
Answer: (d) protons and neutrons
In simple words: The nucleus of an atom is composed of positively charged protons and neutral neutrons, collectively known as nucleons.

🎯 Exam Tip: Clearly differentiate between the particles found in the nucleus (protons, neutrons) and those orbiting it (electrons).

State Whether The Following Statements Are True Or False:

Question 1. An atom as a whole is electrically neutral
Answer: True.
In simple words: A neutral atom has an equal number of positively charged protons and negatively charged electrons, balancing out the total charge to zero.

🎯 Exam Tip: This is a fundamental concept of atomic structure; the electrical neutrality of an atom is due to the balance between protons and electrons.

Question 2. The mass of an atom is distributed evenly within it.
Answer: False. (The mass of atom is not distributed evenly, practically all the mass is concentrated in the nucleus.)
In simple words: The mass of an atom is primarily concentrated in its tiny, dense nucleus, with electrons contributing very little to the overall mass.

🎯 Exam Tip: Remember Rutherford's model, which proposed a concentrated nucleus, disproving Thomson's uniform mass distribution model.

Question 3. The electron has the same mass as that of the proton.
Answer: False. (The mass of the electron is much less than that of the proton.)
In simple words: Electrons are about 1800 times lighter than protons, making their contribution to an atom's total mass negligible.

🎯 Exam Tip: Understand the significant mass difference between electrons, protons, and neutrons; protons and neutrons have roughly equal masses, much larger than an electron's mass.

Question 4. The electron in the K shell has maximum energy.
Answer: False. (The electrons in the K shell has minimum energy.)
In simple words: The K shell is the innermost shell, and electrons in shells closer to the nucleus have lower energy levels.

🎯 Exam Tip: Electron energy levels increase as the shells move further away from the nucleus, meaning the outermost electrons have the highest energy.

Question 5. Isotopes have same atomic number but different atomic mass number.
Answer: True.
In simple words: Isotopes are atoms of the same element, meaning they have the same number of protons (atomic number), but they differ in the number of neutrons, leading to different atomic masses.

🎯 Exam Tip: This is the precise definition of isotopes; focus on the consistent atomic number and varying mass number due to neutron count.

Question 6. Matter is composed of molecules and molecules are made of atoms.
Answer: True.
In simple words: Atoms are the fundamental building blocks that combine to form molecules, and collections of these molecules make up matter.

🎯 Exam Tip: Understand the hierarchy of matter: atoms combine to form molecules, which then aggregate to form substances.

Question 7. In India, total 22 nuclear reactors in eight places are functioning.
Answer: True.
In simple words: India operates 22 nuclear power reactors across eight sites, contributing to the nation's electricity generation.

🎯 Exam Tip: While specific numbers can change, knowing the general scale of nuclear power infrastructure in India is useful for general knowledge questions.

Question 8. The number of electrons in a given orbit is given by the formula n
Answer: False. (The number of electrons in a given orbit is given by the formula \(2n^2\))
In simple words: The capacity of an electron shell to hold electrons is determined by the formula \(2n^2\), not just 'n'.

🎯 Exam Tip: Correctly recall and apply the \(2n^2\) rule for electron shell capacities to avoid errors in electron configuration problems.

Question 9. Atomic masses are measured in a unit called the dalton (u).
Answer: True.
In simple words: Atomic mass unit (u) or Dalton (Da) is the standard unit used to express atomic and molecular masses, based on 1/12th the mass of a carbon-12 atom.

🎯 Exam Tip: Be familiar with the units used for atomic properties; 'u' (atomic mass unit) or 'Da' (Dalton) are standard for mass, while charge is measured in elementary charge 'e'.

Question 10. The chemical properties of isotopes are different.
Answer: False. (The chemical properties of isotopes are the same.)
In simple words: Isotopes of an element have the same number of electrons and hence exhibit identical chemical properties because chemical reactions involve interactions of electrons.

🎯 Exam Tip: Emphasize that chemical properties are governed by the electron configuration, which is the same for isotopes of a given element, while physical properties (like density, melting point) can vary due to mass difference.

Question 11. The maximum capacity of the N shell is 18 electrons.
Answer: False. (The maximum capacity of the N shell is 32 electrons.)
In simple words: The N shell is the fourth shell (n=4), and its maximum electron capacity is calculated as \(2 \times 4^2 = 32\).

🎯 Exam Tip: Double-check the shell number when applying the \(2n^2\) rule to ensure you calculate the correct maximum capacity.

Question 12. Deuterium is an isotope of hydrogen.
Answer: True.
In simple words: Deuterium is an isotope of hydrogen that contains one proton and one neutron, distinguishing it from common hydrogen (protium) which has no neutrons.

🎯 Exam Tip: Remember the common isotopes of hydrogen-protium (1 proton, 0 neutrons), deuterium (1 proton, 1 neutron), and tritium (1 proton, 2 neutrons)-as they are frequently used examples.

Question 13. Isotopes are used in the treatment of cancer.
Answer: True.
In simple words: Certain radioactive isotopes, like Cobalt-60, are used in radiotherapy to target and destroy cancerous cells.

🎯 Exam Tip: Be aware of the practical applications of radioactive isotopes in medicine (diagnosis, treatment), industry, and other fields.

Question 14. Atoms of all elements, except normal hydrogen contain neutrons.
Answer: True.
In simple words: The most common isotope of hydrogen, protium (\({}^{1}_{1}\mathrm{H}\)), consists of only one proton and no neutrons, making it unique among all elements.

🎯 Exam Tip: Normal hydrogen (protium) is the only element whose most abundant isotope has no neutrons, a fact often tested to check understanding of atomic composition.

Question 15. Electrons moving in different orbits possess the same amount of energy.
Answer: False. (Electrons moving in different orbits possess different amounts of energy.)
In simple words: Each electron orbit or shell corresponds to a distinct energy level, with higher orbits having greater energy.

🎯 Exam Tip: Bohr's model introduced the concept of quantized energy levels (orbits), meaning electrons in different orbits have specific, discrete energy values.

Question 16. Rutherford discovered the neutron.
Answer: False. (Chadwick discovered the neutron.)
In simple words: James Chadwick discovered the neutron in 1932, while Rutherford is credited with the discovery of the atomic nucleus and the proton.

🎯 Exam Tip: Accurately attribute discoveries of subatomic particles to their respective scientists: electron (Thomson), proton (Rutherford), neutron (Chadwick), nucleus (Rutherford).

Question 17. The nth orbit contains at the most \(2n^2\) electrons.
Answer: True.
In simple words: The formula \(2n^2\) correctly defines the maximum number of electrons that can occupy any given electron shell 'n'.

🎯 Exam Tip: This rule is crucial for determining electron capacities in various shells (K, L, M, N) and understanding electron configurations.

Question 18. Electrons have different energies according to their orbits.
Answer: True.
In simple words: Electrons occupying different shells around the nucleus are at distinct energy levels, with energy increasing as the distance from the nucleus increases.

🎯 Exam Tip: Recall that each electron shell represents a specific energy level, which is a core principle of Bohr's atomic model.

Question 19. The capacity of the second orbit is 18 electrons.
Answer: False. (The capacity of the second orbit is 8 electrons.)
In simple words: The second orbit, or L shell (n=2), has a maximum capacity of \(2 \times 2^2 = 8\) electrons.

🎯 Exam Tip: Be careful not to confuse the capacities of different shells; apply the \(2n^2\) formula accurately for each shell (K=2, L=8, M=18, N=32).

Question 20. The radioactive isotope Sodium-24 is used in the medical treatment of cancer.
Answer: False. (The radioactive isotope Sodium-24 is used for detection of cracks in the underground pipes.)
In simple words: Sodium-24 is a radioactive tracer used to detect leaks in pipelines, whereas Cobalt-60 is commonly used in cancer therapy.

🎯 Exam Tip: Differentiate the specific uses of various radioactive isotopes; each isotope has unique properties that make it suitable for particular applications.

Question 21. Uranium-235 is used in the production of electricity.
Answer: True.
In simple words: Uranium-235 undergoes nuclear fission in reactors, releasing large amounts of energy that can be harnessed to generate electricity.

🎯 Exam Tip: Understand the role of Uranium-235 as a primary fuel source in nuclear power plants due to its fissile nature.

Consider The Relation Between I The Items In The First Pair And Write The Correlation For Second Pair:

Question 1. K:2:M:...........
Answer: 18
In simple words: The K shell (first shell) has a maximum capacity of 2 electrons, and similarly, the M shell (third shell) has a maximum capacity of 18 electrons according to the \(2n^2\) rule.

🎯 Exam Tip: Understand the relationship between shell letter (K, L, M, N) and its corresponding number (n=1, 2, 3, 4) to correctly apply the electron capacity formula.

Question 2. Carbon : 2, 4 :: Fluorine : ............
Answer: 2, 7
In simple words: Carbon (atomic number 6) has an electronic configuration of 2 electrons in the K shell and 4 in the L shell. Fluorine (atomic number 9) has 2 electrons in the K shell and 7 in the L shell.

🎯 Exam Tip: To find the electronic configuration, distribute the total number of electrons (equal to atomic number for neutral atoms) into successive shells based on their capacities (2, 8, 18, etc.).

Question 3. Nitrogen: Valency three :: Fluorine : ............
Answer: valency one
In simple words: Nitrogen (electronic configuration 2, 5) needs 3 electrons to complete its octet, so its valency is 3. Fluorine (electronic configuration 2, 7) needs 1 electron for its octet, thus its valency is 1.

🎯 Exam Tip: Valency is typically determined by the number of electrons an atom needs to gain or lose to achieve a stable outer shell (usually 8 electrons, or 2 for hydrogen/helium).

Question 4. Atomic radius : pm : Atomic mass : ............
Answer: u
In simple words: Atomic radius is measured in picometers (pm), a very small unit of length, while atomic mass is measured in atomic mass units (u) or Daltons.

🎯 Exam Tip: Be familiar with the appropriate units for different atomic measurements; precise units are important for scientific communication.

Question 5. NaH: Valency of Na : 1 : : \(MgCl_2\) : Valency of Mg ............
Answer: 2
In simple words: In sodium hydride (NaH), hydrogen has a valency of 1, implying sodium also has a valency of 1. In magnesium chloride (\(MgCl_2\)), since chlorine has a valency of 1 and there are two chlorine atoms, magnesium must have a valency of 2 to balance the charges.

🎯 Exam Tip: To determine valency from a chemical formula, use the known valency of one element and the stoichiometry to deduce the valency of the other element, ensuring the compound is electrically neutral.

Question 6. \({}^{35}_{17}\mathrm{Cl}\): number of neutrons 18 : : \({}^{37}_{17}\mathrm{Cl}\) : ............
Answer: number of neutrons 20
In simple words: For \({}^{35}_{17}\mathrm{Cl}\), the number of neutrons is 35 (mass number) - 17 (atomic number) = 18. For \({}^{37}_{17}\mathrm{Cl}\), the number of neutrons is 37 (mass number) - 17 (atomic number) = 20.

🎯 Exam Tip: Practice calculating the number of neutrons for different isotopes by using the formula: Neutrons = Mass Number - Atomic Number.

Question 7. Protons: Positive :: ............ : Neutral.
Answer: Neutrons.
In simple words: Protons carry a positive electrical charge, while neutrons are electrically neutral, meaning they have no net charge.

🎯 Exam Tip: Recall the charges of the subatomic particles: protons are positive, electrons are negative, and neutrons are neutral.

Match The Following:

Question 1.

Column I	Column II
1. Thomson	a. Well defined orbits
2. Rutherford	b. Neutron
3. Chadwick	c. Scattering experiment
4. Bohr	d. Electron

Answer:

Column I	Column II
1. Thomson	d. Electron
2. Rutherford	c. Scattering experiment
3. Chadwick	b. Neutron
4. Bohr	a. Well defined orbits

In simple words: This matching exercise connects atomic theorists with their key discoveries or concepts: Thomson discovered the electron, Rutherford conducted the scattering experiment leading to the nucleus, Chadwick discovered the neutron, and Bohr proposed well-defined electron orbits.

🎯 Exam Tip: Create a timeline or a concept map linking scientists to their major contributions in atomic theory to easily recall them during exams.

Question 2.

Column I	Column II
1. Atom	a. Treatment of goitre
2. Isotopes of iodine	b. Protons + Neutrons
3. Atomic mass number	c. Different number of neutrons
4. Isotopes	d. Electrically neutral

Answer:

Column I	Column II
1. Atom	d. Electrically neutral
2. Isotopes of iodine	a. Treatment of goitre
3. Atomic mass number	b. Protons + Neutrons
4. Isotopes	c. Different number of neutrons

In simple words: Atoms are typically neutral. Iodine isotopes are used for goitre treatment. The atomic mass number is the sum of protons and neutrons. Isotopes differ in neutron count.

🎯 Exam Tip: Review the definitions of basic atomic terms and the applications of radioactive isotopes, as these are common areas for matching questions.

Question 3.

Column I	Column II
1. Alpha particles	a. Inert element
2. Uranium-235	b. Negatively charged particles
3. Helium	c. Positively charged particles
4. Isotopes	d. Krypton - 92, Barium 141
	e. U-233, Th-232
	f. \({}^{12}\mathrm{C}\), \({}^{13}\mathrm{C}\), \({}^{14}\mathrm{C}\)

Answer:

Column I	Column II
1. Alpha particles	c. Positively charged particles
2. Uranium-235	d. Krypton - 92, Barium 141
3. Helium	a. Inert element
4. Isotopes	f. \({}^{12}\mathrm{C}\), \({}^{13}\mathrm{C}\), \({}^{14}\mathrm{C}\)

In simple words: Alpha particles are positively charged. Uranium-235 fission produces Krypton-92 and Barium-141. Helium is an inert element. Carbon-12, -13, and -14 are isotopes.

🎯 Exam Tip: This question tests knowledge of various concepts including properties of subatomic particles, nuclear reactions, types of elements, and definitions of isotopes.

Distinguish Between The Following:

Question 1. Proton and Neutron:
Answer:

Proton	Neutron
1. The proton is positively charged particle	1. The neutron does not have any charge.
2. In an atom, the number of protons is always equal to the number of electrons.	2. In general, in an atom, the number of neutrons is not equal to the number of electrons.

In simple words: Protons are positive particles that define an element's identity and equal the number of electrons in a neutral atom, while neutrons are neutral particles whose count can vary without changing the element.

🎯 Exam Tip: When distinguishing, focus on contrasting properties like charge, location, and role in defining the element or its mass.

Question 2. Neutron and Electron:
Answer:

Neutron	Electron
1. The neutron is an electrically neutral particle.	1. The electron is a negatively charged particle.
2. Neutrons are present in the nucleus of an atom.	2. Electrons revolve around the nucleus of the atom in specific orbits.

In simple words: Neutrons are neutral and found in the nucleus, contributing significantly to mass, whereas electrons are negative, orbit the nucleus, and have negligible mass.

🎯 Exam Tip: Highlight the fundamental differences in charge, location, and relative mass when comparing subatomic particles.

Question 3. Proton and Electron:
Answer:

Proton	Electron
1. The proton is a positively charged particle	1. The electron is a negatively charged particle.
2 Protons are present in the nucleus of an atom.	2. Electrons revolve around the nucleus of the atom in specific orbits.

In simple words: Protons are positively charged particles located in the atom's nucleus, while electrons are negatively charged particles that orbit the nucleus.

🎯 Exam Tip: Focus on the contrasting charges and distinct locations of protons and electrons, as these are their most defining characteristics.

Question 4. Atomic number and Atomic mass number:
Answer:

Atomic number	Atomic mass number
1. The number of electrons or protons in an atom is called the atomic number.	1. The sum of the number of protons and neutrons in the nucleus of an atom is called the atomic mass number.
2. All the isotopes of an element have the same atomic number.	2. Different isotopes of the same element have different mass numbers.
3. The atomic number is represented by the letter Z.	3. The atomic mass number is represented by the letter A.

In simple words: Atomic number (Z) defines an element by its protons, remaining constant for isotopes, while atomic mass number (A) is the sum of protons and neutrons, which varies among isotopes.

🎯 Exam Tip: Clearly differentiate between atomic number (identity, Z) and atomic mass number (mass, A) and how they relate to protons, neutrons, and isotopes.

Answer The Following Questions In One Sentence Each:

Question 1. Name the particles which are present in the nucleus of an atom.
Answer: Protons and neutrons are present in the nucleus of an atom.
In simple words: The nucleus of an atom contains protons and neutrons.

🎯 Exam Tip: Remember that protons carry positive charge and neutrons are neutral, and together they form the dense core of the atom.

Question 2. State the relation between the number of protons, the number of neutrons and the atomic mass number (A) of an element.
Answer: Atomic mass number (A) = Number of protons (p) + Number of neutrons (n)
In simple words: The atomic mass number is the total count of protons and neutrons in an atom's nucleus.

🎯 Exam Tip: This fundamental equation is critical for calculating any of these three values if the other two are known.

Question 3. Chlorine contains 17 protons and 18 neutrons. What is its atomic mass number?
Answer: The atomic mass number of chlorine is 35.
In simple words: Adding the 17 protons and 18 neutrons gives a total atomic mass number of 35 for chlorine.

🎯 Exam Tip: Atomic mass number is simply the sum of protons and neutrons in the nucleus.

Question 4. Carbon contains 6 protons and 6 neutrons. State its atomic number and atomic mass number.
Answer: The atomic number of carbon is 6 and the atomic mass number is 12.
In simple words: Carbon's atomic number is 6 (equal to its protons), and its atomic mass number is 12 (6 protons + 6 neutrons).

🎯 Exam Tip: The atomic number determines the element, and the atomic mass number is the sum of its nucleons.

Question 5. State one use of isotopes of cobalt.
Answer: Isotopes of Cobalt-60 are used in the treatment of cancer.
In simple words: Cobalt-60 is a radioactive isotope primarily used in medicine for radiation therapy to treat cancer.

🎯 Exam Tip: Remember specific applications of radioactive isotopes, especially in medical fields like oncology.

 

Question 6. State one use of isotopes of i uranium.
Answer: Isotopes of uranium are used for production of electrical energy.
In simple words: Uranium isotopes are key for generating electricity in nuclear power plants.

🎯 Exam Tip: Focus on the specific application of isotopes, as these are frequently tested for their practical relevance.

 

Question 7. Write the electronic configuration of oxygen.
Answer: Electronic configuration of oxygen is 2, 6.
In simple words: Oxygen's electrons are arranged with 2 in the first shell and 6 in the second shell.

🎯 Exam Tip: Remember the maximum capacity of the K (2) and L (8) shells when writing electronic configurations.

 

Question 8. Write the electronic configuration of chlorine.
Answer: Electronic configuration of chlorine is 2, 8, 7.
In simple words: Chlorine's electrons are distributed as 2 in the first shell, 8 in the second, and 7 in the outermost third shell.

🎯 Exam Tip: Ensure the total number of electrons in the configuration matches the atomic number of the element.

 

Question 9. State the number of electrons in the L shell of lithium.
Answer: There is one electron in the L shell of lithium.
In simple words: Lithium, having 3 electrons in total, places 2 in the K shell and 1 in its L shell.

🎯 Exam Tip: Understand that the K shell fills first with 2 electrons before any electrons occupy the L shell.

 

Question 10. State the number of electrons in the M shell of argon.
Answer: There are 8 electrons in the M shell of argon.
In simple words: Argon has 18 electrons, with 2 in K, 8 in L, and 8 in the M shell, making its outermost shell complete.

🎯 Exam Tip: Noble gases like Argon have full outermost shells, contributing to their inertness.

 

Question 11. State the number of electrons in the K shell of helium.
Answer: There are two electrons in the K shell of helium.
In simple words: Helium, with only 2 electrons, fills its first and only K shell completely.

🎯 Exam Tip: The K shell is the innermost shell and can hold a maximum of 2 electrons.

 

Question 12. Name isotopes of hydrogen.
Answer: Hydrogen, Deuterium and Tritium are the isotopes of hydrogen.
In simple words: The three common forms of hydrogen with different neutron counts are hydrogen (protium), deuterium, and tritium.

🎯 Exam Tip: Know the difference between these isotopes, particularly their neutron count, as it defines them.

 

Question 13. Name two isotopes of carbon.
Answer: \(_{6}^{12} \mathrm{C}\) and \(_{6}^{14} \mathrm{C}\) are the isotopes of carbon.
In simple words: Two common isotopes of carbon are Carbon-12 and Carbon-14, which differ in their neutron count while having the same number of protons.

🎯 Exam Tip: Isotopes of an element always have the same atomic number (protons) but different mass numbers (neutrons).

 

Question 14. Name two elements in which the K and L shells of an atom are completely filled with electrons.
Answer: The elements are neon and argon.
In simple words: Neon and Argon are noble gases that have their first two electron shells (K and L) completely filled, leading to their stability.

🎯 Exam Tip: Elements with completely filled electron shells are chemically stable and unreactive.

 

Question 15. From the symbol \(_{8}^{16}\text{O}\), state the electronic configuration of oxygen and the atomic mass number of isotope oxygen.
Answer: Electronic configuration of oxygen : 2, 6. Atomic mass number of oxygen : 16.
In simple words: For Oxygen-16, its electron arrangement is 2, 6, and its atomic mass number is 16, derived from 8 protons and 8 neutrons.

🎯 Exam Tip: The subscript in \(_{Z}^{A}\text{X}\) represents the atomic number (Z, also proton number), and the superscript represents the mass number (A).

 

Question 16. The atomic mass number of an element is 18, and the element contains 8 electrons. What is the number of protons and neutrons in it?
Answer: Number of protons : 8, Number of neutrons : 10.
In simple words: An atom with 8 electrons implies 8 protons (for neutrality), and with an atomic mass of 18, it must contain 10 neutrons (18 - 8 = 10).

🎯 Exam Tip: Remember that in a neutral atom, the number of electrons equals the number of protons. Atomic mass number = protons + neutrons.

 

Question 17. An atom contains 2 protons, 2 electrons and 3 neutrons. State its atomic number and atomic mass number.
Answer: Atomic number: 2, Atomic mass number: 5.
In simple words: With 2 protons, the atomic number is 2. The mass number is the sum of protons and neutrons, which is \(2 + 3 = 5\).

🎯 Exam Tip: Atomic number is solely determined by the number of protons, while atomic mass number is protons plus neutrons.

 

Question 18. How many electrons could there be in the outermost orbit of an element whose valency is 3?
Answer: If the valency of an element is 3 then there is a possibility of 3 or 5 electrons in the outermost orbit.
In simple words: An element with a valency of 3 can either have 3 valence electrons (which it loses) or 5 valence electrons (which it gains 3 to complete its octet).

🎯 Exam Tip: Valency can be determined by the number of electrons an atom needs to lose or gain to achieve a stable outer shell (usually 8 electrons, or 2 for the first shell).

 

Question 19. Which element is used as fuel in atomic reactors?
Answer: Isotopes of uranium are used as fuel in atomic reactors.
In simple words: Uranium, specifically certain isotopes, is the primary fuel source for atomic reactors, undergoing fission to produce energy.

🎯 Exam Tip: Recall that Uranium-235 is the most common isotope used for nuclear fission in reactors.

 

Question 20. Name the place and the first nuclear reactor in India.
Answer: Apsara at Bhabha Atomic Research Centre in Mumbai is the first nuclear reactor in India.
In simple words: India's first nuclear reactor, named Apsara, is located at the Bhabha Atomic Research Centre in Mumbai.

🎯 Exam Tip: This is a factual question; remembering names and locations is key.

Answer The Following Questions

 

Question 1. Explain Dalton's atomic theory. OR Write the postulates of Dalton's atomic theory.
Answer: Dalton's atomic theory:
1. According to Dalton's atomic theory matter is made up of atoms and atoms are indivisible and indestructible.
2. All atoms of an element are alike while different element have different atom with different mass.
In simple words: Dalton's theory proposed that all matter consists of tiny, indivisible atoms, with atoms of the same element being identical and atoms of different elements having different masses.

🎯 Exam Tip: Highlight the key concepts of indivisibility and the distinct nature of atoms for different elements.

Try This

 

Question a. Take a solid ball and a Bundi Laddu. Press both these spheres with your palms. What did you find?
Answer: Solid ball does not break and Bundi Laddu breaks into pieces if you press hard.
In simple words: When pressed, a solid ball retains its shape, while a Bundi Laddu, being less cohesive, breaks apart easily.

🎯 Exam Tip: This is an observation-based question, focusing on the physical properties of the objects.

 

Question b. Cut the solid ball with a sharp knife. What did you find?
Answer: There is no cavity inside the ball.
In simple words: Cutting a solid ball reveals that it is uniformly filled inside, without any hollow spaces.

🎯 Exam Tip: This experiment highlights the "solid" nature of the ball, which contrasts with atomic models that propose empty space.

 

Question 2. How will you think about atomic mass distribution according to Thomson's I model? Whether this distribution is uniform or non uniform as per Dalton's atomic theory? (Use your brainpower!
Answer: In Thomson's model the distribution of atomic mass is uniform. Dalton's atomic theory does not say anything about atomic mass distribution.
In simple words: Thomson's model suggests a uniform mass distribution within the atom (like a plum pudding), whereas Dalton's theory doesn't specify how mass is distributed, only that atoms are solid and indivisible.

🎯 Exam Tip: Differentiate clearly between the ideas of different atomic models regarding mass distribution. Thomson's model implies uniformity.

 

Question 3. If the striker flicked by you misses the coin that you aimed at, where would the striker go? Can you tell?
Answer: The striker will continue to move practically with the same velocity till it strikes some other coin or the edge of the board or enter the pocket.
In simple words: If a carrom striker misses its target, it will continue in a straight line with roughly constant speed until another object or boundary stops it.

🎯 Exam Tip: This question relates to Newton's first law of motion (inertia) in a practical context.

 

Question 4. If the striker hits the coin, in which direction would it go? Straight forward to a side or in the reverse direction?
Answer: Striker may go straight forward or it may be deflected depending upon the exact direction of the motion of the striker.
In simple words: When a striker hits a coin, its direction can change depending on the angle of impact-it could continue forward, deflect to the side, or even bounce back if hit directly.

🎯 Exam Tip: The outcome of a collision depends heavily on the angle of impact and the relative masses, illustrating principles similar to Rutherford's scattering experiment.

 

Question 5. Draw a neat labelled diagram of Thomson's atomic model.
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र थॉमसन के परमाणु मॉडल को दर्शाता है, जिसे "प्लम पुडिंग" मॉडल भी कहा जाता है। इसमें एक धनात्मक रूप से आवेशित गोलाकार 'जेल' दिखाया गया है जिसके भीतर नकारात्मक रूप से आवेशित इलेक्ट्रॉन (माइनस चिह्न वाले छोटे गोले) समान रूप से वितरित या धंसे हुए हैं। यह मॉडल दर्शाता है कि परमाणु में धनात्मक और ऋणात्मक आवेश एक दूसरे को संतुलित करते हैं, जिससे परमाणु विद्युत रूप से उदासीन रहता है।
In simple words: Thomson's atomic model, also known as the plum pudding model, depicts an atom as a sphere of uniformly distributed positive charge with negatively charged electrons embedded within it, like plums in a pudding.

🎯 Exam Tip: For diagrams, clear labels for positive "gel" and embedded electrons are crucial for full marks.

 

Question 6. What were the conclusions drawn from the alpha particle experiment performed hy Rutherford?
Answer: On the basis of the alpha particle experiment the following conclusions were drawn by Rutherford:
1. An atom has tiny, dense positively charged nucleus at centre of an atom.
2. Most of the mass of the atom is concentrated in the nucleus.
3. Negatively charged particles called electrons revolve around the nucleus.
4. The total negative charge on all the ; electron is equal to the positive charge on' the nucleus. As the opposite charges are balanced, the atom is electrically neutral.
5. There is an empty space between the i revolving electron and the atomic nucleus.
In simple words: Rutherford's alpha particle experiment concluded that an atom has a tiny, dense, positively charged nucleus where most of its mass is concentrated, with electrons revolving around it, and a significant portion of the atom is empty space.

🎯 Exam Tip: Memorize these key conclusions as they fundamentally changed the understanding of atomic structure and are frequently asked.

 

Question 7. Explain Rutherford's atomic model.
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र रदरफोर्ड के परमाणु मॉडल को दर्शाता है, जिसे सौर मंडल मॉडल भी कहते हैं। इसमें केंद्र में एक छोटा, सघन, धनात्मक आवेशित नाभिक (न्यूक्लियस) है। इसके चारों ओर, इलेक्ट्रॉन (नकारात्मक आवेश) विशिष्ट कक्षाओं में घूमते हुए दिखाए गए हैं। चित्र में प्रोटॉन (धनात्मक), न्यूट्रॉन (उदासीन), और इलेक्ट्रॉन (ऋणात्मक) को मूलभूत कणों के रूप में लेबल किया गया है, तथा यह भी बताया गया है कि इलेक्ट्रॉन का द्रव्यमान प्रोटॉन और न्यूट्रॉन की तुलना में बहुत कम होता है।
1. An atom has tiny, dense, positively charged core called the nucleus.
2. Most of the mass of the atom is concentrated in the nucleus.
3. Electrons bear negative charge. They revolve around the nucleus in orbits.
4. The structure of an atom may be regarded as a miniature of the solar system, the nucleus as the sun and the electrons as the planets.
In simple words: Rutherford's atomic model proposes that an atom has a very small, dense, positively charged nucleus at its center, with negatively charged electrons orbiting it in a vast empty space, much like planets orbiting the sun.

🎯 Exam Tip: Focus on the central, dense nucleus and the orbiting electrons as the defining characteristics of Rutherford's model.

 

Question 8. Which discovery did point out that an atom has internal structure? (Use your brainpower!
Answer: The discovery of radioactivity (1896) pointed out that an atom has internal structure.
In simple words: The observation of radioactivity in 1896 provided the first evidence that atoms are not indivisible but possess internal components and structure.

🎯 Exam Tip: Connecting specific discoveries (like radioactivity) to changes in atomic theory is important.

 

Question 9. What is the difference between the solid atom in Dalton's atomic theory and Thomson's atomic model? (Use your brain power!
Answer: The solid atom in Dalton's atomic theory is a single particle it does not have any structure and also does not have any constituents. In Thomson's model, atom is made of positive and negative charges. Thus it is not a single particle. It has constituents and has internal structure.
In simple words: Dalton's atom was a simple, indivisible solid sphere, while Thomson's atom was a composite structure with positive charge distributed throughout and negative electrons embedded within, showing internal parts.

🎯 Exam Tip: Emphasize that Dalton's model lacked internal structure, while Thomson's introduced subatomic particles (electrons) and a positive matrix.

 

Question 10. Explain the difference between the distribution of positive charge in Thomson's atomic model and Rutherford's atomic model. (Use your brain power!
Answer: According to Thomson's atomic model, the atom is a homogeneous sphere of positive charge. According to Rutherford's atomic model, the positive charge is concentrated in the nucleus.
In simple words: Thomson's model diffused positive charge uniformly across the atom, whereas Rutherford's model localized all positive charge and most of the mass in a tiny central nucleus.

🎯 Exam Tip: The key distinction is between a distributed (Thomson) vs. a concentrated (Rutherford) positive charge.

 

Question 11. What is the point difference between the place of electron in the atomic models of Thomson and Rutherford? (Use your brain power!
Answer: According to Thomson's atomic model, the negatively charged electrons are embedded in a gel of positive charge and they are stationary. According to Rutherford's atomic model, the negatively charged electrons revolve around the nucleus.
In simple words: In Thomson's model, electrons are stationary, embedded within a positive sphere. In contrast, Rutherford's model has electrons actively revolving in orbits around a central nucleus.

🎯 Exam Tip: Note the change from stationary (Thomson) to orbiting (Rutherford) electrons as a crucial development in atomic theory.

 

Question 12. What is the thing which is present in Rutherford's atomic model and not present in Dalton's and Thomson's atomic models? (Use your brain power!
Answer: 1. Dalton's atomic model: Dalton's atomic theory does not tell anything about the structure of the atom. There is no mention of electron in Dalton's atomic theory.
Rutherford's atomic model: Rutherford's atomic model tells about the structure of the atom, i.e. the electron revolving around the nucleus.
2. Thomson's atomic model: The electrons are embedded in a gel of positive charge. Here the electrons are stationary and there is no concept of nucleus.
In simple words: Rutherford's model uniquely introduced the concept of a dense, positively charged nucleus with electrons orbiting it, a feature absent in Dalton's indivisible atom and Thomson's plum pudding model lacking a distinct nucleus.

🎯 Exam Tip: The discovery of the nucleus and planetary electron motion is the hallmark of Rutherford's model over its predecessors.

 

Question 13. Explain Niels Rohr's atomic model. OR Write the postulates of Bohr's atomic model.
Answer: The important postulates of Bohr's atomic model are as follows:
1. The electrons revolving around the atomic nucleus lie in the concentric circular orbits at certain distance from the nucleus.
2. Energy of an electron is constant while it is in a particular orbit.
3. When an electron jumps from an inner orbit to an outer orbit it absorbs specific amount of energy, and when it jumps from an outer orbit to an inner orbit it emits specific amount of energy.
4. The energy emitted or absorbed during these transitions is equal to energy difference between the initial state and the final state of the electron.
In simple words: Bohr's model states that electrons orbit the nucleus in fixed, quantized energy levels without losing energy. They can only jump between these levels by absorbing or emitting a specific amount of energy equal to the energy difference between the orbits.

🎯 Exam Tip: Key points are 'quantized energy levels' or 'discrete orbits' and the absorption/emission of specific energy during transitions.

 

Question 14. How many types of subatomic particles are found in atom? Use your brainpower!
Answer: There are three types of subatomic particles found in atom, viz. the proton, neutron and electron.
In simple words: Atoms are composed of three main subatomic particles: protons (positive), neutrons (neutral), and electrons (negative).

🎯 Exam Tip: Always list all three fundamental subatomic particles with their respective charges when asked.

 

Question 15. Which subatomic particles are electrically charged? Use your brain power!
Answer: The proton is a positively charged subatomic particle and the electron is a negatively charged particle.
In simple words: Protons carry a positive charge, while electrons carry a negative charge, making them the electrically charged subatomic particles.

🎯 Exam Tip: Clearly state both the particle and its corresponding charge.

 

Question 16. Which subatomic particles are present in the nucleus? Use your brain power!
Answer: The proton and neutron are present in the nucleus.
In simple words: The nucleus of an atom contains both positively charged protons and neutral neutrons.

🎯 Exam Tip: Protons and neutrons are collectively called nucleons and reside in the nucleus.

 

Question 17. Where are electrons revolving around the nucleus placed? Use your brain power!
Answer: Electrons revolve in the discrete orbits around the nucleus.
In simple words: Electrons orbit the nucleus in specific, discrete energy shells or orbits, not just randomly.

🎯 Exam Tip: Use terms like 'discrete orbits' or 'energy shells' to accurately describe electron location.

 

Question 18. State the characteristics of the neutron.
Answer:
1. The neutron is electrically neutral and is denoted by the symbol 'N'.
2. It is present in the nucleus along with the proton.
3. The mass of the neutron is almost equal to lu which is almost equal to that of the proton.
In simple words: Neutrons are electrically neutral particles found in the atomic nucleus, possessing a mass nearly identical to that of a proton.

🎯 Exam Tip: Remember its neutrality, nuclear location, and mass relative to a proton.

 

Question 19. State the characteristics of protons.
Answer:
1. Protons are positively charged particles and are denoted by the symbol 'P'.
2. They are present in the nucleus of an atom.
3. The mass of the proton is very nearly equal to lu which is almost equal to that of the hydrogen atom.
4. All the elements contain protons.
In simple words: Protons are positively charged particles located in the atom's nucleus, with a mass approximately 1 atomic mass unit (lu), and are fundamental to all elements.

🎯 Exam Tip: Focus on its positive charge, nuclear location, mass, and presence in all atoms as defining features.

 

Question 20. State the characteristics of electrons.
Answer:
1. Electrons are negatively charged particles and are denoted by the symbol 'e'.
2. They have negligible mass.
3. They revolve around the nucleus in certain discrete orbits.
4. In an atom, the number of electrons is equal to the number of protons in its nucleus (Z).
In simple words: Electrons are negatively charged particles with negligible mass that revolve in discrete orbits around the nucleus. In a neutral atom, their number equals the number of protons.

🎯 Exam Tip: Emphasize their negative charge, negligible mass, orbital motion, and role in atomic neutrality.

 

Question 21. The symbol used for oxygen is 'O'. There are 8 protons and 8 neutrons in its nucleus. From this determine the 1 atomic number (Z) and mass number (A) of oxygen and arrange these in a 1 conventional symbol. Use your brain power!
Answer: Atomic mass number
= Protons + Neutrons
= \(8 + 8\)
Atomic mass number = 16
Atomic number, i.e. the proton number = 8
The conventional symbol = AO
Z = Atomic number
i.e. number of protons is 8.
A = Atomic mass number = 16.
The conventional symbol = \(_{8}^{16}\text{O}\).
In simple words: For oxygen with 8 protons and 8 neutrons, the atomic number (Z) is 8, the mass number (A) is 16, and its conventional symbol is \(_{8}^{16}\text{O}\).

🎯 Exam Tip: Clearly show the calculation for the mass number and the correct placement of A and Z in the isotopic symbol.

 

Question 22. Atomic number of carbon is 6. How many electrons are there in a carbon atom? Use your brainpower!
Answer: The atomic number of carbon, i.e. the proton number of carbon is 6.
The number of electron in the extranuclear part is equal to the number of protons in the nucleus.
. The number of electrons in a carbon atom = 6.
In simple words: Since carbon's atomic number is 6, indicating 6 protons, a neutral carbon atom also has 6 electrons.

🎯 Exam Tip: In a neutral atom, the number of protons always equals the number of electrons.

 

Question 23. A sodium atom contains 11 electrons. What is the atomic number of sodium? Use your brain power!
Answer: The number of electron in the extranuclear part is equal to the number of protons, i.e. atomic number. The atomic number of sodium is 11.
In simple words: A sodium atom with 11 electrons, being electrically neutral, must also have 11 protons, making its atomic number 11.

🎯 Exam Tip: This reinforces the concept of atomic neutrality, where electrons determine the proton count and thus the atomic number.

 

Question 24. The atomic number and mass number of magnesium are 12 and 24 respectively. How will you show this by the convention symbol? Use your brainpower!
Answer: The atomic number, atomic mass number and the symbol of an element are written as:

ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एक परमाणु के पारंपरिक प्रतीक को दर्शाता है। शीर्ष पर (A) द्रव्यमान संख्या (Atomic mass number) लिखी जाती है, जो प्रोटॉन और न्यूट्रॉन का कुल योग होती है। नीचे (Z) परमाणु संख्या (Atomic number) लिखी जाती है, जो प्रोटॉन की संख्या दर्शाती है। केंद्र में (X) तत्व का प्रतीक (Symbol of the element) होता है। यह मानक संकेतन किसी भी परमाणु की द्रव्यमान संख्या, परमाणु संख्या और तत्व प्रतीक को एक साथ प्रस्तुत करता है।
\(_{12}^{24}\text{Mg}\) the conventional symbol for magnesium.
In simple words: For magnesium with an atomic number of 12 and a mass number of 24, its conventional symbol is written as \(_{12}^{24}\text{Mg}\).

🎯 Exam Tip: Accurately place the mass number (superscript) and atomic number (subscript) relative to the element symbol.

 

Question 25. The atomic number and mass number of calcium are 20 and 40 respectively. Deduce the number of neutron present in the calcium nucleus. Use your brain power!
Answer: There are 20 neutrons in the nucleus of calcium.
[Note: The number of neutrons N = A – Z = \(40 – 20 = 20\)]
In simple words: To find the number of neutrons, subtract the atomic number (protons) from the mass number: \(40 - 20 = 20\) neutrons for calcium.

🎯 Exam Tip: The formula N = A - Z is fundamental for calculating the number of neutrons in an atom.

 

Question 26. Write a note on distribution of electrons in orbits. OR What is the maximum number of ( electrons that can be accommodated in each of the orbits (shells) K, L, M, N, etc.?
Answer: The number of maximum electrons in different orbits of the atom are fixed. The orbit (shell) closest to the nucleus is: given the number 1, the next orbit the ; number 2... etc. The orbits one are designated; by letters K, L, M, N,... corresponding to the shell numbers n = 1, 2, 3, 4, etc. The maximum number of electrons in a given orbit is \(2n^2\), when n is the number of orbit (shell), thus the first shell has the capacity of 2 electrons, the second 8, the third 18, the fourth 32 and so on. The electrons in the K shell have minimum energy. The electrons in the subsequent shells possess higher energy.

Shell/Orbit Symbol	Number n	\(2n^2\)	Maximum number of electrons in the orbit (shell)
K	1	\(2 \times 1^2\)	2
L	2	\(2 \times 2^2\)	8
M	3	\(2 \times 3^2\)	18
N	4	\(2 \times 4^2\)	32

Write the maximum number of electron in a shell using the above table. K Shell : 2, L Shell : 8, M Shell : 18, N Shell : 32.
In simple words: Electrons are distributed in discrete energy shells (K, L, M, N, corresponding to n=1, 2, 3, 4) around the nucleus. The maximum capacity of each shell is given by the formula \(2n^2\), meaning K can hold 2, L 8, M 18, and N 32 electrons, with innermost shells having lower energy.

🎯 Exam Tip: Clearly state the \(2n^2\) formula and apply it correctly to determine the maximum electron capacity for each shell (K, L, M, N).

 

Question 27. There is a similarity in atomic structure and solar system. The planets revolve around the sun due to the gravitational force. Which force might be acting in the atomic structure? Can you tell?
Answer: The electric force might be acting in the atomic structure. In the atomic structure, electric force is much stronger than gravitational force.
[Note: Here the gravitational force is very very small compared to the electric force, therefore, the gravitational force can be ignored.]
In simple words: In the atomic structure, the electrons are held in orbit around the nucleus primarily by the electrostatic force of attraction between the negatively charged electrons and the positively charged nucleus, rather than gravity.

🎯 Exam Tip: Recognize that electrostatic (coulombic) force is the dominant interaction in atomic systems, not gravitational force.

 

Question 28. Positively charged proton are together in the nucleus. What might be, one of the function of the neutrons in the nucleus? Can you tell?
Answer: One of the function of the neutrons in the nucleus is to keep the protons and neutrons together by nuclear force.
In simple words: Neutrons help stabilize the nucleus by providing the strong nuclear force, which counteracts the electrostatic repulsion between positively charged protons, holding the nucleus together.

🎯 Exam Tip: The strong nuclear force, mediated by neutrons, is crucial for holding the nucleus together against proton-proton repulsion.

 

Question 29. What do you understand by electronic configuration?
Answer: The shellwise distribution of the electron in an atom of an element is called the electronic configuration.?
In simple words: Electronic configuration is the systematic arrangement of electrons in different energy shells or orbits within an atom.

🎯 Exam Tip: Define electronic configuration clearly as the distribution of electrons in specific shells.

 

Question 30. Write the electronic configuration of the following elements: (1) Hydrogen, (2) Magnesium. (3) Aluminium, (4) Phosphorus, (5) Sulphur.
Answer:

Atom	Symbol	Electron number in the atom	Distribution of electrons shell symbol K (2)	L (8)	M (18)	N (32)	Electronic configuration in the numerical form
Hydrogen	H	1	1				1
Magnesium	Mg	12	2	8	2		2, 8, 2
Aluminium	Al	13	2	8	3		2, 8, 3
Phosphorus	P	15	2	8	5		2, 8, 5
Sulphur	S	16	2	8	6		2, 8, 6

In simple words: The electronic configurations for Hydrogen, Magnesium, Aluminium, Phosphorus, and Sulphur are 1; 2, 8, 2; 2, 8, 3; 2, 8, 5; and 2, 8, 6 respectively, showing how their electrons are distributed across shells.

🎯 Exam Tip: Practice writing electronic configurations by correctly filling each shell according to its maximum capacity (2, 8, 18, 32...).

 

Question 31. Use the following molecular formulae to determine the valencies of H, Cl, O, S, N, C, Br, I, Na. Molecular formulae - H2, HCl, H2O, H2S, NH3, CH4, HBr, HI, NaH. Can you recall?
Answer:

Molecular formula	Valencies
HCl	H = 1, Cl = 1
H2	H = 1
H2O	H = 1, O = 2
H2S	H = 1, S = 2
NH3	H = 1, N = 3
CH4	H = 1, C = 4
HBr	H = 1, Br = 1
HI	H = 1, I = 1
NaH	H = 1, Na = 1

In simple words: By analyzing the given molecular formulas, we can determine the valencies of the elements: Hydrogen, Chlorine, Bromine, Iodine, and Sodium all have a valency of 1; Oxygen and Sulfur have a valency of 2; Nitrogen has a valency of 3; and Carbon has a valency of 4.

🎯 Exam Tip: Valency is the combining capacity of an element; in these examples, hydrogen typically has a valency of 1, and other valencies are derived from how many hydrogen atoms (or equivalent) they combine with.

 

Question 32. Draw suitable diagrams to show the electronic configuration of the atoms of the following elements: Hydrogen, helium, carbon, neon, sodium, chlorine.
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र हाइड्रोजन (Z=1), हीलियम (Z=2), कार्बन (Z=6), नियॉन (Z=10), सोडियम (Z=11), और क्लोरीन (Z=17) परमाणुओं के इलेक्ट्रॉनिक विन्यास को दर्शाता है। प्रत्येक परमाणु के केंद्र में नाभिक (प्रोटॉन और न्यूट्रॉन दर्शाते हुए) दिखाया गया है, और इसके चारों ओर वृत्त विभिन्न इलेक्ट्रॉन कक्षाओं (शेल्स) का प्रतिनिधित्व करते हैं, जिनमें इलेक्ट्रॉन (ऋणात्मक चिह्न) वितरित हैं। यह चित्र स्पष्ट रूप से इन तत्वों में इलेक्ट्रॉन वितरण के कोश-वार पैटर्न को दिखाता है।
In simple words: The diagrams illustrate the electronic configurations for Hydrogen, Helium, Carbon, Neon, Sodium, and Chlorine, showing the central nucleus with protons and neutrons, and electrons orbiting in distinct shells according to their respective atomic numbers.

🎯 Exam Tip: For diagrams, clearly label the nucleus (with proton and neutron count if possible) and show the correct number of electrons in each shell, represented by concentric circles.

 

Question 33. What are the symbols used for the shells which accommodate the electrons in various atoms? Use your brain power!
Answer: The symbols used for the shells which accommodate the electrons are K, L, M, N, ... respectively.
In simple words: The electron shells around an atom's nucleus are conventionally labeled with capital letters, starting from the innermost shell as K, then L, M, N, and so on.

🎯 Exam Tip: Remember the alphabetical order (K, L, M, N) for the electron shells starting from the one closest to the nucleus.

The symbols used for the shells which accommodate the electrons are K, L, M, N, ... respectively.

Question 34. What is the symbol and ordinal number of the innermost shell?
Answer: The symbol of the innermost shell is K and ordinal number of the innermost shell is 1.
In simple words: The K shell is the closest to the nucleus and is designated as the 1st shell.

🎯 Exam Tip: Remember the K, L, M, N notation and their corresponding principal quantum numbers (1, 2, 3, 4) for electron shells.

Question 35. Write symbol of electron distribution in shell of fluorine atom?
Answer: Symbol for fluorine atom = F,
Electronic configuration of fluorine
K L
2-7
In simple words: Fluorine (F) has an atomic number of 9, meaning it has 9 electrons. Its electronic configuration is 2 electrons in the K shell and 7 electrons in the L shell.

🎯 Exam Tip: Practice writing electronic configurations for common elements to easily recall them.

Question 36. Which is the outermost shell of fluorine atom?
Answer: The outermost shell of fluorine atom is L.
In simple words: The L shell contains the valence electrons for fluorine, making it the outermost shell.

🎯 Exam Tip: The outermost shell is where chemical reactions occur, making it crucial for understanding an element's reactivity.

Question 37. Which is the outermost shell of sodium atom?
Answer: The outermost shell of sodium atom is M shell.
In simple words: Sodium (atomic number 11) has an electronic configuration of 2, 8, 1, so its single valence electron is in the M shell.

🎯 Exam Tip: Knowing the electronic configuration helps identify the outermost shell and thus, the valency of an element.

Question 38. Which is the outermost shell of hydrogen atom?
Answer: The outermost shell of hydrogen is K shell.
In simple words: Hydrogen has only one electron, which occupies the first (K) shell, making it its outermost and only shell.

🎯 Exam Tip: Hydrogen is unique with its single electron in the K shell, influencing its role in bonding.

Question 39. What is meant by the atomic number (Z) of an element?
Answer: Atomic number (Z)- The number of electrons or protons in an atom is called the atomic number. It is denoted by Z.
In simple words: The atomic number (Z) uniquely identifies an element by counting its protons (or electrons in a neutral atom).

🎯 Exam Tip: The atomic number is fundamental as it determines an element's identity and its position in the periodic table.

Question 40. Atomic numbers (Z) of some elements are given here. Write down the number of electron present in the outermost shell of each of them.
Answer:

Element	H	C	Na	Cl	N
Z	1	6	11	17	7
Number of electrons in the outermost shell	1	4	1	7	5

In simple words: For neutral atoms, the atomic number equals the number of electrons. The outermost shell electron count is determined by their electronic configuration (K, L, M, N shells).

🎯 Exam Tip: To find outermost shell electrons, write the electronic configuration; the last number represents the valence electrons.

Question 41. The number of electrons of some elements is given here. By using it write the electronic configuration, number of valence electron and valency of the respective elements.
Answer:

Element	Li	C	Mg	O
Number of electrons	3	6	12	8
Electronic configuration	2, 1	2, 4	2, 8, 2	2, 6
Number of valence electrons	1	4	2	6
Valency	1	4	2	2

In simple words: The electronic configuration shows how electrons are arranged in shells. Valence electrons are in the outermost shell, and valency is determined by how many electrons an atom needs to gain, lose, or share to achieve a stable octet (or duplet).

🎯 Exam Tip: Valency helps predict how atoms will combine. For valence electrons 1-4, valency is equal to the number of valence electrons. For 5-7, valency is 8 minus the number of valence electrons.

Question 42. Why are the atomic numbers and atomic mass numbers always in whole numbers?
Answer: The total number of protons and neutrons in the nucleus of the atom is called the atomic mass number (A). The number of electrons or protons in an atom is called the atomic number (Z). Both protons and neutrons are whole numbers, therefore, atomic numbers and atomic mass numbers are always in whole numbers.
In simple words: Atomic number (protons) and atomic mass number (protons + neutrons) are always whole numbers because protons and neutrons are discrete, indivisible particles.

🎯 Exam Tip: Atomic number (Z) and mass number (A) are counts of whole particles, ensuring they are always integers.

Question 43. Sulphur contains 16 protons and 16 neutrons. What would be its atomic number and mass number?
Answer: The atomic number of sulphur is 16 and the atomic mass number is 32.
In simple words: The atomic number is the count of protons (16), and the mass number is the sum of protons and neutrons (16 + 16 = 32).

🎯 Exam Tip: Atomic number = number of protons; Mass number = number of protons + number of neutrons. Always show your calculation for mass number.

Question 44. State the uses of isotopes.
Answer: Isotopes of some elements are radioactive. Isotopes are used in various fields such as industry, agriculture, medicine, research field.
1. Isotopes of uranium is used for nuclear fission and production of electricity.
2. Cobalt - 60 are used in the treatment of cancer.
3. Iodine - 131 is used in the treatment of goitre.
4. The radioactive isotopes, Sodium - 24 are used for detection of cracks (leakage) in the underground pipes.
5. Radioactive isotopes are used for food preservation from microbes.
6. The radioactive C - 14 is used for determining the age of archaeological objects.
In simple words: Isotopes, particularly radioactive ones, have diverse applications including energy generation (uranium), medical treatments (cobalt-60, iodine-131), industrial uses (sodium-24), food preservation, and archaeological dating (carbon-14).

🎯 Exam Tip: Provide specific examples for the uses of isotopes, categorizing them by field (medical, industrial, etc.) to demonstrate comprehensive knowledge.

Question 45. Write a note on nuclear reactor.
Answer: 1. A machine that generates electricity on large scale by using atomic energy is called a nuclear reactor. In a nuclear reactor, the nuclear energy in atom is released by bringing about nuclear reactions on the nuclear fuel.
2. When uranium - 235 is bombarded with a slow speed neutron, it undergoes nuclear fission. Various elements are produced. For example- Krypton - 92 and Barium - 141 along with 2 to 3 neutrons are emitted on fission, these neutron have high speed. Their speed is reduced and they are used for bombarding more Uranium - 235 nuclei.
The process is repeated many times. In this way a chain reaction of nuclear fission takes place (See the figure). A large amount of nuclear energy is released during a chain reaction of fission. The chain reaction is controlled to prevent the probable explosion.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र यूरेनियम-235 के विखंडन की श्रृंखला अभिक्रिया को दर्शाता है। इसमें एक न्यूट्रॉन यूरेनियम-235 नाभिक से टकराता है, जिससे यह दो छोटे नाभिकों (जैसे क्रिप्टन-92 और बेरियम-141) में टूट जाता है और कई न्यूट्रॉन मुक्त होते हैं, जो आगे और नाभिकों से टकराकर अभिक्रिया जारी रखते हैं।
Neutrons are slowed down using graphite or heavy water as moderator. The chain reaction is controlled by absorbing neutron with the help of rods of boron, cadmium and beryllium. The heat produced in the fission is taken out by water as coolent. Water is converted into steam. The available heat is used to drive turbines to produce electricity.
In simple words: A nuclear reactor uses controlled nuclear fission of uranium-235 to generate electricity, where slow neutrons split atoms, releasing energy and more neutrons in a self-sustaining chain reaction, which is managed by moderators and control rods.

🎯 Exam Tip: Emphasize the role of moderators (to slow neutrons) and control rods (to absorb excess neutrons) in maintaining a controlled chain reaction in a nuclear reactor.

Give Scientific Reasons:

Question 1. In Rutherford's experiment, some alpha particles colliding with the thin gold foil are turned back.
Answer: 1. All the positive charges in an atom is concentrated in a very small region at the centre of the atom. This region is called the nucleus. Electrons revolve around the nucleus, but most of the atom is empty. Alpha particles are positively charged.
2. When a thin sheet of gold is bombarded by alpha particles, most of the particles pass through it without deviation as the atom is almost empty. But some alpha particles move directly towards the positive nuclei, collide with them and due to the electric repulsion, turn back. Thus, in Rutherford's experiment, some alpha particles colliding with the thin gold foil are turned back.
In simple words: Alpha particles are positively charged, and when they encounter the dense, positively charged nucleus in the gold foil, the strong electrostatic repulsion causes them to deflect or even bounce back.

🎯 Exam Tip: The deflection of alpha particles was crucial evidence for the existence of a small, dense, positively charged nucleus within the atom, contradicting the plum pudding model.

Question 2. Two electrons in helium atom are placed in only one shell while three electrons in lithium atom occupy two shells.
Answer: 1. The electronic configuration of helium is (2). It indicates that helium atom has two electrons. Both the electrons are accommodated in the K shell. The maximum capacity of the K shell is two electrons and is maintained in all the elements. Hence, two electrons in helium are placed in only one shell.
2. The electronic configuration of lithium is (2, 1). It indicates that lithium atom has three electrons. The first shell K accommodates two electrons, i.e., the maximum capacity of the K shell. The remaining electron is accommodated in the next shell, i.e., the L shell. Thus, three electrons in lithium occupy two shells (K and L).
In simple words: Helium's two electrons fill its K shell (capacity 2). Lithium's three electrons fill the K shell with two, and the third electron must occupy the next available shell, the L shell, due to the K shell's limited capacity.

🎯 Exam Tip: Electron shell capacities (K=2, L=8, M=18, N=32) dictate how electrons are distributed, explaining why elements like lithium occupy multiple shells despite having few electrons.

Complete The Following:

Question 1. Complete the table by putting tick mark in appropriate box.
Answer: Types of substances:

Name of substance	Formula	Smallest particle of the substance			Type of matter
		is an atom	is a molecule	single type of atoms in the molecule	Element	Compound
				different types of atoms in the molecule
Water	\( \text{H}_2\text{O} \)		✓			✓
Oxygen	\( \text{O}_2 \)		✓	✓	✓
Helium	He	✓		✓	✓
Hydrogen	\( \text{H}_2 \)		✓	✓	✓
Ammonia	\( \text{NH}_3 \)		✓			✓
Nitrogen	\( \text{N}_2 \)		✓	✓	✓
Methane	\( \text{CH}_4 \)		✓			✓
Argon	Ar	✓		✓	✓
Neon	Ne	✓		✓	✓
Chlorine	\( \text{Cl}_2 \)		✓	✓	✓

In simple words: This table classifies substances based on their smallest particles (atoms or molecules) and whether they are elements (one type of atom) or compounds (different types of atoms chemically bonded).

🎯 Exam Tip: Understand the basic definitions of atom, molecule, element, and compound to correctly classify different substances.

Question 2. Complete the table:
Answer:

Shell		Electron capacity of the shell
Symbol	n	Formula : \( 2\text{n}^2 \)	Number of electrons
K	1	\( 2 \times (1)^2 \)	2
L	2	\( 2 \times (2)^2 \)	8
M	3	\( 2 \times (3)^2 \)	18
N	4	\( 2 \times (4)^2 \)	32

Write the maximum number of electron in a shell using the above table. K Shell - 2, L Shell - 8, M Shell - 18, N Shell - 32.
In simple words: The table shows the maximum number of electrons each principal electron shell (K, L, M, N) can hold, calculated using the formula \( 2\text{n}^2 \), where \( n \) is the shell number.

🎯 Exam Tip: Memorize the \( 2\text{n}^2 \) formula for electron capacity to quickly determine the maximum electrons in any given shell.

Question 3. Complete the table:
Answer: Electronic configuration of some elements:

Atom	Symbol	Electron number in the atom	Distribution of electrons Shell symbol (maximum capacity)				Electronic configuration in the numerical form
			K (2)	L (8)	M (18)	N (32)
Hydrogen	H	1	1				1
Helium	He	2	2				2
Lithium	Li	3	2	1			2, 1
Carbon	C	6	2	4			2, 4
Nitrogen	N	7	2	5			2, 5
Oxygen	O	8	2	6			2, 6
Fluorine	F	9	2	7			2, 7
Neon	Ne	10	2	8			2, 8
Sodium	Na	11	2	8	1		2, 8, 1
Chlorine	Cl	17	2	8	7		2, 8, 7
Argon	Ar	18	2	8	8		2, 8, 8
Bromine	Br	35	2	8	18	7	2, 8, 18, 7

In simple words: This table systematically presents the electronic configuration for various elements, showing how electrons are distributed across K, L, M, and N shells based on the element's atomic number.

🎯 Exam Tip: Accurately filling this table requires a strong understanding of atomic number and electron shell capacities (2, 8, 18, 32) in sequence.

Question 4. Complete the following table:
Answer: Relationship between valency and electronic configuration:

Sr. No.	Symbol of element	Molecular formula of compound	Valency of the element	Electronic configuration of the element	Number of valence electrons \( \text{X} \)	\( 8-\text{X} \) (For \( \text{X} \geq 4 \))
1	H	HCl	1	1	1
2	Cl	HCl	1	2, 8, 7	7	8-7=1
3	Ne	No compound	0	2, 8	0
4	F	HF	1	2, 7	7	8-7=1
5	Na	NaH	1	2, 8, 1	1
6	Mg	\( \text{MgCl}_2 \)	2	2, 8, 2	2
7	C	\( \text{CH}_4 \)	4	2, 4	4
8	Al	\( \text{AlCl}_3 \)	3	2, 8, 3	3

In simple words: This table illustrates the relationship between an element's valency, its electronic configuration, and the number of electrons in its outermost shell (valence electrons). For elements with four or more valence electrons, valency is often calculated as \( 8-\text{X} \).

🎯 Exam Tip: Understanding how to determine valency from electronic configuration is key to predicting chemical bonding patterns.

Question 5. In table 4 column you have written of that element? How many electrons identified valency from its molecular are used to complete the octet? formulae. When the number of the valency electrons in an element '\( \text{x} \)' is 4 or less than 4, does '\( \text{x} \)' match with the valency of that element?
Answer: When the number of the valence electrons in an element \( \text{x} \) is 4 or less than 4, then \( \text{x} \) matches with the valency of that element.
In simple words: If an element has 4 or fewer valence electrons, its valency is directly equal to that number, as it tends to lose those electrons to achieve stability.

🎯 Exam Tip: This rule is a quick way to determine valency for elements on the left side of the periodic table.

Question 6. When the number of the valence electrons in an element '\( \text{x} \)' is 4 or more than 4, does'(\( 8-\text{x} \))' match with the valency of that element? How many electrons are used to complete the octet?
Answer: When the number of the valence electrons in an element \( \text{x} \) is 4 or more than 4, then (\( 8-\text{x} \)) matches with the valency of that element. (\( 8-\text{x} \)) electrons are used to complete the octet.
In simple words: For elements with 5, 6, or 7 valence electrons, their valency is calculated by subtracting the valence electrons from 8, as they tend to gain electrons to achieve a stable octet.

🎯 Exam Tip: This rule helps determine valency for elements on the right side of the periodic table, focusing on electron gain.

Question 7. Complete the table:
Answer:

Isotopes	Proton number	Neutron number
\( {}_{1}^{1}\text{H} \)	1	-
\( {}_{1}^{2}\text{H} \)	1	1
\( {}_{17}^{35}\text{Cl} \)	17	18
\( {}_{17}^{37}\text{Cl} \)	17	20

In simple words: This table illustrates isotopes, which are atoms of the same element (same proton number) but with different neutron numbers, leading to different mass numbers.

🎯 Exam Tip: For isotopes, remember that the proton number (atomic number) stays constant, while the neutron number (and thus mass number) varies.