Samacheer Kalvi Class 9 Science Solutions Chapter 11 Atomic Structure

Tamilnadu Samacheer Kalvi 9th Science Solutions Chapter 11 Atomic Structure

9th Science Guide Atomic Structure Text Book Back Questions and Answers

I. Choose the correct answer:

 

Question 1. Among the following the odd pair is
(a) \( {}_{8}^{18}\mathrm{O}, {}_{17}^{37}\mathrm{Cl} \)
(b) \( {}_{18}^{40}\mathrm{Ar}, {}_{7}^{14}\mathrm{N} \)
(c) \( {}_{14}^{30}\mathrm{Si}, {}_{15}^{31}\mathrm{P} \)
(d) \( {}_{24}^{54}\mathrm{Cr}, {}_{19}^{39}\mathrm{K} \)
Answer: (b) \( {}_{18}^{40}\mathrm{Ar}, {}_{7}^{14}\mathrm{N} \)
In simple words: We need to identify the pair that doesn't fit the pattern of isotopes, isobars, or isotones. The pair \( {}_{18}^{40}\mathrm{Ar} \) and \( {}_{7}^{14}\mathrm{N} \) has different atomic numbers and different mass numbers, making it the odd one out compared to the other options which are pairs of isotopes or isotones.

🎯 Exam Tip: To find the odd pair, calculate protons, neutrons, and electrons for each element in the options. This helps classify them as isotopes (same Z, different A), isobars (same A, different Z), or isotones (same N, different Z and A).

 

Question 2. Change in the number of neutrons in an atom changes it to
(a) an ion.
(b) an isotope.
(c) an isobar.
(d) another element.
Answer: (b) an isotope
In simple words: When an atom gains or loses neutrons, it becomes a different version of the same element, called an isotope. The number of protons stays the same, so it's still the same element.

🎯 Exam Tip: Remember that changing protons changes the element, changing electrons makes an ion, and changing neutrons creates an isotope.

 

Question 3. The term nucleons refer to
(a) protons and electrons
(b) only neutrons
(c) electrons and neutrons
(d) protons and neutrons
Answer: (d) protons and neutrons
In simple words: Nucleons are the particles found inside the nucleus of an atom. These are protons, which have a positive charge, and neutrons, which have no charge.

🎯 Exam Tip: Understand that "nucleon" refers to all particles within the nucleus, while "electron" is found outside the nucleus.

 

Question 4. The number of protons, neutrons and electrons present respectively in \( {}_{35}^{80} \mathrm{Br} \) are
(a) 80,80,35
(b) 40,35,40
(c) 35,35,80
(d) 35,45,35
Answer: (d) 35,45,35
In simple words: For \( {}_{35}^{80} \mathrm{Br} \), the atomic number (bottom number) is 35, which means there are 35 protons and, in a neutral atom, 35 electrons. The mass number (top number) is 80. To find neutrons, subtract protons from the mass number: \( 80 - 35 = 45 \) neutrons.

🎯 Exam Tip: Always remember: Atomic Number = Number of Protons = Number of Electrons (in a neutral atom). Mass Number = Number of Protons + Number of Neutrons.

 

Question 5. The correct electronic configuration of potassium is
(a) 2,8,9
(b) 2,8,1
(c) 2, 8, 8, 1
(d) 2, 8, 8, 3
Answer: (c) 2, 8, 8, 1
In simple words: Potassium has an atomic number of 19. This means it has 19 electrons. These electrons fill up shells in a specific order: 2 electrons in the K shell, 8 in the L shell, 8 in the M shell, and the last 1 electron goes into the N shell.

🎯 Exam Tip: Electron shells fill according to rules like \( 2n^2 \), but for elements up to Calcium, the third shell (M) often fills with 8 electrons before the next shell starts. The K shell holds up to 2, L up to 8, M up to 18, and N up to 32.

 

II. State whether true or false. If false, correct the statement:

 

Question 1. In an atom, electrons revolve around the nucleus in fixed orbits.
Answer: True.
In simple words: Electrons move in special paths or energy levels around the center of an atom, just like planets orbit the sun.

🎯 Exam Tip: This statement describes Bohr's model of the atom, which suggests electrons stay in specific energy shells or orbits without losing energy.

 

Question 2. Isotopes of an element have different atomic numbers.
Answer: False.
Correct statement: Same atomic numbers but different mass numbers.
In simple words: Isotopes are different versions of the same element, so they must have the same number of protons (and thus the same atomic number). They only differ in the number of neutrons, which changes their mass.

🎯 Exam Tip: The definition of an element is based on its atomic number (number of protons). Isotopes have the same chemical identity, hence the same atomic number.

 

Question 3. Electrons have negligible mass and charge.
Answer: False.
Correct statement: Electrons have negligible mass but have a definite negative charge.
In simple words: Electrons are very, very light, almost weightless compared to protons and neutrons. However, they definitely carry a negative electrical charge, which is a key part of how atoms interact.

🎯 Exam Tip: While electrons have very little mass, their charge is fundamental to chemistry. Remember that "negligible" mass doesn't mean "zero" mass, and the charge is always definite.

 

Question 4. Smaller the size of the orbit, lower is the energy of the orbit.
Answer: True.
In simple words: Electrons closer to the nucleus are held more tightly and have less energy. As you move to larger orbits further away, the electrons have more energy.

🎯 Exam Tip: This is a key principle of Bohr's atomic model. Electrons in the innermost shells (like K-shell) are in a lower energy state, making them more stable.

 

Question 5. The maximum number of electron in L Shell is 10.
Answer: False.
Correct statement: The maximum number of electrons in L Shell is 8.
In simple words: The L shell is the second electron shell, and it can hold a maximum of 8 electrons. This is based on the \( 2n^2 \) rule, where for the L shell, n equals 2, so \( 2 \times 2^2 = 8 \).

🎯 Exam Tip: Memorize the maximum electron capacities for the first few shells: K-shell (n=1) is 2, L-shell (n=2) is 8, M-shell (n=3) is 18, and N-shell (n=4) is 32.

 

III. Fill in the Blanks:

 

Question 1. Calcium and Argon are examples of a pair of ...........
Answer: isobars
In simple words: Isobars are elements that have different atomic numbers (different elements) but the same mass number. For example, Calcium-40 and Argon-40 both have a mass number of 40 but are different elements.

🎯 Exam Tip: Distinguish between isotopes (same element, different neutrons), isobars (different elements, same mass number), and isotones (different elements, same number of neutrons).

 

Question 2. Total number of electrons that can be accommodated in an orbit is given by .........
Answer: the formula \( 2n^2 \)
In simple words: The formula \( 2n^2 \) tells us how many electrons each electron shell can hold. Here, 'n' stands for the number of the shell (like 1 for the first shell, 2 for the second, and so on).

🎯 Exam Tip: This formula applies to the maximum capacity of a shell. However, the outermost shell of an atom usually holds a maximum of 8 electrons, regardless of its \( 2n^2 \) capacity, for stability.

 

Question 3. .............isotope is used in the nuclear reactors.
Answer: Uranium-235
In simple words: Uranium-235 is a special type of uranium that is split in nuclear reactors to create a lot of energy. This process is called nuclear fission.

🎯 Exam Tip: Remember common applications of specific isotopes, such as Carbon-14 for dating and Cobalt-60 for cancer treatment.

 

Question 4. The number of neutrons present in \( {}_{3}^{7} \mathrm{Li} \) is ...........
Answer: 4
In simple words: For the lithium isotope \( {}_{3}^{7} \mathrm{Li} \), the mass number is 7 and the atomic number is 3. To find the number of neutrons, subtract the atomic number from the mass number: \( 7 - 3 = 4 \).

🎯 Exam Tip: Practice calculating protons, neutrons, and electrons for various elements given their mass and atomic numbers to avoid errors.

 

Question 5. The valency of Argon is ...........
Answer: Zero
In simple words: Argon is a noble gas, meaning its outermost electron shell is completely full (8 electrons). Because it has a full outer shell, it is very stable and does not easily react or combine with other elements, so its combining capacity, or valency, is zero.

🎯 Exam Tip: Elements with completely filled outermost shells are noble gases, which are chemically inert and have zero valency.

 

IV. Match the following:

 

Question 1. Match the following.
Answer:

In simple words: Dalton proposed the first atomic theory, Chadwick found neutrons, Rutherford discovered the nucleus with his gold foil experiment, and Niels Bohr created a model for the hydrogen atom.

🎯 Exam Tip: Associate key scientists with their major contributions to atomic theory, as this is a common type of question to test historical understanding.

 

V. Complete the following table:

 

Question 1. Complete the following table.
Answer:

In simple words: To fill this table, use the rules: Atomic number is the number of protons and electrons. Mass number is the sum of protons and neutrons. Knowing two values helps you find the rest. For instance, if you have 9 protons, the atomic number is 9 and there are 9 electrons. If mass number is 19, then \( 19 - 9 = 10 \) neutrons.

🎯 Exam Tip: Always remember the fundamental relationships: Atomic Number = Protons = Electrons (for neutral atoms). Mass Number = Protons + Neutrons. This will help you fill any missing values in such tables.

 

VI. Answer very briefly:

 

Question 1. Name an element which has the same number of electrons in its first and second shell.
Answer: Beryllium. Atomic number - 4 (K-shell-2, L-shell-2). Beryllium has 2 electrons in its first shell and 2 electrons in its second shell.
In simple words: Beryllium is an element that has two electrons in its first shell and two electrons in its second shell, so both shells have the same number of electrons.

🎯 Exam Tip: To answer such questions, write out the electron configuration of lighter elements (H, He, Li, Be, etc.) until you find one that matches the condition. Beryllium is a common example.

 

Question 2. Write the electronic configuration of K and Cl
Answer:

Potassium (atomic number 19) has 2 electrons in the K shell, 8 in the L shell, 8 in the M shell, and 1 in the N shell. Chlorine (atomic number 17) has 2 electrons in the K shell, 8 in the L shell, and 7 in the M shell.
In simple words: For Potassium (K), the electrons are arranged as 2 in the first shell, 8 in the second, 8 in the third, and 1 in the fourth. For Chlorine (Cl), the electrons are arranged as 2 in the first shell, 8 in the second, and 7 in the third.

🎯 Exam Tip: When writing electronic configurations, ensure the total number of electrons equals the atomic number of the element. Follow the 2, 8, 8, 18... rule for filling shells, keeping in mind the octet rule for stability in outer shells.

 

Question 3. Write down the names of the particles represented by the following symbols and explain the meaning of superscript and subscript numbers attached. \( {}_{1}^{1}\mathrm{H} \), \( {}_{0}^{1}\mathrm{n} \), \( {}_{-1}^{0}\mathrm{e} \)
Answer:
\( {}_{1}^{1}\mathrm{H} \) - This represents a hydrogen atom. The superscript '1' is the mass number, meaning it has 1 nucleon (proton + neutron). The subscript '1' is the atomic number, meaning it has 1 proton. This isotope of hydrogen is also called protium, and its mass is approximately 1 amu.
\( {}_{0}^{1}\mathrm{n} \) - This represents a neutron. The superscript '1' is the mass number, indicating it has a mass similar to a proton. The subscript '0' is the atomic number or charge, indicating it has no charge.
\( {}_{-1}^{0}\mathrm{e} \) - This represents an electron. The superscript '0' is the mass number, indicating its mass is negligible (almost zero) compared to protons and neutrons. The subscript '-1' is the charge, indicating it has a negative charge of -1.
In simple words: The top number is the mass (how heavy it is), and the bottom number is the charge or atomic number (how many protons). So, \( {}_{1}^{1}\mathrm{H} \) is a hydrogen atom with 1 mass and 1 proton. \( {}_{0}^{1}\mathrm{n} \) is a neutron with 1 mass and no charge. \( {}_{-1}^{0}\mathrm{e} \) is an electron with almost no mass and a negative charge of -1.

🎯 Exam Tip: Clearly differentiate between superscript (mass number) and subscript (atomic number/charge) when interpreting chemical symbols. Remember their standard values for common subatomic particles.

 

Question 4. For an atom 'X', K, L, and M shells are completely filled. How many electrons will be present in it?
Answer:
K-shell capacity = 2 electrons
L-shell capacity = 8 electrons
M-shell capacity = 18 electrons
If K, L, and M shells are completely filled, the total number of electrons will be \( 2 + 8 + 18 = 28 \) electrons. This element is Nickel.
In simple words: The K shell holds 2 electrons, the L shell holds 8, and the M shell holds 18. If all three are full, the atom will have \( 2 + 8 + 18 = 28 \) electrons in total. This element is Nickel.

🎯 Exam Tip: Remember the maximum capacities of electron shells: K (2), L (8), M (18), N (32). Ensure you sum them correctly for a completely filled atom.

 

Question 5. What is the same about the electron structures of: a. Lithium, Sodium and Potassium. b. Beryllium, Magnesium and Calcium.
Answer:
a. Lithium, Sodium, and Potassium all have 1 electron in their outermost shell. These elements belong to Group 1 of the periodic table, known as alkali metals.
b. Beryllium, Magnesium, and Calcium all have 2 electrons in their outermost shell. These elements belong to Group 2 of the periodic table, known as alkaline earth metals.
In simple words: For Lithium, Sodium, and Potassium, they all have just one electron in their very last electron shell. For Beryllium, Magnesium, and Calcium, they all have two electrons in their very last electron shell. This similarity makes them behave alike in chemical reactions.

🎯 Exam Tip: The number of electrons in the outermost shell (valence electrons) largely determines an element's chemical properties and its position in the periodic table (group number).

 

VII. Answer briefly:

 

Question 1. How was it shown that atom has empty space?
Answer: In Rutherford's Alpha ray scattering experiment, he bombarded a thin gold foil with very small, positively charged alpha particles. He observed that most of the alpha particles passed straight through the foil without deflection. This led him to conclude that most of the space within an atom is empty. Only a small fraction of particles were deflected or bounced back, indicating a small, dense, positively charged nucleus.
In simple words: Rutherford shot tiny positive particles at a thin gold sheet. Most particles went straight through, like passing through an empty room. This showed that atoms are mostly empty space, with a small, dense center.

🎯 Exam Tip: When describing Rutherford's experiment, focus on the key observation (most particles passed through) and the inference (atoms are mostly empty space) directly linked to that observation.

 

Question 2. Why do \( {}_{17}^{35} \mathrm{Cl} \) and \( {}_{17}^{37} \mathrm{Cl} \) have the same chemical properties? In what respect do these atoms differ?
Answer:
\( {}_{17}^{35} \mathrm{Cl} \) and \( {}_{17}^{37} \mathrm{Cl} \) are isotopes of chlorine. They have the same chemical properties because their atomic number (number of protons) is the same (17). This means they have the same number of electrons (17) in a neutral atom, and it is these valence electrons that determine an element's chemical behavior. The chemical reactivity depends on the electronic configuration, which is identical for these isotopes.
These atoms differ in their number of neutrons and thus their mass number. \( {}_{17}^{35} \mathrm{Cl} \) has \( 35 - 17 = 18 \) neutrons, while \( {}_{17}^{37} \mathrm{Cl} \) has \( 37 - 17 = 20 \) neutrons. This difference in neutron count leads to different physical properties, such as mass and density.
In simple words: These two types of chlorine atoms have the same chemical properties because they both have 17 protons and 17 electrons. The electrons are what make atoms react with others. They are different because they have a different number of neutrons (one has 18, the other has 20), which makes one slightly heavier than the other.

🎯 Exam Tip: Chemical properties are determined by electron configuration (primarily valence electrons), while physical properties like mass and density are affected by the total number of protons and neutrons (mass number).

 

Question 3. Draw the structure of oxygen and sulphur atoms.
Answer:The diagrams show the central nucleus with protons (P) and neutrons (N), surrounded by electron shells. Oxygen has 2 electrons in the first shell and 6 in the second. Sulphur has 2 in the first, 8 in the second, and 6 in the third. The outer shells for both elements have 6 electrons, placing them in the same group of the periodic table.
In simple words: The drawings show Oxygen with 8 protons and 8 neutrons in the middle, and 2 electrons in the first ring, 6 in the second. Sulphur has 16 protons and 16 neutrons in its core, with 2 electrons in the first ring, 8 in the second, and 6 in the third.

🎯 Exam Tip: When drawing atomic structures, clearly label the nucleus with protons and neutrons, and show the correct number of electrons in each shell according to the atomic number.

 

Question 4. Calculate the number of neutrons, protons and electrons: (i) atomic number 3 and mass number 7 (ii) atomic number 92 and mass number 238.
Answer:
(i) For an atom with atomic number 3 and mass number 7 (e.g., Lithium-7):
Number of protons = Atomic number = 3
Number of electrons = Atomic number = 3 (for a neutral atom)
Number of neutrons = Mass number - Atomic number = \( 7 - 3 = 4 \)
(ii) For an atom with atomic number 92 and mass number 238 (e.g., Uranium-238):
Number of protons = Atomic number = 92
Number of electrons = Atomic number = 92 (for a neutral atom)
Number of neutrons = Mass number - Atomic number = \( 238 - 92 = 146 \)
In simple words: For any atom, the atomic number tells you how many protons and electrons it has. To find neutrons, you just subtract the atomic number from the mass number.

🎯 Exam Tip: Always state whether you are considering a neutral atom when determining the number of electrons. The number of protons is always equal to the atomic number, regardless of charge.

 

Question 5. What are nucleons? How many nucleons are present in Phosphorous? Draw its structure.
Answer: Nucleons are the subatomic particles that make up the atomic nucleus. These include both protons and neutrons. The mass number of an atom represents the total number of nucleons.
Phosphorus (P) has an atomic number of 15 and a mass number of 31. Therefore, it has 15 protons and \( 31 - 15 = 16 \) neutrons. The total number of nucleons in Phosphorus is the sum of protons and neutrons, which is \( 15 + 16 = 31 \).
The diagram shows the phosphorus atom with 15 protons and 16 neutrons in the nucleus. It has 2 electrons in the K shell, 8 in the L shell, and 5 in the M shell.
In simple words: Nucleons are the particles in the center of an atom, namely protons and neutrons. Phosphorus has 15 protons and 16 neutrons, so it has 31 nucleons in total. The picture shows its electrons in shells: 2 in the first, 8 in the second, and 5 in the third.

🎯 Exam Tip: The term "nucleons" is synonymous with "mass number." When asked to draw the structure, remember to include the nucleus (protons and neutrons) and correctly filled electron shells.

 

VIII. Answer in detail:

 

Question 1. What conclusions were made from the observations of Gold foil experiment?
Answer: In Rutherford's Alpha ray scattering experiment, he observed the following, which led to important conclusions about atomic structure:
1. Most of the alpha particles (small, positively charged particles) passed straight through the gold foil without any deflection. This indicated that most of the space inside an atom is empty.
2. A small fraction of the alpha particles were deflected through small angles. This suggested that there must be a small, positively charged region at the center of the atom, repelling the positively charged alpha particles.
3. A very few alpha particles (about 1 in 20,000) were deflected through large angles, or even bounced back. This indicated that the positively charged region (nucleus) is extremely dense and occupies a very tiny volume within the atom, concentrating almost all the atom's mass.
From these observations, Rutherford concluded that an atom has a very small, dense, positively charged nucleus at its center, with electrons revolving around it in the empty space. The entire mass of an atom is concentrated in this nucleus.
In simple words: When Rutherford shot tiny positive particles at gold foil, most went right through, meaning atoms are mostly empty space. Some particles bent a little, showing there's a small positive part in the middle. A very few bounced back, meaning this central positive part, called the nucleus, is very tiny but very heavy.

🎯 Exam Tip: Clearly state each observation and its corresponding conclusion. Emphasize the idea of empty space, a small dense nucleus, and that the nucleus is positively charged, as these are the main takeaways from Rutherford's experiment.

 

Question 2. Explain the postulates of Bohr's atomic model.
Answer: Niels Bohr proposed a model for the atom in 1913. His main postulates are:
1. Electrons revolve around the nucleus in specific, fixed circular paths called orbits or shells or energy levels. Each orbit is associated with a definite amount of energy.
2. While revolving in these orbits, electrons do not radiate (lose) energy. These orbits are therefore called stationary states or stable orbits.
3. The circular orbits are numbered as 1, 2, 3, 4, starting from the one closest to the nucleus, or designated as K, L, M, N shells. These numbers are called principal quantum numbers (n).
4. The K shell (n=1) is the orbit closest to the nucleus and has the lowest energy. As the distance from the nucleus increases, the energy of the orbits also increases.
5. An electron can move from a lower energy level to a higher energy level by absorbing a fixed amount of energy, or from a higher energy level to a lower energy level by emitting a fixed amount of energy. The energy difference between orbits is fixed, meaning energy is quantized.

The diagram illustrates the nucleus at the center, surrounded by distinct energy shells or orbits (K, L, M, N) where electrons are found.
In simple words: Bohr said electrons go around the center of an atom in certain circular paths, like lanes on a track. They don't lose energy when they're in these paths. Each path has a different energy level, and electrons can jump between these levels by taking in or giving out energy. The paths are numbered, with the closest one having the lowest energy.

🎯 Exam Tip: When explaining Bohr's model, highlight the key concepts of fixed orbits, quantized energy levels, and no energy emission in stationary states. A simple diagram showing concentric shells can enhance your answer.

 

Question 3. State Gay Lussac's law of combining volumes, explain with an illustration.
Answer: Gay-Lussac's law of combining volumes states that when gases react together, the volumes of the reacting gases and the gaseous products bear a simple whole number ratio to one another, provided that all volumes are measured under the same conditions of temperature and pressure.
Example: Formation of Ammonia (\( \mathrm{NH}_3 \))
Nitrogen gas (\( \mathrm{N}_2 \)) reacts with hydrogen gas (\( \mathrm{H}_2 \)) to form ammonia gas (\( \mathrm{NH}_3 \)). The balanced chemical equation for this reaction is:
\( \mathrm{N}_{2}(\mathrm{g}) + 3\mathrm{H}_{2}(\mathrm{g}) \rightarrow 2\mathrm{NH}_{3}(\mathrm{g}) \)
According to Gay-Lussac's law, the ratio of the volumes of reacting gases and product gas is:
1 volume of Nitrogen + 3 volumes of Hydrogen \( \rightarrow \) 2 volumes of Ammonia
Thus, the ratio by volume of Nitrogen : Hydrogen : Ammonia is \( 1 : 3 : 2 \). This is a simple whole number ratio, confirming Gay-Lussac's law.
This law is an important aspect of stoichiometry for gaseous reactions. It shows that chemical reactions happen in fixed ratios, even when dealing with gas volumes.
In simple words: Gay-Lussac's law says that when gases react and form new gases, their volumes (how much space they take up) always mix in simple, whole number amounts, as long as the temperature and pressure are the same. For example, 1 cup of nitrogen gas reacts with 3 cups of hydrogen gas to make 2 cups of ammonia gas. The ratio \( 1:3:2 \) is a simple whole number ratio.

🎯 Exam Tip: Clearly state the conditions (constant temperature and pressure) under which Gay-Lussac's law applies. Use a balanced chemical equation to illustrate the simple whole number ratio of volumes.

 

Intext Activities

ACTIVITY - 1

 

Question 1. Symbolically represent the following atoms using the atomic number and mass number, a) Carbon b) Oxygen c) Silicon d) Beryllium
Answer:
a) Carbon: \( {}_{6}^{12}\mathrm{C} \)
b) Oxygen: \( {}_{8}^{16}\mathrm{O} \)
c) Silicon: \( {}_{14}^{28}\mathrm{Si} \)
d) Beryllium: \( {}_{4}^{9}\mathrm{Be} \)
In simple words: For each element, we write the chemical symbol. The smaller number below it is the atomic number (number of protons), and the larger number above it is the mass number (total protons and neutrons).

🎯 Exam Tip: Remember the standard notation: \( {}_{\text{Atomic Number}}^{\text{Mass Number}}\mathrm{Symbol} \). Ensure you use the correct atomic and mass numbers for each element.

 

ACTIVITY -2

 

Question 1. Assign the valency for Phosphorus, Chlorine, Silicon and Argon
Answer:
Phosphorus (P): Atomic number 15. Electronic configuration: 2, 8, 5. To achieve a stable octet, it can gain 3 electrons or lose 5. Valency is typically 3 (by gaining 3 electrons) or 5 (by losing 5 electrons to form bonds). Common valencies are 3 and 5.
Chlorine (Cl): Atomic number 17. Electronic configuration: 2, 8, 7. To achieve a stable octet, it needs to gain 1 electron. Valency is 1.
Silicon (Si): Atomic number 14. Electronic configuration: 2, 8, 4. It can either gain 4 or lose 4 electrons to complete its octet. Valency is 4.
Argon (Ar): Atomic number 18. Electronic configuration: 2, 8, 8. Its outermost shell is completely filled (octet). Thus, it is a noble gas and chemically inert. Valency is 0.
In simple words: Phosphorus has valency 3 or 5. Chlorine has valency 1. Silicon has valency 4. Argon has valency 0 because its outer shell is full. Valency shows how many bonds an atom can make.

🎯 Exam Tip: Valency is primarily determined by the number of valence electrons. For elements trying to achieve an octet, valency is either the number of electrons to be gained, lost, or shared to reach 8 in the outermost shell.

 

ACTIVITY -4

 

Question 1. Draw the structures of the isotopes of oxygen O16 and O18
Answer: Oxygen has an atomic number of 8. This means it always has 8 protons.For Oxygen-16 (\( {}_{8}^{16}\mathrm{O} \)): Protons = 8, Neutrons = \( 16 - 8 = 8 \), Electrons = 8. Configuration: 2, 6.
For Oxygen-18 (\( {}_{8}^{18}\mathrm{O} \)): Protons = 8, Neutrons = \( 18 - 8 = 10 \), Electrons = 8. Configuration: 2, 6.
The diagrams illustrate that both isotopes have the same number of protons and electrons but differ in their number of neutrons.
In simple words: The pictures show two types of Oxygen atoms. Both have 8 protons and electrons, meaning they are both Oxygen. But one has 8 neutrons (Oxygen-16), and the other has 10 neutrons (Oxygen-18). This difference in neutrons makes them isotopes.

🎯 Exam Tip: When drawing isotopes, ensure the number of protons and electron configuration remains the same for both, and only the number of neutrons changes, which affects the mass number.

 

ACTIVITY –5

 

Question 1. Draw the model of the following pairs of isotones: (i) Fluorine & Neon (ii) Sodium & Magnesium (iii) Aluminum and Silicon.
Answer: Isotones are atoms of different elements that have the same number of neutrons but different atomic numbers and mass numbers.
(i) Fluorine & Neon:
Fluorine (F): Atomic number 9, Mass number 19 (\( {}_{9}^{19}\mathrm{F} \)). Neutrons = \( 19 - 9 = 10 \). Electronic configuration: 2, 7.
Neon (Ne): Atomic number 10, Mass number 20 (\( {}_{10}^{20}\mathrm{Ne} \)). Neutrons = \( 20 - 10 = 10 \). Electronic configuration: 2, 8.

(ii) Sodium & Magnesium:
Sodium (Na): Atomic number 11, Mass number 23 (\( {}_{11}^{23}\mathrm{Na} \)). Neutrons = \( 23 - 11 = 12 \). Electronic configuration: 2, 8, 1.
Magnesium (Mg): Atomic number 12, Mass number 24 (\( {}_{12}^{24}\mathrm{Mg} \)). Neutrons = \( 24 - 12 = 12 \). Electronic configuration: 2, 8, 2.

(iii) Aluminum & Silicon:
Aluminum (Al): Atomic number 13, Mass number 27 (\( {}_{13}^{27}\mathrm{Al} \)). Neutrons = \( 27 - 13 = 14 \). Electronic configuration: 2, 8, 3.
Silicon (Si): Atomic number 14, Mass number 28 (\( {}_{14}^{28}\mathrm{Si} \)). Neutrons = \( 28 - 14 = 14 \). Electronic configuration: 2, 8, 4.

In simple words: Isotones are atoms of different elements that have the same number of neutrons. For Fluorine and Neon, both have 10 neutrons. For Sodium and Magnesium, both have 12 neutrons. For Aluminum and Silicon, both have 14 neutrons. The drawings show their protons, neutrons, and electron shells.

🎯 Exam Tip: To draw isotones, ensure the nucleus of each atom has the same number of neutrons, but different numbers of protons and electrons, and thus different electron shell arrangements.

 

ACTIVITY -6

 

Question 1. Nitrogen combines with hydrogen to form ammonia (NH3). Illustrate Gay Lussac's law using this example.
Answer: Gay-Lussac's law of combining volumes states that when gases react together, the volumes of the reacting gases and the gaseous products bear a simple whole number ratio to one another, provided that all volumes are measured under the same conditions of temperature and pressure.
For the reaction of nitrogen and hydrogen to form ammonia:
Step 1: Write the word equation:
Nitrogen gas + Hydrogen gas \( \rightarrow \) Ammonia gas
Step 2: Write the unbalanced chemical equation:
\( \mathrm{N}_{2}(\mathrm{g}) + \mathrm{H}_{2}(\mathrm{g}) \rightarrow \mathrm{NH}_{3}(\mathrm{g}) \)
Step 3: Balance the chemical equation:
\( \mathrm{N}_{2}(\mathrm{g}) + 3\mathrm{H}_{2}(\mathrm{g}) \rightarrow 2\mathrm{NH}_{3}(\mathrm{g}) \)
From the balanced equation, we can see the molar ratio of the reacting gases and product gas:
1 mole of Nitrogen + 3 moles of Hydrogen \( \rightarrow \) 2 moles of Ammonia
According to Avogadro's law, equal moles of gases at the same temperature and pressure occupy equal volumes. Therefore, the volume ratio is:
1 volume of Nitrogen + 3 volumes of Hydrogen \( \rightarrow \) 2 volumes of Ammonia
The ratio by volume of Nitrogen : Hydrogen : Ammonia is \( 1 : 3 : 2 \). This demonstrates a simple whole number ratio, as stated by Gay-Lussac's law.
In simple words: When nitrogen gas and hydrogen gas mix to make ammonia gas, their volumes always combine in easy, whole number amounts. For example, one part of nitrogen gas always mixes with three parts of hydrogen gas to make two parts of ammonia gas. This rule works if the temperature and pressure stay the same.

🎯 Exam Tip: To illustrate Gay-Lussac's law, always start with a balanced chemical equation for gases. Then, directly relate the stoichiometric coefficients (moles) to the volumes of the gases involved, showing the simple whole-number ratio.

 

Test Yourself:

 

Question 1. Calculate the number of neutrons in the following atoms:
a) \( {}_{13}^{27}\mathrm{Al} \)
b) \( {}_{15}^{31}\mathrm{P} \)
c) \( {}_{76}^{190}\mathrm{Os} \)
d) \( {}_{24}^{54}\mathrm{Cr} \)
Answer: The number of neutrons (n) is calculated as: Mass number (A) - Atomic number (Z).
a) For Aluminum (\( {}_{13}^{27}\mathrm{Al} \)): Number of neutrons = \( 27 - 13 = 14 \)
b) For Phosphorus (\( {}_{15}^{31}\mathrm{P} \)): Number of neutrons = \( 31 - 15 = 16 \)
c) For Osmium (\( {}_{76}^{190}\mathrm{Os} \)): Number of neutrons = \( 190 - 76 = 114 \)
d) For Chromium (\( {}_{24}^{54}\mathrm{Cr} \)): Number of neutrons = \( 54 - 24 = 30 \)
In simple words: To find the number of neutrons, simply subtract the smaller number (atomic number) from the larger number (mass number) for each atom.

🎯 Exam Tip: Always remember that the superscript represents the mass number (protons + neutrons) and the subscript represents the atomic number (protons). Practice this calculation to avoid simple errors.

 

9th Science Guide Atomic Structure Additional Important Questions and Answers

I. Choose the correct answer:

 

Question 1. Hydrogen atom does not have
(a) Proton
(b) Neutron
(c) Electron
(d) Proton and electron
Answer: (b) Neutron
In simple words: A regular hydrogen atom has one proton and one electron, but it usually doesn't have any neutrons. Some types of hydrogen, called isotopes, can have neutrons, but the most common one does not.

🎯 Exam Tip: The most common isotope of hydrogen (\( {}_{1}^{1}\mathrm{H} \)), also known as protium, uniquely lacks neutrons. Be mindful of which specific isotope a question might refer to if not explicitly stated.

 

Question 2. An electron has
(a) Mass \( 1/1837^{\text{th}} \) of proton and positively charged
(b) Mass equal to proton and positively charged.
(c) Mass \( 1/1837^{\text{th}} \) of proton and negatively charged.
(d) Mass equal to proton and negatively charged.
Answer: (c) Mass \( 1/1837^{\text{th}} \) of proton and negatively charged.
In simple words: An electron is much lighter than a proton, about 1837 times lighter. It also carries a negative electrical charge.

🎯 Exam Tip: Remember the relative masses and charges of subatomic particles: protons (+1 charge, 1 amu mass), neutrons (0 charge, 1 amu mass), and electrons (-1 charge, negligible mass, approximately \( 1/1837^{\text{th}} \) of a proton).

 

Question 3. Which statement is not correct about isotopes of an element?
(a) Their chemical properties are same.
(b) Their atomic number is same.
(c) Number of electrons are different.
(d) Number of neutrons are different.
Answer: (c) Number of electrons are different
In simple words: Isotopes are atoms of the same element, meaning they have the same number of protons and electrons. This means their chemical behaviors are identical, but they differ in the number of neutrons.

🎯 Exam Tip: Focus on the key difference between isotopes: same atomic number, different mass number due to varying neutrons. Chemical properties depend only on electron configuration.

 

Question 4. Co-60 is used in the treatment of ....................
(a) Cancer
(b) Thyroid disorders
(c) Leukemia
(d) Blockage of arteries.
Answer: (a) Cancer
In simple words: Cobalt-60 is a special kind of cobalt that gives off radiation. Doctors use this radiation to treat cancer by targeting and destroying cancerous cells.

🎯 Exam Tip: Remember specific medical applications of radioisotopes, like Co-60 for cancer therapy and I-131 for thyroid conditions.

 

Question 5. Every atom has an equal number of protons and electrons. The nature of the atom is ....................
(a) Positive
(b) Neutral
(c) Negative
(d) None of the options
Answer: (b) Neutral
In simple words: Atoms normally have an equal count of positive protons and negative electrons. Because these charges cancel each other out, the atom has no overall charge and is called neutral.

🎯 Exam Tip: Understand that the balance between positive protons and negative electrons makes a neutral atom; any imbalance creates an ion.

 

Question 6. Gold foil alpha particle scattering experiment was performed by ....................
(a) Dalton
(b) Goldstein
(c) Thomson
(d) Rutherford
Answer: (d) Rutherford
In simple words: The famous experiment where alpha particles were shot at a thin gold sheet, which showed that atoms have a small, dense nucleus, was carried out by Rutherford.

🎯 Exam Tip: Associate the gold foil experiment and the discovery of the atomic nucleus directly with Rutherford.

 

Question 7. Which subatomic particles have no charge ....................
(a) Proton
(b) Neutron
(c) Electron
(d) Proton and electron
Answer: (b) Neutron
In simple words: Inside an atom, protons have a positive charge, electrons have a negative charge, but neutrons have no charge at all; they are electrically neutral.

🎯 Exam Tip: Distinguish the charges of the three main subatomic particles: proton (+1), electron (-1), and neutron (0).

 

Question 8. The outermost shell of an atom cannot accommodate electrons more than ....................
(a) 2
(b) 8
(c) 18
(d) 32
Answer: (b) 8
In simple words: Even if inner shells can hold more electrons, the very last or outermost shell of an atom can never hold more than 8 electrons. This important rule is known as the octet rule.

🎯 Exam Tip: Remember the octet rule for the outermost shell, which is crucial for understanding chemical bonding and stability. It applies to most elements, with hydrogen and helium being exceptions.

 

Question 9. The above is the law of ....................
(a) Law of multiple proportions
(b) Law of reciprocal proportions
(c) Gay Lussac's Law of combining volumes
(d) None of the options
Answer: (a) Law of multiple proportions
In simple words: This principle describes how if two elements can combine in different ways to form more than one compound, the weights of one element that combine with a fixed weight of the other will be in simple whole-number ratios.

🎯 Exam Tip: Recognize the conditions for the Law of Multiple Proportions: two elements forming multiple compounds, with one element's mass kept constant.

 

Question 10. .................... is a radioactive isotope present in our body.
(a) Cobalt 60
(b) Uranium 235
(c) Iodine 131
(d) Pottassium 40
Answer: (d) Pottassium 40
In simple words: Potassium-40 is a naturally occurring radioactive form of potassium that is found inside the human body in small amounts.

🎯 Exam Tip: Be aware of common radioactive isotopes and their presence, both natural (like K-40) and artificial (like Co-60).

 

Question 11. Proton was discovered by ....................
(a) Rutherford
(b) Goldstein
(c) Chadwick
(d) J.J. Thomson
Answer: (b) Goldstein
In simple words: Goldstein was the scientist who first observed and identified the proton, which is a positively charged particle found in the atom's nucleus.

🎯 Exam Tip: Remember the key scientists associated with the discovery of subatomic particles: Electron (J.J. Thomson), Proton (Goldstein), Neutron (Chadwick), and the Nucleus (Rutherford).

 

Question 12. Electron was discovered by ....................
(a) J.J. Thomson
(b) Rutherford
(c) Goldstein
(d) Chadwick
Answer: (a) J.J. Thomson
In simple words: J.J. Thomson discovered the electron, which is the tiny negatively charged particle that circles around the atom's center.

🎯 Exam Tip: Keep the main discoveries of subatomic particles and their discoverers clear in your mind for quick recall.

 

Question 13. \(\alpha\) – particles are ....................
(a) Negative charged particles
(b) Positive charged particles
(c) Beam of Neutrons
(d) gamma radiation
Answer: (b) Positive charged particles
In simple words: Alpha particles are made up of two protons and two neutrons, which means they carry a positive electrical charge.

🎯 Exam Tip: Alpha particles are essentially helium nuclei, hence they are positively charged and relatively heavy.

 

Question 14. An \(\alpha\) – particle is ....................
(a) hydrogen Nucleu
(b) a Helium Nucleus
(c) a Proton
(d) an electron
Answer: (b) a Helium Nucleus
In simple words: An alpha particle is the same as the nucleus of a helium atom, which has two protons and two neutrons, making it positively charged.

🎯 Exam Tip: Understand that an alpha particle is equivalent to a helium nucleus, lacking electrons.

 

Question 15. The electronic configuration of an atom is 2,8,3. The valency of the atom is ....................
(a) 13
(b) 10
(d) 8
Answer: (c) 3
In simple words: The valency of an atom is determined by the number of electrons it needs to gain, lose, or share to have a full outer shell (usually 8 electrons). For an electron configuration of 2,8,3, the atom will easily lose 3 electrons to become stable.

🎯 Exam Tip: Valency is determined by the number of electrons in the outermost shell; if it's 1, 2, or 3, valency is usually that number (tendency to lose); if it's 5, 6, or 7, valency is 8 minus that number (tendency to gain).

 

Question 16. The No. of electrons in the outermost shell of the atom of an inert gas ....................
(a) 0
(b) 1
(c) 2
(d) 8
Answer: (d) 8
In simple words: Inert gases like Neon and Argon have a full outer shell, meaning they usually have 8 electrons in their outermost shell (except Helium, which has 2). This makes them very stable and unreactive.

🎯 Exam Tip: Remember that inert gases (noble gases) are stable because their outermost electron shell is full, typically with 8 electrons (octet rule).

 

Question 17. Which one of the following will have the maximum charge / mass ratio?
(a) Electron
(b) Proton
(c) Neutron
(d) a Particle
Answer: (a) Electron
In simple words: An electron is much lighter than a proton or an alpha particle, but carries the same basic charge as a proton. So, its charge-to-mass ratio is the largest.

🎯 Exam Tip: The charge-to-mass ratio is greatest for electrons because they have the smallest mass among charged subatomic particles.

 

Question 18. The maximum No. of electrons that can be accommodated in M shell is ....................
(a) 2
(b) 8
(d) 32
Answer: (c) 18
In simple words: The M shell is the third electron shell (n=3). Using the formula \(2n^2\), it can hold a maximum of \(2 \times 3^2 = 18\) electrons.

🎯 Exam Tip: Use the \(2n^2\) formula to calculate the maximum number of electrons in any given shell (K=2, L=8, M=18, N=32).

 

Question 19. Elements with valency 1 are ....................
(a) always metals
(b) always metaloids
(c) either metals (or) non-metals
(d) always non - metals
Answer: (a) always metals
In simple words: Elements that have a valency of 1 (meaning they usually lose one electron) are typically metals, like sodium or lithium, found in Group 1 of the periodic table.

🎯 Exam Tip: Group 1 elements (alkali metals) are highly reactive metals with a valency of 1, as they readily lose their single valence electron.

 

Question 20. The electronic configuration of 'Cl' is ....................
(a) 2,7
(b) 2,8,8,7
(c) 2,8,7
(d) 2,7,8
Answer: (c) 2, 8, 7
In simple words: Chlorine has an atomic number of 17. Its electrons are arranged in shells as 2 in the first shell, 8 in the second, and 7 in the outermost shell.

🎯 Exam Tip: To find the electronic configuration, distribute electrons into shells (K, L, M, N) following the 2, 8, 18, 32 rule, filling from inner to outer shells.

 

Question 21. The isotope used to remove the brain tumour and treatment of Cancer is ....................
(a) Cobalt 60
(b) Uranium 235
(c) Iodine 131
Answer: (a) Cobalt 60
In simple words: Cobalt-60 is a radioactive isotope specifically used in medical treatments, particularly for destroying cancerous cells and brain tumors with its radiation.

🎯 Exam Tip: Distinguish between different radioisotopes and their specific medical or industrial applications (e.g., Co-60 for cancer, I-131 for thyroid, C-14 for dating).

 

II. Fill In The Blanks:

 

Question 1. .................... are the building blocks of matter.
Answer: Atoms
In simple words: Atoms are the tiny basic units that make up everything around us; they are the fundamental building blocks of all matter.

🎯 Exam Tip: Define fundamental terms like 'atom' clearly and concisely.

 

Question 2. The combination of different elements to form a compound is governed by certain basic rules called ....................
Answer: Laws of chemical combination
In simple words: When different elements join to create a new compound, they follow specific rules, which are known as the laws of chemical combination.

🎯 Exam Tip: Recall the basic laws of chemical combination, such as the Law of Conservation of Mass and the Law of Definite Proportions.

 

Question 3. Volume of the nucleus of an atom is .................... compared to the volume of the extranuclear part of the atom.
Answer: very small
In simple words: The nucleus of an atom is incredibly tiny compared to the total space occupied by the electrons orbiting around it. Most of the atom is empty space.

🎯 Exam Tip: Emphasize the small, dense nature of the nucleus relative to the overall size of the atom.

 

Question 4. the Atomic number of an element is 20. It has.................... valence electrons.
Answer: 2
In simple words: An atom with atomic number 20 (Calcium) has an electron arrangement of 2, 8, 8, 2. This means it has 2 electrons in its outermost shell, which are its valence electrons.

🎯 Exam Tip: To find valence electrons, determine the electron configuration and count the electrons in the outermost shell.

 

Question 5. Atoms are so tiny their mass number cannot be expressed in grams but expressed in....................
Answer: atomic mass unit (amu)
In simple words: Because atoms are extremely small and light, their mass is measured using a special tiny unit called the atomic mass unit (amu) instead of grams.

🎯 Exam Tip: Remember the appropriate units for atomic scale measurements; 'amu' is specific to atomic and molecular masses.

 

Question 6. Elements having completely filled outermost shell, show....................... valency.
Answer: Zero
In simple words: Elements like noble gases, which have a full outer electron shell, do not need to gain or lose electrons. Because of this, they have a valency of zero and are very unreactive.

🎯 Exam Tip: Elements with full outermost shells (octet rule satisfied) are chemically stable and have zero valency.

 

Question 7. A .................... number describes a specific aspect of an electron.
Answer: quantum
In simple words: A quantum number is a special number that describes the unique properties and state of an electron within an atom, such as its energy level and orbital shape.

🎯 Exam Tip: Understand that quantum numbers uniquely define an electron's state in an atom.

 

Question 8. When an atom gives electrons from its outermost shell, it becomes .................... charged.
Answer: positively
In simple words: If an atom loses negatively charged electrons from its outer shell, it will have more positive protons than negative electrons, making the atom as a whole positively charged.

🎯 Exam Tip: Loss of electrons leads to a positive ion (cation), while gain of electrons leads to a negative ion (anion).

 

Question 9. The elementary particles such as protons and neutrons are collectively called as....................
Answer: nucleons
In simple words: Protons and neutrons are the particles found inside the nucleus of an atom. Together, they are known as nucleons.

🎯 Exam Tip: Nucleons are the particles residing in the nucleus (protons and neutrons).

 

Question 10. In two isotopic species number of protons is same but the number of.................... is different.
Answer: Neutrons
In simple words: Isotopes are atoms of the same element, meaning they have the same number of protons. However, they have different numbers of neutrons, which changes their mass.

🎯 Exam Tip: The defining characteristic of isotopes is having the same atomic number but different neutron numbers.

 

Question 11. The combination of different elements form a....................
Answer: Compound
In simple words: When two or more different elements chemically combine in a fixed ratio, they create a new substance called a compound.

🎯 Exam Tip: Clearly distinguish between elements, compounds, and mixtures based on their composition and bonding.

 

Question 12. A compound is governed by basic rules are known as ....................
Answer: Laws of chemical combination
In simple words: The ways elements combine to form compounds are explained by fundamental scientific principles known as the laws of chemical combination.

🎯 Exam Tip: Recall the key laws governing chemical reactions and compound formation to understand how substances combine.

 

Question 13. The ratio of masses of H and O in H2O is....................
Answer: 1:8
In simple words: In a water molecule (\(H_2O\)), there are two hydrogen atoms (each with an atomic mass of 1) and one oxygen atom (with an atomic mass of 16). So the mass ratio of H to O is \((2 \times 1) : 16\), which simplifies to 2:16 or 1:8.

🎯 Exam Tip: Calculate mass ratios in compounds by using the atomic masses of the elements and their proportions in the chemical formula.

 

Question 14. \(\alpha\), \(\beta\), \(\gamma\) rays are .................... during the ratio active decay of an atom.
Answer: emitted
In simple words: During radioactive decay, unstable atomic nuclei break down and release energy in the form of different types of radiation, such as alpha, beta, and gamma rays.

🎯 Exam Tip: Remember the three main types of radiation (\(\alpha\), \(\beta\), \(\gamma\)) and that they are emitted during radioactive decay as an atom tries to achieve stability.

 

Question 15. Beryllium + \(\alpha\) – rays \(\rightarrow\) Carbon +....................
Answer: Neutron
In simple words: When beryllium is bombarded with alpha particles, it changes into carbon and also releases a neutron. This reaction shows how a new element can be created and how neutrons are produced.

🎯 Exam Tip: This is a classic nuclear reaction that led to the discovery of the neutron.

 

Question 16. .................... = No. of protons = No. of the electrons.
Answer: Atomic No.
In simple words: The atomic number of an element is equal to the number of protons in its nucleus. In a neutral atom, this is also equal to the number of electrons.

🎯 Exam Tip: Understand that atomic number uniquely identifies an element and equals the number of protons and electrons in a neutral atom.

 

Question 17. 'A' is denoted by ....................
Answer: Mass No
In simple words: In chemical notation, the letter 'A' is used to represent the mass number of an atom, which is the total count of protons and neutrons in its nucleus.

🎯 Exam Tip: Recall the standard notation: A for mass number (top), Z for atomic number (bottom), and X for the element symbol.

 

Question 18. 'Z' is denoted by ....................
Answer: Atomic No
In simple words: In chemical notation, the letter 'Z' stands for the atomic number, which tells us how many protons are in an atom's nucleus.

🎯 Exam Tip: 'Z' for atomic number (protons), 'A' for mass number (protons + neutrons).

 

Question 19. Mass No = .................... + No. of Neutrons
Answer: No. of protons
In simple words: The mass number of an atom is calculated by adding the number of protons and the number of neutrons found in its nucleus.

🎯 Exam Tip: The mass number is the sum of protons and neutrons, which are the main contributors to an atom's mass.

 

Question 20. The Mass No. of 27 is for the element....................
Answer: Aluminium
In simple words: The element with a mass number of 27 and an atomic number of 13 is Aluminum.

🎯 Exam Tip: Relate specific mass numbers and atomic numbers to their corresponding elements using the periodic table.

 

Question 21. U-235 Isotope is used as ....................
Answer: Fuel in nuclear reactors
In simple words: Uranium-235 is a special type of uranium that can be split to release a large amount of energy, making it a key fuel in nuclear power plants.

🎯 Exam Tip: Uranium-235 is critical for nuclear energy production due to its fissionable properties.

 

Question 22. The symbol of phosphorus is ....................
Answer: P
In simple words: The chemical symbol for the element phosphorus is 'P'.

🎯 Exam Tip: Know the common chemical symbols for elements, as they are fundamental in chemistry.

 

Question 23. Na \(\rightarrow\) .................... Na\(^+\).
Answer: e-
In simple words: When a neutral sodium atom (Na) loses one electron (\(e^-\)), it becomes a positively charged sodium ion (\(Na^+\)).

🎯 Exam Tip: Losing electrons creates positive ions (cations), while gaining electrons creates negative ions (anions).

 

Question 24. Elements with 4 to 7 in their valence shells are ....................
Answer: non -metals
In simple words: Elements that have 4, 5, 6, or 7 electrons in their outermost shell are typically non-metals, as they tend to gain electrons to achieve a stable outer shell.

🎯 Exam Tip: The number of valence electrons helps classify elements as metals (typically 1-3 valence electrons) or non-metals (typically 4-7 valence electrons).

 

Question 25. Valency of the elements having .................... is 1, 2, 3 respectively.
Answer: Valence electrons 1, 2, 3
In simple words: If an element has 1, 2, or 3 valence electrons, its valency is usually that same number, as it tends to lose these electrons to become stable.

🎯 Exam Tip: Direct correlation exists between low valence electron count and valency (tendency to lose electrons) for many elements.

 

Question 26. Atomic orbitals allow atoms to make .................... bonds
Answer: Co-valent
In simple words: Atomic orbitals, which are regions where electrons are likely to be found, overlap to allow atoms to share electrons and form covalent bonds.

🎯 Exam Tip: Covalent bonds involve the sharing of electrons between atoms, often facilitated by overlapping atomic orbitals.

 

Question 27. The symbol of Azimuthal quantum number is....................
Answer: l
In simple words: The azimuthal quantum number, also known as the angular momentum or subsidiary quantum number, is represented by the symbol 'l' and describes the shape of an electron's orbital.

🎯 Exam Tip: Recall the symbols and what each of the four quantum numbers (n, l, m_l, m_s) represents.

 

Question 28. The symbol of the Principal quantum number is....................
Answer: n
In simple words: The principal quantum number, symbolized by 'n', indicates the main energy level or shell of an electron in an atom.

🎯 Exam Tip: The principal quantum number 'n' primarily determines the energy and size of an electron orbital.

 

Question 29. Valency is the combining capacity of ....................
Answer: an atom
In simple words: Valency refers to how many bonds an atom can typically form, or how readily it combines with other atoms.

🎯 Exam Tip: Valency is a measure of an atom's combining power with other atoms.

 

III. State Whether True Or False. If False, Correct The Statement:

 

Question 1. In Rutherford's experiment, bouncing of alpha particles suggests that nucleus of an atom is negatively charged.
Answer: False. Correct statement: Nucleus of an atom is positively charged.
In simple words: In Rutherford's experiment, alpha particles bounced back because they hit a tiny, dense, positively charged nucleus, not a negatively charged one. Like charges repel each other.

🎯 Exam Tip: The deflection of alpha particles in Rutherford's experiment proved the existence of a small, dense, positively charged nucleus.

 

Question 2. Electrons revolve around the nucleus in definite open shells.
Answer: False. Correct statement: Electrons revolve around the nucleus in definite closed shells.
In simple words: Electrons orbit the nucleus in specific energy levels or "shells" that are fixed and quantized, not open. They follow defined paths and energy states.

🎯 Exam Tip: Bohr's model introduced the concept of electrons occupying specific, quantized energy levels or shells.

 

Question 3. K and L shell can accommodate a maximum 2 and 8 electrons respectively.
Answer: True
In simple words: The K shell (n=1) can hold a maximum of \(2 \times 1^2 = 2\) electrons, and the L shell (n=2) can hold a maximum of \(2 \times 2^2 = 8\) electrons, as per the \(2n^2\) rule.

🎯 Exam Tip: Apply the \(2n^2\) rule to determine the maximum electron capacity for each shell (K=2, L=8).

 

Question 4. Radio activity of Carbon 14 is considered to determine the age of object, especially of archeological importance.
Answer: True
In simple words: Carbon-14 dating is a scientific method used to find out the age of old objects and fossils by measuring the remaining radioactive carbon-14 in them.

🎯 Exam Tip: Carbon-14 dating is a key application of radioisotopes in archaeology and geology for age determination.

 

Question 5. In a neutral atom number of protons is equal to the number of neutrons.
Answer: False. Correct statement: In a neutral atom number of protons is equal to the number of electrons.
In simple words: A neutral atom has an equal number of protons (positive charges) and electrons (negative charges), which balance each other out. The number of neutrons can be different.

🎯 Exam Tip: Protons determine the atomic number, electrons balance the charge in a neutral atom, while neutrons contribute to mass but not charge.

 

Question 6. If an atom has 6 electrons in its outermost shell, it is inert.
Answer: False. Correct statement: If an atom has 8 electrons in its outermost shell, it is inert.
In simple words: An atom with 6 electrons in its outer shell is usually reactive, as it wants to gain 2 more electrons to complete its octet. Only atoms with a full outermost shell (usually 8 electrons) are inert.

🎯 Exam Tip: Atoms are chemically inert when their outermost electron shell is completely filled, achieving a stable electron configuration (e.g., octet rule).

 

Question 7. The valency of Chlorine is 7.
Answer: False. Correct statement: The valency is 1.
In simple words: Chlorine has 7 electrons in its outermost shell, so it tends to gain just one electron to achieve a stable octet. Therefore, its valency is 1, not 7.

🎯 Exam Tip: Valency is the combining capacity; for non-metals with 5, 6, or 7 valence electrons, it's typically 8 minus the number of valence electrons.

 

Question 8. Smaller the size of the orbit, the smaller the energy of the orbit.
Answer: True
In simple words: Electrons in orbits closer to the nucleus have lower energy levels because they are more tightly held by the nucleus's positive charge.

🎯 Exam Tip: Electron shells closer to the nucleus correspond to lower energy states.

 

Question 9. The stability of the nucleus is determined by the Neutron – Proton ratio.
Answer: True
In simple words: The balance between neutrons and protons inside the nucleus is very important for its stability. If this ratio is not right, the nucleus can become unstable and radioactive.

🎯 Exam Tip: An optimal neutron-to-proton ratio is crucial for nuclear stability; deviations lead to radioactivity.

 

Question 10. Isotopes of elements have the same chemical properties.
Answer: True
In simple words: Isotopes have the same number of electrons and the same electron arrangement, which means they react chemically in exactly the same way. Their different neutron numbers do not affect their chemical behavior.

🎯 Exam Tip: Chemical properties depend on the electron configuration, which is identical for isotopes of the same element.

 

IV. Match The Following:

 

Question 1. Match the following quantum numbers with their descriptions.

Answer: 1) – c, 2) – d, 3) – a, 4) – b
In simple words: Each quantum number describes a specific feature of an electron: the principal quantum number determines the main energy level, the azimuthal quantum number describes the subshell shape, the magnetic quantum number indicates orbital orientation, and the spin quantum number describes the electron's spin direction.

🎯 Exam Tip: Memorize what each of the four quantum numbers signifies and which properties of an electron they describe.

 

Question 2. Match the following shells with their maximum electron capacities.

Answer: 1) – c, 2) – a, 3) – d, 4) – b
In simple words: The maximum number of electrons each shell can hold increases with its shell number: K (n=1) holds 2, L (n=2) holds 8, M (n=3) holds 18, and N (n=4) holds 32 electrons, following the \(2n^2\) rule.

🎯 Exam Tip: Remember the \(2n^2\) rule for electron capacity of shells (K=2, L=8, M=18, N=32) and associate each shell letter with its corresponding principal quantum number 'n'.

 

Question 3. Match the following properties with their correct values.

Answer: a) 4, b) 3, c) 2, d) 1
In simple words: Protons and electrons have opposite charges, with protons being positive (+1) and electrons negative (-1). Protons are significantly heavier than electrons, with their masses expressed in very small gram values.

🎯 Exam Tip: Know the approximate masses and charges of protons and electrons, and remember that protons are about 1836 times heavier than electrons.

 

Question 1. If an element M has mass number 24 and atomic number 12, how many neutrons does its atom contain?
Answer: To find the number of neutrons, subtract the atomic number (number of protons) from the mass number. The mass number is 24, and the atomic number is 12. So, the number of neutrons is \( 24 - 12 = 12 \). Neutrons are important because they add to the atom's mass but do not affect its charge.
In simple words: An atom has 12 neutrons. You get this by taking the mass number (24) and subtracting the atomic number (12).

🎯 Exam Tip: Remember that mass number = number of protons + number of neutrons, and atomic number = number of protons (or electrons in a neutral atom).

 

Question 2. Atomic number of A and B are 18 and 19 respectively. What is the electronic configuration of A and B?
Answer: The electronic configuration shows how electrons are arranged in shells around the nucleus. For element A with atomic number 18, the configuration is 2, 8, 8. For element B with atomic number 19, the configuration is 2, 8, 8, 1. The first shell holds up to 2 electrons, the second up to 8, and the third can hold up to 18, but elements tend to have 8 in their outer shell for stability.
In simple words: Element A has electrons arranged as 2, 8, 8. Element B has electrons arranged as 2, 8, 8, 1.

🎯 Exam Tip: When writing electronic configurations, always fill the inner shells first (K, L, M, N) according to the 2n² rule, making sure the outermost shell doesn't exceed 8 electrons for stable elements.

 

Question 3. How will you define the term 'electronic configuration'?
Answer: Electronic configuration is the way electrons are arranged in different energy shells or orbits inside an atom. This arrangement helps us understand how an atom will react with other atoms. The electrons fill the shells in a specific order, starting from the closest shell to the nucleus.
In simple words: Electronic configuration is how electrons are placed in different energy levels around an atom's center.

🎯 Exam Tip: Clearly state that it's the arrangement of electrons in *shells* or *orbits* (energy levels) to define electronic configuration accurately.

 

Question 4. Why the chemical properties of an element are determined by valence electrons?
Answer: The chemical properties of an element are decided by its valence electrons. These are the electrons in the outermost shell of an atom. They are the ones that take part in chemical reactions by being shared, gained, or lost, which causes elements to form bonds. This is why elements in the same group of the periodic table, having the same number of valence electrons, show similar chemical behavior.
In simple words: Valence electrons are the outer electrons of an atom. They decide how an element will react with others because they are involved in forming chemical bonds.

🎯 Exam Tip: Focus on the keywords "outermost shell" and "take part in chemical reactions" when explaining the role of valence electrons.

 

Question 5. What are the elements which have valence electrons 1 or 2 or 3 are called and nts with valence Electrons 4 to 7 in their valence shells?
Answer: Elements that have 1, 2, or 3 valence electrons are generally called metals. These elements tend to lose electrons to become positive ions. Elements with 4 to 7 valence electrons are typically called non-metals. These elements usually gain or share electrons to form bonds.
In simple words: Elements with 1, 2, or 3 outer electrons are metals. Elements with 4 to 7 outer electrons are non-metals.

🎯 Exam Tip: Linking the number of valence electrons to whether an element is a metal or non-metal is key, as it explains their general behavior.

 

Question 6. How is an atom of the element represented? Give an example.
Answer: An atom of an element is represented by a symbol, like 'X'. The atomic mass (mass number) is written at the upper left side, and the atomic number is written at the lower left side of the symbol. For example, for Sodium (Na): \( {}_{11}^{23} \mathrm{Na} \). Here, 23 is the atomic mass (mass number), and 11 is the atomic number. This standard notation quickly tells us about the number of protons, neutrons, and electrons in an atom.
In simple words: An element's symbol shows its atomic mass (top left) and atomic number (bottom left). For example, \( {}_{11}^{23} \mathrm{Na} \) means Sodium has a mass of 23 and an atomic number of 11.

🎯 Exam Tip: Always place the mass number as a superscript and the atomic number as a subscript to the left of the element symbol for correct representation.

 

Question 7. Why an atom is considered neutral?
Answer: An atom is considered neutral because it has an equal number of positively charged protons and negatively charged electrons. The positive charges of the protons in the nucleus cancel out the negative charges of the electrons orbiting around it. This balance of charges makes the overall atom have no net electrical charge. If an atom gains or loses electrons, it becomes an ion, which is charged.
In simple words: An atom is neutral because it has the same number of positive protons and negative electrons, so their charges cancel each other out.

🎯 Exam Tip: The key to neutrality is the balance between positive protons and negative electrons; make sure to mention both.

 

Question 8. Name the scientist and his experiment to prove that nucleus of an atom is positively charged.
Answer: The scientist who proved that the nucleus of an atom is positively charged was Ernest Rutherford. He conducted the Gold foil alpha particle scattering experiment. In this experiment, positive alpha particles were fired at a thin gold foil, and some were deflected back, showing a dense, positively charged center. This experiment totally changed our understanding of the atom's structure.
In simple words: Ernest Rutherford showed the nucleus is positive using his Gold foil experiment.

🎯 Exam Tip: Remember to name both the scientist (Rutherford) and the experiment (Gold foil alpha particle scattering experiment) for a complete answer.

 

Question 9. Name the particles which determine the mass of an atom?
Answer: The mass of an atom is mainly determined by the protons and neutrons. Both of these particles are found in the nucleus of an atom and are much heavier than electrons. Electrons have very little mass and contribute almost nothing to the atom's total weight. Therefore, the atomic mass is equal to the number of protons plus the number of neutrons.
In simple words: The mass of an atom comes from its protons and neutrons, which are located in the center of the atom.

🎯 Exam Tip: Focus on protons and neutrons as the primary contributors to atomic mass, while noting electrons have negligible mass.

 

Question 10. what are the shells?
Answer: Shells are different energy levels or orbits where electrons revolve around the nucleus of an atom. These shells are designated by letters like K, L, M, N, and so on, or numbered as 1, 2, 3, 4, respectively. Each shell can hold a specific maximum number of electrons, with inner shells having lower energy.
In simple words: Shells are the paths or energy levels where electrons move around the center of an atom. They are named K, L, M, N, or numbered 1, 2, 3, 4.

🎯 Exam Tip: Clearly define shells as energy levels or orbits for electrons and mention their common designations (K, L, M, N or 1, 2, 3, 4).

 

Question 11. Rutherford's model of an atom has a very small ............ at the center.
Answer: Rutherford's model of an atom has a very small Nucleus at the center. This central part contains all the positive charge and most of the atom's mass, as discovered through his gold foil experiment. The electrons orbit this tiny, dense nucleus.
In simple words: Rutherford's atom model has a tiny, dense nucleus in the middle.

🎯 Exam Tip: Remember that Rutherford's key contribution was identifying the tiny, dense, and positively charged nucleus at the atom's core.

 

Question 12. Niels Bohr was Born on ............
Answer: Niels Bohr was Born on October 7, 1885. He was a Danish physicist who made big contributions to understanding atomic structure and quantum theory. His model of the atom, which proposed that electrons orbit the nucleus in fixed energy levels, was a major step forward.
In simple words: Niels Bohr was born on October 7, 1885.

🎯 Exam Tip: For specific dates, ensure accuracy. Briefly mentioning his key contribution reinforces the context.

 

Question 13. When an electron returns from high energy level to lower energy level it gives ............
Answer: When an electron returns from a high energy level to a lower energy level, it gives off energy. This energy is released as electromagnetic radiation, often in the form of light. This process is how atoms emit light and forms the basis of spectroscopy. The specific color of light depends on the amount of energy released.
In simple words: When an electron moves from a higher energy path to a lower one, it releases energy, often as light.

🎯 Exam Tip: The key concept here is "emission of energy" or "radiation." Connect it to light emission if possible for better understanding.

 

Question 14. In \( {}_{\mathrm{Z}}^{\mathrm{A}} \mathrm{X} \) what are the letters A, Z, X stand for?
Answer: In the notation \( {}_{\mathrm{Z}}^{\mathrm{A}} \mathrm{X} \):
X represents the symbol of the element.
Z represents the atomic number (which is the number of protons).
A represents the mass number (which is the total number of protons and neutrons).
This standard notation helps scientists quickly understand the basic composition of an atom.
In simple words: X is the element symbol, Z is the atomic number, and A is the mass number in this atomic notation.

🎯 Exam Tip: Clearly define each letter: X for element symbol, Z for atomic number (protons), and A for mass number (protons + neutrons).

VI. Answer briefly :

 

Question 1. State the law of reciprocal proportion.
Answer: The law of reciprocal proportions states that if two different elements combine separately with the same weight of a third element, then the ratio of their masses in which they do so is either the same or a simple multiple of the mass ratio in which they combine with each other. This law helps explain how elements react in set ratios. For example, if carbon combines with hydrogen and oxygen separately, the ratio of carbon's mass in these compounds will relate simply to its mass when carbon and oxygen combine directly.
In simple words: If two elements each combine with a third element in a certain amount, the ratio of their masses will be simple when they combine with each other.

🎯 Exam Tip: Define the law precisely, emphasizing the "same weight of a third element" and the "simple multiple" relationship between the ratios.

 

Question 2. Why proton is considered to be the fingerprint of an atom?
Answer: A proton is considered the fingerprint of an atom because the number of protons in an atom is unique to each element. This number is called the atomic number, and it defines the element. For example, every atom with 6 protons is carbon, and every atom with 8 protons is oxygen. The number of protons never changes for a given element, unlike electrons (which can be gained or lost to form ions) or neutrons (which can vary to form isotopes).
In simple words: Protons are like an atom's fingerprint because their number (atomic number) is unique to each element and never changes.

🎯 Exam Tip: Highlight that the *number* of protons is unique and unchangeable for an element, making it its defining characteristic.

 

Question 3. How can an element be identified on the Periodic Table? What makes an element different from another element?
Answer: An element can be identified on the Periodic Table by its atomic number. The atomic number is always unique to each element. What makes one element different from another element is its atomic number, which represents the exact number of protons in the nucleus of its atoms. This number gives each element its distinct chemical identity and properties. For example, carbon always has 6 protons, while nitrogen always has 7.
In simple words: An element is identified by its atomic number on the Periodic Table. Elements are different because they have a different number of protons.

🎯 Exam Tip: Emphasize "atomic number" and "number of protons" as the key to identifying and distinguishing elements on the Periodic Table.

 

Question 4. Because atoms are so small, what do scientists do?
Answer: Because atoms are incredibly small and cannot be seen directly, scientists create models to describe them. These models can be diagrams, mental pictures, mathematical statements, or physical objects that help explain how atoms behave and interact in the natural world. Models allow scientists to study and predict atomic behavior even without direct observation, continuously improving as new discoveries are made.
In simple words: Since atoms are too small to see, scientists make models, like drawings or equations, to understand and explain them.

🎯 Exam Tip: The core idea is that models are used to study things too small to be seen directly; specify different types of models (diagrams, mathematical statements).

 

Question 5. What are isotopes?
Answer: Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. This means they have the same atomic number but different mass numbers. For example, Carbon-12, Carbon-13, and Carbon-14 are isotopes of carbon. They all have 6 protons but have 6, 7, and 8 neutrons, respectively. Isotopes of an element have similar chemical properties because their electron configurations are the same.
In simple words: Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have different masses.

🎯 Exam Tip: Clearly state "same number of protons" and "different number of neutrons" as the defining characteristics of isotopes.

 

Question 6. Draw the diagram of laws of chemical combination.
Answer:Laws of chemical combination explain how elements combine to form compounds. The main laws are shown in the diagram below.

In simple words: This diagram shows the main rules that explain how different elements combine with each other to form new compounds.

🎯 Exam Tip: When drawing diagrams, ensure all labels are clear and placed correctly to accurately represent the concepts.

 

Question 7. Define Isobars.
Answer: Isobars are atoms of different elements that have the same mass number but different atomic numbers. This means they have a different number of protons (since they are different elements) but the total number of protons and neutrons combined is the same. For example, \( {}_{18}^{40} \mathrm{Ar} \) (Argon) and \( {}_{20}^{40} \mathrm{Ca} \) (Calcium) are isobars because both have a mass number of 40, but different atomic numbers. They have different chemical properties due to the varying number of protons.
In simple words: Isobars are atoms of different elements that have the same mass (same mass number) but different atomic numbers.

🎯 Exam Tip: Distinguish isobars from isotopes by remembering that isobars are *different* elements with the *same mass number*.

 

Question 8. What is a combination reaction?
Answer: A combination reaction is a type of chemical reaction where two or more reactants combine to form a single, more complex product. It's like putting several ingredients together to make one new dish. A general form of a combination reaction is \( A + B \rightarrow AB \). For instance, when hydrogen gas and oxygen gas combine to form water, it is a combination reaction. This kind of reaction often releases energy.
In simple words: A combination reaction is when two or more simple things join to make one new, more complex thing.

🎯 Exam Tip: The key characteristic of a combination reaction is "two or more reactants forming a *single* product."

 

Question 9. What are the salient features of Rutherford's model of an atom?
Answer: The important features of Rutherford's model of an atom are:

- An atom has a very small, dense, positively charged center called the nucleus.
- Most of the atom's mass is concentrated in this nucleus.
- There is a large empty space around the nucleus.
- Electrons are distributed in this vacant space and revolve around the nucleus in circular paths. This model is often compared to a miniature solar system.
In simple words: Rutherford's model says an atom has a tiny, heavy, positive center called the nucleus. Electrons orbit this nucleus, and most of the atom is empty space.

🎯 Exam Tip: The key takeaways for Rutherford's model are the presence of a small, dense, positive nucleus and a large empty space where electrons orbit.

 

Question 10. What are the Limitations of Bohr's Model?
Answer: Bohr's model was a big step, but it had some limits:
- Its main limitation was that it could only explain the spectrum of hydrogen and hydrogen-like ions (atoms with only one electron).
- It could not be extended to atoms with more than one electron (multi-electron atoms).
- It also could not explain the fine structure of spectral lines or the Zeeman and Stark effects. This means it couldn't fully explain why some colors of light split into even finer lines when a magnetic or electric field is applied. Later models helped fix these issues.
In simple words: Bohr's model only worked for simple atoms like hydrogen. It couldn't explain how more complex atoms behaved or why light split into very thin lines in a magnetic field.

🎯 Exam Tip: The most critical limitation to remember is that Bohr's model was applicable only to single-electron systems (like hydrogen).

 

Question 11. Explain the Composition of the Nucleus.
Answer: The nucleus is the small, dense center of an atom. It is composed of two main types of particles:
- Protons: These are positively charged particles. The number of protons determines the atomic number of an element and its identity.
- Neutrons: These are neutral particles, meaning they have no electrical charge. Neutrons contribute to the mass of the atom and help to hold the positively charged protons together, reducing the repulsive force between them. Without neutrons, a nucleus with multiple protons would fly apart due to electrical repulsion.
Electrons have negligible mass compared to protons and neutrons, so the nucleus contains almost all of the atom's mass.
In simple words: The nucleus is made of positively charged protons and neutral neutrons. Neutrons help keep the protons together, and these two particles make up almost all of the atom's weight.

🎯 Exam Tip: Clearly state that the nucleus contains protons (positive charge, define element) and neutrons (no charge, stabilize nucleus, add mass).

 

Question 12. Define the Nucleons.
Answer: Nucleons are the collective name for the elementary particles that make up the atomic nucleus. These particles are protons and neutrons. Both protons and neutrons are very similar in mass and are much heavier than electrons. Therefore, the total number of nucleons determines the mass number of an atom. The term 'nucleon' simply provides a general name for particles found in the nucleus.
In simple words: Nucleons are the particles found inside an atom's nucleus, which are protons and neutrons.

🎯 Exam Tip: The definition of nucleons is straightforward: they are protons and neutrons when referred to collectively as components of the nucleus.

 

Question 13. What are the relation between mass no. & atomic no?
Answer: There's a clear relationship between mass number and atomic number:
- Atomic Number (Z): This is the number of protons (or electrons in a neutral atom). It identifies the element.
- Mass Number (A): This is the total number of protons and neutrons in the nucleus.
The relationship is: Mass Number (A) = Number of Protons + Number of Neutrons (n).
Since the number of protons is equal to the atomic number (Z), we can also write it as:
\( \implies \) Mass Number (A) = Atomic Number (Z) + Number of Neutrons (n).
So, \( A = Z + n \). This equation helps calculate any of these values if the other two are known.
In simple words: The mass number is the total of protons and neutrons. The atomic number is just the number of protons. So, Mass Number = Atomic Number + Neutrons.

🎯 Exam Tip: Provide both verbal definitions and the formula \( A = Z + n \) to clearly show the relationship between mass number, atomic number, and neutrons.

 

Question 14. What are the types of Quantum Number?
Answer: Quantum numbers describe the properties of electrons in an atom, such as their energy, shape of their orbital, and orientation. There are four main types of quantum numbers:

These numbers help to uniquely identify each electron within an atom.
In simple words: Quantum numbers are like special codes (principal, azimuthal, magnetic, and spin) that tell us everything about an electron's energy, shape, and direction in an atom.

🎯 Exam Tip: List all four quantum numbers, their symbols, and the specific property of the electron each one describes.

 

Question 15. Draw the atomic structure of Mg (At. No. 12, Mass No. 24)
Answer: For Magnesium (Mg), with atomic number 12 and mass number 24, it means it has 12 protons, 12 electrons, and 12 neutrons (\( 24 - 12 = 12 \)). The electronic configuration is 2, 8, 2. Here's how its atomic structure looks:

The electronic configuration shows 2 electrons in the K shell, 8 in the L shell, and 2 in the M shell.
In simple words: The Magnesium atom has 12 protons and 12 neutrons in its center. Its 12 electrons are arranged as 2 in the first shell, 8 in the second, and 2 in the third.

🎯 Exam Tip: For atomic structure diagrams, ensure the nucleus is clearly labeled with protons and neutrons, and electrons are correctly distributed in their respective shells.

 

Question 16. Compare the charge and mass of protons and electrons.
Answer: Here's a comparison of protons and electrons based on their charge and mass:

Protons have a positive charge and are much heavier, found in the nucleus. Electrons have a negative charge and are extremely light, orbiting the nucleus. This huge difference in mass means electrons contribute very little to an atom's total mass.
In simple words: Protons are positive and heavy. Electrons are negative and very, very light.

🎯 Exam Tip: Always specify both the relative charge and approximate mass for protons and electrons, highlighting the significant mass difference.

 

Question 17. \( \mathrm{Ca}^{2+} \) has completely filled outer shell. Justify your answer.
Answer: Calcium (Ca) has an atomic number of 20. Its electronic configuration is 2, 8, 8, 2 (K=2, L=8, M=8, N=2). When calcium loses its two outermost electrons from the N shell to form a \( \mathrm{Ca}^{2+} \) ion, its electron configuration becomes 2, 8, 8. In this configuration, the M shell, which is now the outermost shell, has 8 electrons. Having 8 electrons in the outermost shell is considered a stable, completely filled (octet) configuration for most atoms. This is why \( \mathrm{Ca}^{2+} \) is very stable.
In simple words: Calcium (Ca) has 20 electrons. When it loses 2 electrons to become \( \mathrm{Ca}^{2+} \), its new outer shell (M shell) has 8 electrons, which is a full and stable shell.

🎯 Exam Tip: Justify by showing the initial and final electron configurations and explaining why 8 electrons in the outermost shell makes it "completely filled" or stable (octet rule).

 

Question 18. State the law of multiple proportion.
Answer: The Law of Multiple Proportions states that if two elements can combine to form more than one compound, then the masses of one element that combine with a fixed mass of the other element are in a ratio of small whole numbers. For example, carbon and oxygen can combine to form carbon monoxide (CO) and carbon dioxide (\( \mathrm{CO}_{2} \)). In CO, 12 parts of carbon combine with 16 parts of oxygen. In \( \mathrm{CO}_{2} \), 12 parts of carbon combine with 32 parts of oxygen. The ratio of oxygen masses (16:32) is 1:2, a simple whole number ratio. This law highlights the discrete nature of elements combining in compounds.
In simple words: When two elements make more than one compound, the amounts of one element that join with a fixed amount of the other will always be in a simple whole number ratio.

🎯 Exam Tip: Remember to state that the law applies when *more than one* compound is formed between two elements and the ratio of masses is a *simple whole number*.

 

Question 19. List the uses of isotopes.
Answer: Isotopes have many important uses:
- Uranium-235: Used as fuel in nuclear reactors to generate electricity.
- Cobalt-60: Used in the treatment of cancer because it emits gamma rays that can kill cancer cells.
- Iodine-131: Used in the treatment of goitre (a condition of the thyroid gland).
- Carbon-14: Used to find the age of ancient plants, animals, and archaeological artifacts (radiocarbon dating).
These applications show how isotopes are valuable in energy, medicine, and scientific research.
In simple words: Isotopes are used in power plants (Uranium-235), for treating cancer (Cobalt-60), for thyroid problems (Iodine-131), and for dating old things (Carbon-14).

🎯 Exam Tip: List at least three common isotopes and their specific applications in different fields like energy, medicine, or dating.

 

Question 20. What is isotone? Give an example.
Answer: Isotones are atoms of different elements that have the same number of neutrons but different atomic numbers and different mass numbers. For example, Boron-12 (\( {}_{5}^{12} \mathrm{B} \)) has 5 protons and 7 neutrons (\( 12-5=7 \)). Carbon-13 (\( {}_{6}^{13} \mathrm{C} \)) has 6 protons and 7 neutrons (\( 13-6=7 \)). Since both Boron-12 and Carbon-13 have 7 neutrons, they are considered isotones. They have different chemical properties because they are different elements with different numbers of protons.
In simple words: Isotones are atoms of different elements that have the same number of neutrons. For example, Boron-12 and Carbon-13 are isotones because both have 7 neutrons.

🎯 Exam Tip: Define isotones by emphasizing "same number of neutrons" for *different* elements, and provide a clear example with calculation of neutrons.

VII. Answer in detail :

 

Question 1. Prove that in the following example, Sulphur and Oxygen combine using the law of reciprocal proportions.
Answer: To prove the law of reciprocal proportions using Sulphur and Oxygen with Copper:
1. Sulphur and oxygen react with copper to give copper oxide (CuO) and copper sulfide (CuS).
2. We assume the mass of copper (the third element) is fixed at 63.5 units.
- In Copper Sulfide (CuS): Copper (Cu) : Sulphur (S) = 63.5 : 32
- In Copper Oxide (CuO): Copper (Cu) : Oxygen (O) = 63.5 : 16
3. From these, the ratio of masses of Sulphur and Oxygen that combine with a fixed mass of copper (63.5) is S : O = 32 : 16, which simplifies to 2 : 1.
4. Now, let's look at the direct combination of Sulphur and Oxygen to form sulphur dioxide (\( \mathrm{SO}_{2} \)).
- In \( \mathrm{SO}_{2} \): Sulphur (S) : Oxygen (O) = 32 : 32, which simplifies to 1 : 1.
5. Finally, we compare the two ratios: (Ratio from combining with Copper) : (Ratio from direct combination) = (2 : 1) : (1 : 1). The ratio between these two ratios is \( \frac{2}{1} : \frac{1}{1} \), which is 2 : 1. This is a simple whole number ratio.

\( \implies \) Since the ratio between the two ratios (2:1) is a simple whole number, the law of reciprocal proportions is proven. This shows how elements combine in specific, simple ratios.
In simple words: We showed that when sulphur and oxygen combine with copper, their mass ratio is 2:1. When sulphur and oxygen combine directly, their mass ratio is 1:1. The ratio of these two ratios is 2:1, which is a simple whole number. This proves the law.

🎯 Exam Tip: Clearly state the fixed mass of the third element (copper) and show the mass ratios in both scenarios, then calculate and compare the ratios to demonstrate the simple whole number relationship.

 

Question 2. Explain about radioactive decay and radioactive isotopes.
Answer:
**Radioactive Isotopes:** These are atoms of an element that have an unstable nucleus. The nucleus becomes unstable when there is an imbalance in the number of protons and neutrons. Because they are unstable, these isotopes try to become more stable by releasing energy and particles from their nucleus. For example, Carbon-14 and Uranium-235 are common radioactive isotopes.
**Radioactive Decay:** This is the process where an unstable (radioactive) nucleus changes into a more stable nucleus by spontaneously emitting radiation. This radiation can be in the form of alpha particles, beta particles, or gamma rays. This process cannot be predicted for a single atom, but for a large group of atoms, the decay happens at a certain rate called half-life. It's a natural way for elements to transform over time. Every element has specific isotopes, and some of them happen to be radioactive.
In simple words: Radioactive isotopes are unstable atoms that release energy and particles from their center to become stable. This process of releasing radiation is called radioactive decay.

🎯 Exam Tip: Define both terms separately. For radioactive isotopes, mention "unstable nucleus" due to proton-neutron imbalance. For radioactive decay, explain it as the process of emitting radiation to achieve stability.

 

Question 3. Describe the limitations in Rutherford's model.
Answer: Rutherford's model of the atom, while groundbreaking, had a significant limitation related to atomic stability:
- According to classical electromagnetic theory, an electron moving in a circular orbit around the nucleus is constantly accelerating. An accelerating charged particle should continuously radiate energy. If electrons were to continuously lose energy, their path would gradually reduce, causing them to spiral inwards and eventually fall into the positively charged nucleus. This would make the atom unstable and collapse.
- However, atoms are known to be very stable. Rutherford's model could not explain this stability. It failed to account for why electrons do not lose energy and fall into the nucleus. This was a major challenge that was later addressed by Bohr's model, which introduced the concept of fixed energy orbits where electrons do not radiate energy. Despite its limitation, Rutherford's model was crucial for identifying the nucleus.
In simple words: Rutherford's model couldn't explain why atoms are stable. If electrons orbit the nucleus, they should constantly lose energy and fall into the center, but this doesn't happen in real atoms.

🎯 Exam Tip: The primary limitation is its failure to explain atomic stability. Elaborate on why, according to classical physics, orbiting electrons should radiate energy and spiral into the nucleus.

 

Question 4. Explain the electronic configuration of atoms with an illustration.
Answer: Electronic configuration describes how electrons are arranged in different energy levels or shells around an atom's nucleus. This arrangement follows specific rules, often called Bohr and Bury Rules:
1. Electrons occupy different energy levels, called orbits or shells. Each shell has a fixed amount of energy.
2. The maximum number of electrons that can be accommodated in a shell is given by the formula \( 2n^2 \), where 'n' is the principal quantum number (or shell number, starting from 1 for the K-shell).

3. Shells are filled in a step-by-step manner, starting from the innermost shell. Electrons first fill the lowest energy shells.
4. The outermost shell of an atom cannot have more than 8 electrons, even if it has the capacity for more. This is known as the octet rule, which explains chemical stability.
**Illustration (Example for Calcium - 20 electrons):**
The electronic configuration for Calcium (atomic number 20) is 2, 8, 8, 2.
K-shell: 2 electrons
L-shell: 8 electrons
M-shell: 8 electrons
N-shell: 2 electrons
This arrangement dictates how calcium will interact in chemical reactions, as it tends to lose the 2 electrons in its outermost N-shell to achieve a stable configuration.
In simple words: Electronic configuration shows how electrons are placed in different energy shells (K, L, M, N) around the nucleus. Rules like the \( 2n^2 \) formula and the 8-electron limit for the outer shell guide this arrangement. For example, Calcium has 2, 8, 8, 2 electrons in its shells.

🎯 Exam Tip: Explain the 2n² rule and the octet rule clearly. Use a table to illustrate the maximum electrons per shell and provide a specific element example to show the practical application.

 

Question 5. Explain the valence electron and valency with an illustration.
Answer:
**Valence Electrons:** These are the electrons present in the outermost shell of an atom. They are the ones involved in chemical bonding and largely determine an element's chemical properties. For example, in Nitrogen (atomic number 7), its electronic configuration is 2, 5. So, Nitrogen has 5 valence electrons in its outermost shell.

**Valency:** Valency is the combining capacity of an element, which tells us how many bonds an atom can form. It is determined by the number of electrons an atom needs to gain, lose, or share to achieve a stable outermost shell (usually 8 electrons, like in noble gases).
- If an atom has 1, 2, or 3 valence electrons, its valency is usually 1, 2, or 3, respectively (it tends to lose these electrons).
- If an atom has 4, 5, 6, or 7 valence electrons, its valency is typically calculated as (8 - number of valence electrons). For example, if it has 5 valence electrons, its valency is \( 8 - 5 = 3 \).
- Elements with a completely filled outermost shell (like noble gases with 8 valence electrons, or Helium with 2) have a valency of zero, as they are already stable.
**Illustration: Assigning Valency for Magnesium & Sulphur**
- **Magnesium (Mg):** Atomic number 12. Electronic configuration: 2, 8, 2. It has 2 valence electrons. So, its valency is 2 (it loses 2 electrons).
- **Sulphur (S):** Atomic number 16. Electronic configuration: 2, 8, 6. It has 6 valence electrons. So, its valency is \( 8 - 6 = 2 \) (it gains 2 electrons).
This concept is central to understanding how elements interact to form molecules and compounds.
In simple words: Valence electrons are the outer electrons of an atom. Valency is how many bonds an atom can form, based on how many electrons it needs to lose, gain, or share to become stable. For example, Magnesium has 2 outer electrons, so its valency is 2. Sulphur has 6 outer electrons, so its valency is also 2 (because 8 - 6 = 2).

🎯 Exam Tip: Define valence electrons as outermost electrons and valency as combining capacity. Clearly explain how valency is derived from the number of valence electrons, using concrete examples like Magnesium and Sulphur.

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