RBSE Solutions Class 9 Science Chapter 3 Atomic Structure

Objective Type Questions

 

Question 1. Plum pudding model of the atom was proposed by:
(A) Neils Bohr
(B) Thomson
(C) Rutherford
(D) Goldstein James
Answer: (B) Thomson
In simple words: The plum pudding model, which pictured atoms as spheres of positive charge with electrons embedded in them, was suggested by Thomson. It was an early idea about what atoms look like.

🎯 Exam Tip: Remember key scientists and their models/discoveries in atomic structure, as these are common factual questions.

 

Question 2. Neutron was discovered by:
(A) C.V. Raman
(B) Rutherford
(C) J.J. Thomson
(D) Chadwick
Answer: (D) Chadwick
In simple words: The neutron, a particle with no electric charge found in the atom's nucleus, was found by Chadwick. This discovery helped to complete our understanding of atomic structure.

🎯 Exam Tip: Keep a timeline or list of major atomic discoveries and the scientists responsible for them to easily recall facts like these.

 

Question 3. The size of an atom is:
(A) \( 10^{5} \) cm
(B) \( 10^{6} \) cm
(C) \( 10^{-2} \) cm
(D) \( 10^{-8} \) cm
Answer: (D) \( 10^{-8} \) cm
In simple words: Atoms are incredibly tiny, so their size is measured in very small units. A typical atom has a diameter of about \( 10^{-8} \) centimeters.

🎯 Exam Tip: Be precise with powers of ten when dealing with atomic scale measurements; even a small difference can be a wrong answer.

 

Question 4. The number of neutrons in deuterium, an isotope of hydrogen is:
(A) one
(B) two
(C) three
(D) None
Answer: (A) one
In simple words: Deuterium is a special type of hydrogen atom. Unlike normal hydrogen, which has no neutrons, deuterium has one neutron in its nucleus.

🎯 Exam Tip: Understand that isotopes of an element have the same number of protons but different numbers of neutrons, which affects their mass number.

 

Question 6. Define isobars.
Answer: Isobars are atoms of different chemical elements that have the same mass number but different atomic numbers. This means they have a different number of protons but the same total number of protons and neutrons. For example, Calcium (atomic number 20) and Argon (atomic number 18) are isobars if both have a mass number of 40.
In simple words: Isobars are like atomic cousins from different families; they weigh the same but are made of different main parts.

🎯 Exam Tip: Differentiate clearly between isotopes (same element, different neutrons) and isobars (different elements, same total mass number).

 

Question 7. What are the fundamental particles of an atom?
Answer: The fundamental particles of an atom are electrons, protons, and neutrons. Electrons carry a negative charge, protons carry a positive charge, and neutrons have no charge, making them neutral. These three particles combine to form every atom.
In simple words: Atoms are built from three basic pieces: electrons (negative), protons (positive), and neutrons (no charge).

🎯 Exam Tip: Know the charge and approximate mass of each fundamental particle, as these properties are crucial for understanding atomic behavior.

 

Question 8. Define atomic mass.
Answer: Atomic mass is defined as the relative mass of an atom of an element when compared to the mass of one-twelfth of a carbon-12 isotope atom. The carbon-12 isotope is used as a standard for comparison. This comparison helps scientists to assign a specific weight to each atom relative to a known standard.
In simple words: Atomic mass tells us how heavy an atom is by comparing it to a standard carbon atom.

🎯 Exam Tip: Remember that atomic mass is a relative measure, not an absolute one, and the carbon-12 isotope serves as the universal reference point.

 

Question 9. Define atomic number.
Answer: The atomic number (Z) of an element is the total count of protons found in the nucleus of an atom of that element. In a neutral atom, the number of electrons is exactly equal to the number of protons. The atomic number uniquely identifies an element, as each element has a distinct number of protons.
In simple words: The atomic number is simply the number of protons in an atom's center. It's like an element's ID.

🎯 Exam Tip: The atomic number determines an element's identity and its position in the periodic table; a change in atomic number means a different element.

 

Question 11. Write the value of Avogadro's number.
Answer: The value of Avogadro's number is \( 6.022 \times 10^{23} \). This number represents the amount of particles (atoms, molecules, ions, etc.) in one mole of a substance. It's a fundamental constant in chemistry, allowing us to relate the macroscopic world of grams to the microscopic world of atoms.
In simple words: Avogadro's number is a very large number, \( 6.022 \times 10^{23} \), which tells us how many tiny particles are in one 'mole' of anything.

🎯 Exam Tip: Know Avogadro's number by heart and understand its role in converting between moles and the number of particles.

 

Question 12. Who discovered proton?
Answer: The proton was discovered by Goldstein. He carried out experiments with a modified cathode ray tube, which led to the discovery of positively charged particles. These particles were later identified as protons, which are found in the nucleus of an atom.
In simple words: Goldstein found the proton, which is the positively charged particle inside an atom's center.

🎯 Exam Tip: Associate Goldstein with the discovery of the proton, and remember that protons determine an element's atomic number.

 

Atomic Structure Short Answer Type Questions

 

Question 13. What is an electric discharge tube? Explain with a diagram.
Answer: An electric discharge tube is a specially designed glass tube used to study the flow of electricity through gases at very low pressures. It typically has two metal electrodes (cathode and anode) at its ends and a side tube connected to a vacuum pump to control the gas pressure. When high voltage is applied, gases inside the tube become ionized and emit light or glow, allowing scientists to study fundamental particles. A diagram would show the tube with cathode, anode, vacuum pump connection, and high voltage generator, indicating cathode rays moving from cathode to anode and a green glow.
In simple words: An electric discharge tube is a glass tube used to see how electricity moves through gases. It has metal parts at each end and you can remove air from it.

🎯 Exam Tip: When asked to explain with a diagram, clearly label all parts of your drawing to demonstrate full understanding.

 

Question 15. Explain the mole concept.
Answer: The mole is a unit of measurement used in chemistry to express amounts of a chemical substance. One mole of any substance contains Avogadro's number (approximately \( 6.022 \times 10^{23} \)) of particles (atoms, molecules, ions, etc.). It connects the mass of a substance to the number of particles it contains, with its mass in grams being numerically equal to its atomic or molecular mass. For instance, 1 mole of carbon-12 weighs 12 grams.
In simple words: The mole is a way to count huge numbers of atoms or molecules. One mole of anything always has the same vast number of particles and its weight in grams equals its atomic weight.

🎯 Exam Tip: Understand that the mole is a 'counting unit' like a 'dozen', but for incredibly large quantities, bridging the gap between individual atoms and macroscopic quantities.

 

Question 16. Write the main postulates of Dalton's atomic theory.
Answer: Dalton's atomic theory, proposed in 1808, laid the foundation for modern chemistry with several key ideas:

• Every type of matter is made up of extremely tiny particles called atoms.
• Atoms are indivisible; they cannot be created, split, or destroyed during a chemical reaction.
• All atoms of a specific element are identical, possessing the same mass and chemical properties.
• Atoms of different elements have unique masses and different chemical properties.
• Atoms combine in simple, whole-number ratios to form compounds.

This theory helped explain the laws of chemical combination.
In simple words: Dalton said that all matter is made of tiny, unbreakable atoms. All atoms of the same element are identical, different elements have different atoms, and atoms join in simple ways to make new things.

🎯 Exam Tip: Focus on understanding the five main points of Dalton's theory and how they explain fundamental chemical laws.

 

Question 18. Explain covalent radius of an atom with an example.
Answer: The covalent radius of an atom is defined as half the distance between the nuclei of two identical atoms that are bonded together by a single covalent bond in a molecule. This measurement helps estimate the size of an atom when it forms a chemical bond. For example, in a chlorine molecule (\( Cl_2 \)), two chlorine atoms are covalently bonded, and the distance between their nuclei is 198 pm (or 1.98 Å). So, the covalent radius of a chlorine atom is half of this distance, which is 99 pm (or 0.99 Å).
In simple words: Covalent radius is half the length of a single bond between two similar atoms. It shows how big an atom is when it's part of a molecule.

🎯 Exam Tip: Remember that covalent radius is specifically for atoms joined by a covalent bond and is typically half the bond length between identical atoms.

 

Atomic Structure Long Answer Type Questions

 

Question 19. Describe Rutherford's nuclear model of an atom with the help of the gold foil experiment and well-labeled diagram.
Answer: Rutherford's nuclear model of the atom emerged from his famous gold foil experiment. In this experiment, fast-moving alpha-particles were shot at a thin gold foil.
Observations:

• Most alpha-particles passed straight through the foil, suggesting that atoms are mostly empty space.
• Some alpha-particles were slightly deflected, indicating the presence of a positive charge within the atom.
• A very few alpha-particles (about 1 in 12,000) were deflected back by 180 degrees, suggesting that all the positive charge and most of the atom's mass are concentrated in a tiny, dense region.

Conclusions (Rutherford's Model):

• An atom contains a very small, dense, positively charged center called the nucleus.
• Most of the atom's mass is concentrated in this nucleus.
• Electrons, which are negatively charged, revolve around the nucleus in well-defined orbits.
• The size of the nucleus is much smaller than the size of the atom.
• The atom as a whole is electrically neutral because the number of positive charges (protons) in the nucleus equals the number of negative charges (electrons) outside it.

A diagram would show alpha particles directed at a gold foil, with most passing through, some deflecting slightly, and a few bouncing back, illustrating the concept of a dense, positively charged nucleus.
In simple words: Rutherford found that atoms have a tiny, heavy center called a nucleus, which is positively charged. Most of the atom is empty space, and tiny electrons orbit the nucleus. He proved this by shooting small particles at a gold sheet.

🎯 Exam Tip: Clearly state the observations and conclusions from Rutherford's experiment; this helps explain *why* his model was proposed and what it suggests about atomic structure.

 

Question 20. Describe the main postulates of the Neils Bohr model of the atom. Draw and explain the atomic structure of Na and K based on it.
Answer: Bohr's model of an atom built upon Rutherford's model, adding crucial postulates to explain atomic stability and spectral lines:

1. Electrons revolve around the nucleus in specific, stable orbits called 'energy levels' or 'shells'. These shells have fixed energy and are represented by numbers (1, 2, 3...) or letters (K, L, M, N...) starting from the nucleus.
2. Each energy level has a fixed amount of energy. The energy of these orbits increases as their distance from the nucleus increases.
3. As long as an electron stays in a particular orbit, it does not lose or gain energy, making the atom stable.
4. An electron can move to a higher energy level by absorbing a specific amount of energy, or drop to a lower energy level by emitting energy.

**Atomic Structure of Sodium (Na):** Sodium has an atomic number of 11 and a mass number of 23. As a neutral atom, it has 11 protons and 11 electrons. Electronic configuration: K-shell: 2 electrons, L-shell: 8 electrons, M-shell: 1 electron. (2, 8, 1) A diagram would show a central nucleus with 11 protons, surrounded by 3 shells: the innermost K-shell with 2 electrons, the L-shell with 8 electrons, and the outermost M-shell with 1 electron.
**Atomic Structure of Potassium (K):** Potassium has an atomic number of 19 and a mass number of 39. As a neutral atom, it has 19 protons and 19 electrons. Electronic configuration: K-shell: 2 electrons, L-shell: 8 electrons, M-shell: 8 electrons, N-shell: 1 electron. (2, 8, 8, 1) A diagram would show a central nucleus with 19 protons, surrounded by 4 shells: K-shell with 2 electrons, L-shell with 8 electrons, M-shell with 8 electrons, and the outermost N-shell with 1 electron. The number of neutrons would be \( 39 - 19 = 20 \).
In simple words: Bohr's model says electrons spin around the atom's center in special paths called 'shells', each with a set energy. Electrons don't lose energy unless they jump between these shells. For Sodium (11 electrons), they fill up as 2, 8, 1. For Potassium (19 electrons), they fill up as 2, 8, 8, 1.

🎯 Exam Tip: When drawing atomic structures, always label the nucleus, the shells (K, L, M, N), and correctly place the number of electrons in each shell according to the 2n² rule.

 

Question 21. What are positive rays? How are they produced? Write their properties also.
Answer: Positive rays, also known as canal rays or anode rays, are streams of positively charged ions that travel through a discharge tube in the opposite direction to cathode rays.
**Production:** They are produced when high voltage is applied across a gas at low pressure in a discharge tube, especially one with a perforated cathode. The fast-moving electrons of the cathode rays collide with the neutral gas atoms, knocking off electrons from them and creating positive ions. These positive ions are then attracted towards the negatively charged perforated cathode and pass through its holes, forming the positive rays.
**Properties:**

1. They are streams of positively charged particles (ions) and travel in straight lines.
2. They are deflected by electric and magnetic fields, but in the opposite direction to cathode rays, confirming their positive charge.
3. They can cause a greenish glow on the glass wall of the discharge tube.
4. They can produce mechanical effects by rotating a paddle wheel placed in their path, showing they possess kinetic energy.
5. The charge-to-mass ratio (\( e/m \)) of positive ray particles varies depending on the type of gas used in the discharge tube, unlike cathode rays.
6. The particles in positive rays are generally heavier than electrons.

A diagram would illustrate the discharge tube with a perforated cathode, showing positive rays moving towards the cathode and negative rays (cathode rays) moving towards the anode.
In simple words: Positive rays are streams of positive particles made when electrons hit gas atoms inside a special tube, knocking off their own electrons. These rays move towards the negative side and can be bent by magnets or electricity. Their weight changes depending on the gas used.

🎯 Exam Tip: Clearly distinguish positive rays from cathode rays in terms of charge, direction of deflection, and the dependency of their charge-to-mass ratio on the gas used.

 

Numerical Questions Solved

 

Question 1. An isotope of an element having a number of neutrons 9 and its mass number is 17. Write the name and an atomic number of the element.
Answer: We know that: Mass number (A) = Atomic number (Z) + Number of neutrons (n) Given: Mass number (A) = 17 Number of neutrons (n) = 9 So, \( 17 = Z + 9 \)
\( \implies Z = 17 - 9 \)
\( \implies Z = 8 \) The atomic number (Z) is 8. The element with atomic number 8 is Oxygen. This is because each element is uniquely identified by its atomic number.
In simple words: To find the atomic number, we subtract the number of neutrons from the mass number. For this atom, the atomic number is 8, which means it is oxygen.

🎯 Exam Tip: Always remember the relationship: Mass Number = Protons + Neutrons, and Atomic Number = Protons. These two equations are fundamental to solving such problems.

 

Question 2. Calculate the weight of nitrogen (in gms) at 22.4 liters at NTP.
Answer: At Normal Temperature and Pressure (NTP), one mole of any gas occupies a volume of 22.4 liters. For nitrogen gas (\( N_2 \)), the molecular weight is calculated from its atomic mass. Atomic mass of nitrogen (N) is 14 grams. Therefore, the molecular weight of \( N_2 \) is \( 2 \times 14 = 28 \) grams. Since 22.4 liters of \( N_2 \) gas at NTP represents one mole, its weight will be equal to its molar mass. Thus, 22.4 liters of \( N_2 \) at NTP has a weight of 28 grams. This is because the molar mass in grams is equivalent to one mole of a substance.
In simple words: At standard conditions, 22.4 liters of any gas is one mole. For nitrogen gas, one mole weighs 28 grams. So, 22.4 liters of nitrogen gas weighs 28 grams.

🎯 Exam Tip: Remember Avogadro's Law for gases at NTP: 1 mole of any gas occupies 22.4 liters. This is a crucial shortcut for many gas-related calculations.

 

Question 3. How many numbers of atoms of C is present in 1.5 moles of carbon?
Answer: We know that 1 mole of any substance contains Avogadro's number of particles. Avogadro's number = \( 6.022 \times 10^{23} \) atoms/mole. So, 1 mole of Carbon (C) contains \( 6.022 \times 10^{23} \) atoms. To find the number of atoms in 1.5 moles of Carbon, we multiply Avogadro's number by 1.5: Number of atoms = \( 1.5 \times (6.022 \times 10^{23}) \) Number of atoms = \( 9.033 \times 10^{23} \) atoms of carbon. This calculation directly applies the definition of a mole to find the total number of atoms.
In simple words: One mole of carbon has \( 6.022 \times 10^{23} \) atoms. So, 1.5 moles will have 1.5 times that number of atoms.

🎯 Exam Tip: Always use Avogadro's number when converting between moles and the number of individual particles (atoms, molecules, ions).

 

Atomic Structure Additional Questions Solved

I. Multiple Choice Questions (MCQs)

 

Question 1. The accepted unit of atomic and molecular masses is:
(A) kilogram
(B) gram
(C) pound
(D) atomic mass unit
Answer: (D) atomic mass unit
In simple words: The special unit for measuring how heavy atoms and molecules are is called the atomic mass unit, or amu.

🎯 Exam Tip: The atomic mass unit (amu or u) is specifically designed for the tiny masses of atoms and molecules, making it easier to work with than grams or kilograms.

 

Question 2. An element whose gram-atomic mass and gram-molecular mass are the same is:
(A) hydrogen
(B) oxygen
(C) nitrogen
(D) helium
Answer: (D) helium
In simple words: Helium is a special gas where one atom is also one molecule. So, its atomic weight in grams is the same as its molecular weight in grams.

🎯 Exam Tip: Remember that noble gases like helium exist as single atoms (monoatomic), so their atomic and molecular masses are identical. Other options (hydrogen, oxygen, nitrogen) are diatomic molecules (\( H_2, O_2, N_2 \)).

 

Question 3. Mass of Avogadro's number of oxygen (O) atoms is equal to:
(A) 16 u
(B) 16 g
(C) 32 g
(D) 6 kg
Answer: (B) 16 g
In simple words: One mole of oxygen atoms, which is Avogadro's number of atoms, weighs 16 grams. This is because the molar mass of an element is its atomic mass in grams.

🎯 Exam Tip: The mass of one mole of atoms (Avogadro's number of atoms) is always numerically equal to the element's atomic mass, but expressed in grams (molar mass).

 

Question 5. The number of molecules in 8 g of oxygen gas is:
(A) \( 6.02 \times 10^{23} \)
(B) \( 3.01 \times 10^{23} \)
(C) \( 1.5 \times 10^{23} \)
(D) \( 3.01 \times 10^{12} \)
Answer: (C) \( 1.5 \times 10^{23} \)
In simple words: First, find out how many moles are in 8 grams of oxygen gas (\( O_2 \)). Since \( O_2 \) weighs 32 g per mole, 8 g is 0.25 moles. Then multiply by Avogadro's number to get \( 1.5 \times 10^{23} \) molecules.

🎯 Exam Tip: Remember that oxygen gas is diatomic (\( O_2 \)), so its molecular mass is \( 2 \times 16 = 32 \) g/mol. Always use the molecular mass for gases when calculating moles.

 

Question 6. Which of the following is used as the 'reference standard' used at present, for describing the atomic and molecular masses?
(A) Carbon-13
(B) Chlorine-35
(C) Oxygen-8
(D) Carbon-12
Answer: (D) Carbon-12
In simple words: Scientists use a specific type of carbon atom, called carbon-12, as the main reference point to measure the weights of all other atoms and molecules.

🎯 Exam Tip: Carbon-12 is the universally accepted standard for atomic mass units because it is stable and abundant, providing a consistent reference.

 

Question 7. The gas which has a molecular mass twice that of oxygen gas is:
(A) \( CO_2 \)
(B) CO
(C) \( SO_2 \)
(D) \( H_2S \)
Answer: (C) \( SO_2 \)
In simple words: Oxygen gas (\( O_2 \)) has a mass of 32 u. Sulfur dioxide (\( SO_2 \)) has a mass of 64 u, which is exactly double that of oxygen gas.

🎯 Exam Tip: Always calculate the molecular mass of each option: \( O_2 = 32 \), \( CO_2 = 44 \), \( CO = 28 \), \( SO_2 = 64 \), \( H_2S = 34 \). This systematic approach avoids errors.

 

Question 8. The element which can form gases having different atomicity is:
(A) chlorine
(B) oxygen
(C) hydrogen
(D) nitrogen
Answer: (B) oxygen
In simple words: Oxygen can exist as normal oxygen gas (\( O_2 \), two atoms) or as ozone (\( O_3 \), three atoms). This means it can form gases with different numbers of atoms in each molecule.

🎯 Exam Tip: Atomicity refers to the number of atoms in a molecule of an element. Oxygen is a classic example of an element exhibiting allotropy, forming both diatomic oxygen and triatomic ozone.

 

Question 10. If atomic mass of nitrogen is 14 u and atomic mass of hydrogen is 1 u, what is the molar mass of ammonia?
(A) 17 u
(B) 15 u
(C) 34 u
(D) 30 u
Answer: (A) 17 u
In simple words: Ammonia (\( NH_3 \)) has one nitrogen atom and three hydrogen atoms. So, its total mass is \( (1 \times 14) + (3 \times 1) = 17 \) u.

🎯 Exam Tip: To find the molar mass of a compound, sum the atomic masses of all atoms present in its chemical formula.

 

Question 11. Molar mass of \( HNO_3 \) is 63 u, and an atomic mass of hydrogen and nitrogen are 1 u and 14 u respectively. Atomic mass of oxygen will be:
(A) 3 u
(B) 16 u
(C) 48 u
(D) 24 u
Answer: (B) 16 u
In simple words: The total mass of \( HNO_3 \) is 63 u. Hydrogen is 1 u, and Nitrogen is 14 u. So, the three oxygen atoms together weigh \( 63 - (1 + 14) = 48 \) u. This means one oxygen atom weighs \( 48 / 3 = 16 \) u.

🎯 Exam Tip: For compounds, the sum of individual atomic masses must equal the given molar mass. Use this to find an unknown atomic mass by subtraction.

 

Question 12. One mole of \( BaCl_2 \) ionises completely, total moles of barium and chloride ions will be:
(A) 1 mole
(B) 2 moles
(C) 3 moles
(D) \( 0.2 \times 10^{23} \) moles
Answer: (C) 3 moles
In simple words: When \( BaCl_2 \) breaks apart in water, one molecule creates one barium ion and two chloride ions. So, one mole of \( BaCl_2 \) gives one mole of \( Ba^{2+} \) and two moles of \( Cl^- \), totaling three moles of ions.

🎯 Exam Tip: Understand the dissociation of ionic compounds. For \( BaCl_2 \), it dissociates into \( Ba^{2+} \) and \( 2Cl^- \), meaning one formula unit yields three ions.

 

Question 14. Particles present in the nucleus are:
(A) Proton and Electron
(B) Proton and Neutron
(C) Neutron and Electron
(D) Only Neutron
Answer: (B) Proton and Neutron
In simple words: The nucleus, which is the dense center of an atom, is made up of two types of particles: positively charged protons and neutral neutrons.

🎯 Exam Tip: Clearly differentiate between the subatomic particles. Electrons orbit the nucleus, while protons and neutrons reside within it.

 

Question 15. The presence of the nucleus at the centre of the atom was suggested by:
(A) J.J. Thomson
(B) Rutherford
(C) Goldstein
(D) Bohr
Answer: (B) Rutherford
In simple words: Rutherford's gold foil experiment showed that most of an atom's mass and positive charge is packed into a tiny central part, which he called the nucleus.

🎯 Exam Tip: Associate Rutherford's gold foil experiment with the discovery of the atomic nucleus, a pivotal moment in understanding atomic structure.

 

Question 16. A maximum number of electrons which can be accommodated in the \( n^{th} \) energy level is given by:
(A) \( n^2 \)
(B) \( 2n^2 \)
(C) 2n
(D) n
Answer: (B) \( 2n^2 \)
In simple words: The formula \( 2n^2 \) tells us the largest number of electrons that can fit into any electron shell, where 'n' is the shell number. For example, the first shell (n=1) can hold 2 electrons, and the second (n=2) can hold 8 electrons.

🎯 Exam Tip: This formula (\( 2n^2 \)) is essential for writing electron configurations and understanding how electrons are arranged in an atom's shells.

 

Question 17. The fundamental particles with proper electric charge in an atom are:
(A) Electron (-1), Proton (+ 1) and Neutron (1)
(B) Electron (- 1), Proton (0) and Neutron (0)
(C) Electron (+ 1), Proton (0) and Neutron (- 1)
(D) Electron (-1), Proton (+ 1) and Neutron (0)
Answer: (D) Electron (-1), Proton (+ 1) and Neutron (0)
In simple words: Electrons have a negative charge (-1), protons have a positive charge (+1), and neutrons have no charge (0). These are the basic charges of the particles in an atom.

🎯 Exam Tip: Accurately knowing the charge of each subatomic particle is fundamental for understanding atomic behavior and chemical bonding.

 

Question 19. Which of the following is correct about the energy of the shells in an atom?
(A) M > L > K
(B) M < L < K
(C) K > M > L
(D) M > K > L
Answer: (A) M > L > K
In simple words: In an atom, electron shells further from the nucleus have more energy. So, the K shell (closest) has the least energy, then L, and then M (farthest), which has the most energy.

🎯 Exam Tip: Remember that energy levels increase as the distance from the nucleus increases; K is the first shell, L the second, and M the third.

 

Question 20. An atom of an element with atomic number 18 and mass number 40 has the following articles:
(A) 18 protons, 18 electrons, 22 neutrons
(B) 18 protons, 22 electrons, 18 neutrons
(C) 22 protons, 18 electrons, 18 neutrons
(D) 22 protons, 22 electrons, 18 neutrons
Answer: (A) 18 protons, 18 electrons, 22 neutrons
In simple words: An atomic number of 18 means 18 protons. For a neutral atom, there are also 18 electrons. The mass number (40) minus the protons (18) gives 22 neutrons.

🎯 Exam Tip: Clearly remember that atomic number = protons, and for a neutral atom, protons = electrons. Neutrons = Mass Number - Protons.

 

Question 21. Rutherford's scattering experiment resulted in the discovery of:
(A) electron
(B) proton
(C) neutron
(D) nucleus in an atom
Answer: (D) nucleus in an atom
In simple words: Rutherford's famous experiment, where he shot alpha particles at gold foil, proved that atoms have a tiny, dense, positively charged center called the nucleus.

🎯 Exam Tip: This question tests a foundational concept. The gold foil experiment led directly to the nuclear model of the atom, establishing the existence of the nucleus.

 

Question 22. A mass number of an atom is equal to the:
(A) number of protons in the nucleus of the atom
(B) number of electrons and protons in the atom
(C) number of neutrons and protons in the nucleus of an atom
(D) number of neutrons in the nucleus of an atom
Answer: (C) number of neutrons and protons in the nucleus of an atom
In simple words: The mass number of an atom is simply the total count of both protons and neutrons that are found in its nucleus.

🎯 Exam Tip: The mass number accounts for nearly all of an atom's mass, as electrons have negligible mass compared to protons and neutrons.

 

Question 24. Which of the following pairs represents a pair of isotopes?
(a) \( 8p + 8n; 8p + 9n \)
(b) \( 9p + 9n; 8p + 9n \)
(c) \( 18p + 22n; 16p + 24n \)
(d) \( 9p + 9n; 8p + 10n \)
Answer: (a) \( 8p + 8n; 8p + 9n \)
In simple words: Isotopes are atoms of the same element, meaning they have the same number of protons (p) but different numbers of neutrons (n). Option (a) shows both atoms having 8 protons, but one has 8 neutrons and the other has 9 neutrons, fitting the definition of isotopes.

🎯 Exam Tip: Remember, isotopes only differ in their neutron count. The number of protons defines the element, and it must be the same for isotopes.

 

Question 25. The number of electrons in an element X is 15 and the number of neutrons is 16. Which of the following is the correct representation of the element?
(a) \( _{15}^{31}{X} \)
(b) \( _{16}^{31}{X} \)
(c) \( _{16}^{15}{X} \)
(d) \( _{15}^{16}{X} \)
Answer: (a) \( _{15}^{31}{X} \)
In simple words: In a neutral atom, the number of electrons is equal to the number of protons, which is also the atomic number (Z). Here, electrons are 15, so protons are 15 (Z=15). The mass number (A) is protons + neutrons = 15 + 16 = 31. So, the element is written as \( _{Z}^{A}{X} \).

🎯 Exam Tip: The atomic number (Z) is always the subscript (bottom number) and represents protons (and electrons in a neutral atom). The mass number (A) is the superscript (top number) and is the sum of protons and neutrons.

 

Question 26. The ion of an element has 3 unit positive charge. A mass number of its atom is 27 and number of neutrons in it is 14. What is the number of electrons in the ion?
(a) 14
(b) 13
(c) 27
(d) 10
Answer: (d) 10
In simple words: First, find the number of protons using the mass number and neutrons. Then, subtract the positive charge from the number of protons to get the number of electrons in the ion.

🎯 Exam Tip: A positive charge means the atom has lost electrons. Subtract the charge value from the atomic number (number of protons) to find the electrons in the ion.

 

Question 27. The mass number of an element is 7 and number of neutrons in its atom is 4. What is the number of electrons in the atom?
(a) 3
(b) 7
(c) 4
(d) 11
Answer: (a) 3
In simple words: The mass number is the sum of protons and neutrons. If the mass number is 7 and neutrons are 4, then protons are 7 - 4 = 3. In a neutral atom, the number of electrons is equal to the number of protons, so there are 3 electrons.

🎯 Exam Tip: For a neutral atom, electrons = protons = atomic number. This relationship is key to solving many atomic structure problems.

 

Question 28. Which of the following configuration is not possible as per the Bohr's model of an atom?
(a) 2, 5
(b) 2, 8, 8
(c) 2, 7, 3
(d) 2, 8, 2
Answer: (c) 2, 7, 3
In simple words: According to Bohr's model, electrons fill shells from inside to outside, and the second shell (L-shell) can hold a maximum of 8 electrons. The configuration 2, 7, 3 is not possible because the second shell has 7 electrons, but the third shell (M-shell) has already started to fill with 3 electrons. This should only happen if the second shell is full.

🎯 Exam Tip: Remember Bohr's rules for electron distribution: the maximum number of electrons in a shell is \( 2n^2 \), and outer shells generally fill only after inner shells are complete, with an exception for elements with more than 8 electrons in their penultimate shell which can start filling the outermost shell with 2 electrons before the penultimate shell is completely full (e.g., potassium 2,8,8,1). However, the general rule is to fill inner shells first.

 

Question 29. Which of the following pairs represents a pair of isobars?
(a) \( 8p + 8n; 8p + 9n \)
(b) \( 18p + 22n; 20p + 20n \)
(c) \( 18p + 22n; 16p + 22n \)
(d) \( 8p + 9n; 8p + 8n \)
Answer: (b) \( 18p + 22n; 20p + 20n \)
In simple words: Isobars are atoms of different elements that have the same mass number (total protons + neutrons) but different atomic numbers (number of protons). In option (b), the first atom has 18 protons + 22 neutrons = 40 mass number. The second atom has 20 protons + 20 neutrons = 40 mass number. They both have a mass number of 40 but different proton numbers, making them isobars.

🎯 Exam Tip: For isobars, focus on the mass number (protons + neutrons) being the same, and the atomic number (protons) being different. It's the opposite of isotopes.

Atomic Structure Very Short Answer Type Questions

 

Question 1. Define the term atom.
Answer: An atom is the smallest particle of an element. It can take part in a chemical reaction and may or may not exist on its own.
In simple words: An atom is the tiny building block of everything, and it can join with other atoms in chemical changes.

🎯 Exam Tip: Emphasize that atoms are the smallest unit of an element that retains its chemical properties and participates in reactions.

 

Question 2. Define law of conservation of mass for a chemical reaction.
Answer: The law of conservation of mass states that during any chemical reaction, mass can neither be created nor destroyed. The total mass of reactants must equal the total mass of products.
In simple words: This law means that in a chemical change, nothing disappears or magically appears; the total amount of stuff stays the same.

🎯 Exam Tip: State the law clearly and concisely. A good way to remember it is "mass in equals mass out" for any reaction.

 

Question 3. State the law of constant proportion.
Answer: The law of constant proportion, also known as the law of definite proportions, states that in a pure chemical compound, the elements are always present in fixed proportions by mass. This means a compound always has the same elements in the same ratio, no matter how or where it was made.
In simple words: This law says that a chemical compound always has the same ingredients mixed in the same exact amounts, like a recipe.

🎯 Exam Tip: Mentioning "fixed proportions by mass" is crucial. Use water (H₂O) as a simple mental example: it always has hydrogen and oxygen in a 1:8 mass ratio.

 

Question 5. Define atomicity.
Answer: Atomicity is the number of atoms present in one molecule of an element or a compound. For example, a molecule of oxygen gas (\( O_2 \)) has an atomicity of 2 because it contains two oxygen atoms.
In simple words: Atomicity tells us how many atoms are stuck together to make one molecule.

🎯 Exam Tip: Give a simple example like \( H_2 \), \( O_2 \), or \( O_3 \) (ozone) to illustrate how atomicity changes.

 

Question 6. Give one relevant reason why scientists, initially choose 1/16 of the mass of an atom of naturally occurring oxygen as the atomic mass unit.
Answer: Scientists initially chose 1/16th of the mass of a naturally occurring oxygen atom as the atomic mass unit because this choice allowed the atomic masses of most other elements to be expressed as whole numbers. This made calculations and comparisons simpler at the time.
In simple words: They picked 1/16th of oxygen's mass so that other atoms' weights would mostly be nice, round numbers, making them easier to work with.

🎯 Exam Tip: The main advantage was getting whole numbers for most atomic masses, simplifying early atomic theory. Later, carbon-12 became the standard.

 

Question 7. What is molar mass? What are its units?
Answer: Molar mass is defined as the mass of one mole of a substance. It is expressed in grams per mole (g/mol). For instance, the molar mass of water (H₂O) is approximately 18 g/mol, meaning 1 mole of water weighs 18 grams.
In simple words: Molar mass is the weight of a huge group of atoms or molecules, called a mole, and its unit is grams per mole.

🎯 Exam Tip: Clearly state both the definition and the unit. Understanding that molar mass connects atomic/molecular mass (in 'u') to a macroscopic mass (in 'g') is key.

 

Question 8. How many moles of \( CO_2 \) are there in 11g of carbon dioxide? Molar mass of \( CO_2 \) is 44 g/mol.
Answer: To find the number of moles, we use the formula: Moles \( = \frac{\text{Given mass}}{\text{Molar mass}} \).
Given mass of \( CO_2 \) \( = 11 \text{ g} \)
Molar mass of \( CO_2 \) \( = 44 \text{ g/mol} \)
\( n = \frac{m}{M} \)
\( \implies n = \frac{11 \text{ g}}{44 \text{ g/mol}} \)
\( \implies n = 0.25 \text{ mol} \)
Therefore, there are 0.25 moles of \( CO_2 \) in 11g of carbon dioxide.
In simple words: To find how many moles are in 11g of carbon dioxide, we divide the given weight by its molar mass. The calculation shows there are 0.25 moles.

🎯 Exam Tip: Always write down the formula for calculating moles and clearly show the substitution of values. Ensure units cancel out correctly to yield moles.

 

Question 9. Name the scientists who proposed the law of conservation of mass and law of constant proportions.
Answer: Antoine Lavoisier proposed the law of conservation of mass, and Joseph Proust proposed the law of constant proportions. Both these scientists made fundamental contributions to modern chemistry.
In simple words: Lavoisier came up with the idea that mass stays the same in reactions, and Proust discovered that compounds always have elements in fixed amounts.

🎯 Exam Tip: Make sure to correctly associate each scientist with their specific law. These are foundational principles in chemistry.

 

Question 10. Which two fundamental particles are present inside the nucleus?
Answer: The two fundamental particles present inside the nucleus of an atom are protons and neutrons. These particles, collectively called nucleons, make up almost all of the atom's mass.
In simple words: Protons and neutrons are the two tiny bits found inside the center of an atom.

🎯 Exam Tip: Remember that electrons orbit outside the nucleus. The nucleus contains only protons (positive charge) and neutrons (no charge).

 

Question 12. Chemical properties of all the isotopes of an element are similar. Give one reason.
Answer: The chemical properties of all isotopes of an element are similar because they have the same number of protons and thus the same atomic number. This means they have the same electronic configuration and the same number of valence electrons, which primarily determine how an atom reacts chemically.
In simple words: Isotopes act the same in chemistry because they have the same number of electrons on their outer shell, which controls how they react.

🎯 Exam Tip: The key reason is identical electron configuration, especially the number of valence electrons. Neutrons affect mass but not chemical reactivity.

 

Question 13. What is Orbit?
Answer: According to Bohr's model, an orbit (or electron shell) is a fixed, circular path around the nucleus in which electrons move. Each orbit has a specific, fixed amount of energy. Electrons stay in these orbits unless they gain or lose energy.
In simple words: An orbit is like a special, fixed circular track where electrons move around the center of an atom.

🎯 Exam Tip: Highlight that orbits are fixed energy levels where electrons travel, a key departure from Rutherford's model where electrons would spiral into the nucleus.

 

Question 14. An element has mass number 28 and atomic number 14. Find the number of protons, neutrons, and electrons in it. Write the complete symbol of the element.
Answer: Given:
Mass number (A) \( = 28 \)
Atomic number (Z) \( = 14 \)

Number of protons \( = \text{Atomic number} = 14 \).
Number of electrons (in a neutral atom) \( = \text{Number of protons} = 14 \).
Number of neutrons \( = \text{Mass number} - \text{Atomic number} = 28 - 14 = 14 \).
The element with atomic number 14 is Silicon (Si).
The complete symbol of the element is \( _{14}^{28}{Si} \). Silicon is a crucial semiconductor, forming the basis of many electronic devices.
In simple words: The atom has 14 protons, 14 neutrons, and 14 electrons. It is Silicon, and its symbol is \( _{14}^{28}{Si} \).

🎯 Exam Tip: Remember these fundamental relations: Protons = Atomic Number (Z); Electrons = Protons (for neutral atom); Neutrons = Mass Number (A) - Atomic Number (Z).

 

Question 15. If the atomic number of an element is 11 and its mass number is 23, then what is the number of neutrons in its nucleus?
Answer: Given:
Atomic number \( = 11 \)
Mass number \( = 23 \)
Number of neutrons \( = \text{Mass number} - \text{Atomic number} \)
\( \implies \) Number of neutrons \( = 23 - 11 = 12 \).
Thus, the nucleus of this atom contains 12 neutrons. This element is sodium, a highly reactive alkali metal.
In simple words: To find the neutrons, just subtract the atomic number (11) from the mass number (23), which gives 12 neutrons.

🎯 Exam Tip: The mass number represents the total number of particles in the nucleus (protons + neutrons), so subtracting protons gives neutrons.

Atomic Structure Short Answer Type Questions

 

Question 1. Why do the canal rays obtained by using different gases have different e/m values?
Answer: Canal rays are streams of positively charged ions formed when electrons collide with the neutral atoms of the gas inside the discharge tube, knocking off their own electrons. These positive ions are formed from the gas atoms themselves. Since different gases have atoms with different masses and can form ions with different charges, the mass (m) and sometimes the charge (e) of these resulting ions will vary. Therefore, the charge to mass ratio (\( e/m \)) for canal rays depends on the nature of the gas used.
In simple words: Canal rays are made of charged bits of the gas itself. Different gases have different types of atoms and ions, so their mass and charge vary, which changes their \( e/m \) ratio.

🎯 Exam Tip: The key distinction is that canal rays are composed of the ionized atoms of the residual gas, unlike cathode rays which are always electrons.

 

Question 2. How can one determine the nature of the charge of the particles constituting the cathode rays from this experiment? If Yes, how?
Answer: Yes, the nature of the charge of cathode ray particles can be determined from the experiment. When cathode rays are passed through an electric field created by positively and negatively charged plates, they are deflected towards the positively charged plate. This deflection towards the positive electrode clearly indicates that the particles constituting the cathode rays are negatively charged. This behavior helped establish that cathode rays are streams of electrons.
In simple words: Yes, by putting charged plates nearby, we see cathode rays bend towards the positive plate. This bending proves they are negatively charged.

🎯 Exam Tip: The deflection in an electric field is a direct proof of charge. Remember, opposite charges attract, causing the deflection towards the positive plate.

 

Question 3. The electronic configuration of an element X is 2, 8, 7.
1. What is the atomic number of the element X?
2. Is X metal or non-metal?
3. What is the valency of X?
Answer:
1. The atomic number is the sum of electrons in all shells: \( 2 + 8 + 7 = 17 \). So, the atomic number of element X is 17.
2. Element X has 7 valence electrons (electrons in the outermost shell). Elements with 5, 6, or 7 valence electrons are typically non-metals, as they tend to gain electrons to achieve a stable octet. Thus, X is a non-metal (Chlorine).
3. Valency is the combining capacity of an element. Since X has 7 valence electrons, it needs 1 more electron to complete its octet (8 electrons in the outermost shell). Therefore, its valency is 1.
In simple words: 1. Element X has 17 electrons, so its atomic number is 17. 2. It's a non-metal because it has 7 electrons in its outer shell. 3. Its valency is 1 because it needs one more electron to be stable.

🎯 Exam Tip: The number of electrons determines the atomic number. The number of valence electrons helps identify if it's a metal (1, 2, 3 valence electrons, tends to lose) or a non-metal (5, 6, 7 valence electrons, tends to gain).

 

Question 4. If an atom carries one proton and one electron, will it carry any net charge or not?
Answer: If an atom carries one proton and one electron, it will not carry any net charge. A proton has a +1 unit charge, and an electron has a -1 unit charge. When these two charges are equal in magnitude and opposite in sign, they balance each other out, resulting in a net charge of zero for the atom. This forms a hydrogen atom, the simplest element.
In simple words: No, it will not have any net charge. The positive charge of the proton cancels out the negative charge of the electron, making the atom neutral.

🎯 Exam Tip: Remember that protons carry positive charge and electrons carry negative charge. For a neutral atom, the number of protons and electrons must be equal.

 

Question 5. What is the difference between isobar: and isotopes ? Give one example for each

Answer: The main difference is that isobars are atoms of *different* elements with the *same* mass number, while isotopes are atoms of the *same* element with *different* mass numbers. This means isotopes have the same number of protons, but isobars have different numbers of protons. Their unique compositions lead to distinct chemical identities for isobars and similar ones for isotopes.
In simple words: Isobars are like different elements with the same total weight, but isotopes are like the same element with different total weights because of extra neutrons.

🎯 Exam Tip: Remember: IsObar means same O (mass number). IsOtope means same P (protons/atomic number). Always provide a clear example for each definition.

 

Question 6. State the law of conservation of mass. Is this law applicable to the chemical reactions? Elaborate your answer with the help of an example.
Answer: The law of conservation of mass states that mass can neither be created nor destroyed in any chemical reaction. This means the total mass of the reactants before a chemical change must be equal to the total mass of the products formed after the change. This law is indeed applicable to all chemical reactions because it reflects the fundamental principle that atoms are simply rearranged, not lost or gained, during a chemical process. This principle is a cornerstone of stoichiometry.
For example: Consider the reaction between barium chloride and sodium sulfate:
\( \text{BaCl}_2(\text{aq}) + \text{Na}_2\text{SO}_4(\text{aq}) \rightarrow \text{BaSO}_4(\text{s}) + 2\text{NaCl}(\text{aq}) \)
If we take:
Mass of \( \text{BaCl}_2 \) \( = 208.5 \text{ g} \)
Mass of \( \text{Na}_2\text{SO}_4 \) \( = 142 \text{ g} \)
Total mass of reactants \( = 208.5 + 142 = 350.5 \text{ g} \)
And the products formed are:
Mass of \( \text{BaSO}_4 \) \( = 233.5 \text{ g} \)
Mass of \( 2\text{NaCl} \) \( = 117 \text{ g} \)
Total mass of products \( = 233.5 + 117 = 350.5 \text{ g} \)
As shown, the total mass of reactants equals the total mass of products, demonstrating the law of conservation of mass.
In simple words: The law of conservation of mass means that in any chemical reaction, the total weight of everything you start with will be the same as the total weight of everything you end up with. Atoms just get shuffled around, not destroyed or created. This law works for all chemical reactions, as shown by the example where the starting chemicals weighed the same as the ending chemicals.

🎯 Exam Tip: Always define the law clearly, state its applicability to chemical reactions, and provide a balanced chemical equation with example masses to illustrate the concept effectively.

 

Question 7.
(1) What is the difference between molecular mass and molar mass?
(2) What relation exists between the atomic mass of an element and number of atoms? Explain with an example.
Answer:
(1) The difference between molecular mass and molar mass:
Molecular mass is the mass of one molecule of a substance, typically measured in atomic mass units (u). It's a relative mass comparing to carbon-12. Molar mass, on the other hand, is the mass of one mole (\( 6.022 \times 10^{23} \) molecules) of a substance, and it is expressed in grams per mole (g/mol). Both values are numerically the same, but their units differ, reflecting a change from atomic scale to macroscopic scale.
(2) The atomic mass of an element, when expressed in grams, is numerically equal to the mass of one mole of that element. One mole of any element contains Avogadro's number (\( 6.022 \times 10^{23} \)) of atoms. Thus, the atomic mass of every element, when taken in grams, represents the mass of \( 6.022 \times 10^{23} \) atoms of that element.
For example: The atomic mass of magnesium (Mg) is 24 u. This means one atom of magnesium weighs 24 u. Its molar mass is 24 g/mol. Therefore, 24 grams of magnesium contain \( 6.022 \times 10^{23} \) atoms of magnesium. This connection is fundamental for converting between the microscopic and macroscopic world in chemistry.
In simple words: (1) Molecular mass is the weight of one tiny molecule in 'u' units, while molar mass is the weight of a huge group of molecules (one mole) in 'grams'. They have the same number but different units. (2) When you take the atomic mass of an element in grams, that amount will always contain the same huge number of atoms (\( 6.022 \times 10^{23} \)). For example, 24 grams of magnesium will always have \( 6.022 \times 10^{23} \) magnesium atoms because 24 is its atomic mass.

🎯 Exam Tip: Clearly distinguish the units (u vs. g/mol) for molecular and molar mass. For the relationship, emphasize that "gram atomic mass" is equivalent to "one mole of atoms".

 

Question 8. What is Avogadro's number and how is it related to the molecular mass of compounds? Explain with an example?
Answer: Avogadro's number is a fundamental constant in chemistry, approximately \( 6.022 \times 10^{23} \). It represents the number of particles (atoms, molecules, ions, etc.) in one mole of any substance. This immense number is named after Amedeo Avogadro, whose hypothesis laid the groundwork for the concept of the mole.
The molecular mass of a compound is the sum of the atomic masses of all atoms in one molecule, expressed in atomic mass units (u). The molar mass of a compound is numerically equal to its molecular mass but is expressed in grams per mole (g/mol). Avogadro's number connects these two: the mass, in grams, of \( 6.022 \times 10^{23} \) molecules of any substance is equal to its molar mass. This means that if you have the molecular mass (e.g., X u), then X grams of that substance will contain \( 6.022 \times 10^{23} \) molecules. This allows chemists to easily convert between the number of molecules and the mass of a substance.
For example: The molecular mass of water (\( H_2O \)) is 18 u (\( 2 \times 1 \text{ u} + 16 \text{ u} \)). This means that the molar mass of water is 18 g/mol. Therefore, 18 grams of water contains \( 6.022 \times 10^{23} \) molecules of water. This demonstrates how molecular mass, Avogadro's number, and molar mass are intrinsically linked.
In simple words: Avogadro's number is \( 6.022 \times 10^{23} \), which is a very large number that tells us how many atoms or molecules are in one "mole" of any substance. When you know the molecular mass of a compound (like 18 for water), then that same number in grams (18 grams of water) will contain exactly Avogadro's number of molecules. This helps us count tiny molecules by weighing them.

🎯 Exam Tip: Define Avogadro's number precisely. Explain how it bridges molecular mass (in u) to molar mass (in g/mol) for a macroscopic quantity of substance.

 

Question 9.
(a) What is the maximum number of electrons that can be present in the outermost shell of an atom?
(b) Draw a sketch of Bohr's model of an atom with three shells.
Answer:
(a) The maximum number of electrons that can be present in the outermost shell of an atom is 8. This rule, known as the octet rule, contributes to the stability of atoms, making them less reactive when their outermost shell is full. (For the first shell, the maximum is 2 electrons).
(b) Here is a sketch of Bohr's model of an atom with three shells:

In simple words: (a) An atom's outermost ring can hold up to 8 electrons for most stable atoms. (b) The picture shows a simple model of an atom with a central nucleus and three rings (shells) where electrons move.

🎯 Exam Tip: For Bohr's model, ensure the nucleus is central, shells are concentric circles, and label them clearly (K, L, M or 1, 2, 3). For the outermost shell, 8 electrons is the general rule, but remember the first shell only holds 2.

 

Question 10.
(a) How are canal rays different from electrons in terms of charge and mass?
(b) What are canal rays? Who discovered them? What is the charge and mass of canal ray?
Answer:
(a) Canal rays consist of positively charged particles (ions of the residual gas), which have significant mass and vary depending on the gas used. In contrast, electrons (cathode rays) are negatively charged, have a fixed, very small mass, and are identical regardless of the gas in the tube or the electrode material. Electrons are fundamental particles, whereas canal ray particles are composite ions.
(b) Canal rays are streams of positively charged ions that are produced in a discharge tube when electrons (cathode rays) collide with residual gas atoms, ionizing them. They were discovered by E. Goldstein in 1886. The particles in canal rays carry a positive charge, but their mass and specific charge-to-mass ratio (\( e/m \)) are not fixed; they depend on the nature of the gas used in the discharge tube. Generally, their mass is much larger than that of an electron, and they carry charges that are multiples of the elementary positive charge.
In simple words: (a) Canal rays are heavy and positive, made of different gas particles, unlike electrons which are light, negative, and always the same. (b) Canal rays are positive streams of charged gas bits discovered by Goldstein. Their positive charge and weight change depending on the gas used.

🎯 Exam Tip: For canal rays, remember that their properties (charge, mass, \( e/m \) ratio) depend on the gas, unlike cathode rays (electrons) which are universal particles.

 

Question 11. What information do you get from the diagrams given below about the atomic number, mass number, and valency of the atoms X, Y, and Z? Give your answer in a tabular form

Answer: The information about elements X, Y, and Z can be derived from the diagrams as follows:

The diagrams clearly show the number of protons (P) and neutrons (n) in the nucleus. The atomic number is equal to the number of protons. The mass number is the sum of protons and neutrons. For a neutral atom, the number of electrons equals the atomic number. Valency is determined by the number of valence electrons. Element Z (Phosphorus) typically has a valency of 3 or 5 depending on the compound it forms.
In simple words: The diagrams show the protons and neutrons inside each atom. From these, we can figure out the atomic number (same as protons), mass number (protons + neutrons), how many electrons there are, and how many bonds each atom can make (its valency). This information is summarized in the table.

🎯 Exam Tip: Always clearly list the number of protons and neutrons from the diagram. Remember that atomic number = protons, mass number = protons + neutrons, and for a neutral atom, electrons = protons. Valency is determined by the outermost electron shell.

 

Question 12. State the major drawback in Rutherford's model of an atom. Mention two features of Bohr's Model which helped compensate this drawback.
Answer: The major drawback of Rutherford's nuclear model of an atom was its inability to explain the stability of the atom. According to classical electromagnetic theory, an electron revolving in a circular orbit would continuously accelerate and radiate energy. This constant energy loss would cause the electron to spiral inwards and eventually collapse into the nucleus, leading to an unstable atom. However, atoms are known to be stable.

Two features of Bohr's model that helped compensate for this drawback are:
1. Bohr proposed that electrons revolve around the nucleus in specific, discrete orbits or energy shells, also known as stationary states. Electrons are allowed to exist only in these specific orbits.
2. While revolving in these discrete orbits, electrons do not radiate energy. This means that as long as an electron remains in a particular orbit, its energy remains constant, thus providing stability to the atom. This idea was a significant step in explaining atomic stability.
In simple words: Rutherford's model couldn't explain why atoms are stable; it predicted electrons would crash into the nucleus. Bohr's model fixed this by saying electrons only move in special paths called orbits without losing energy, making the atom stable.

🎯 Exam Tip: Clearly articulate the stability issue with Rutherford's model. For Bohr's model, emphasize "discrete orbits" and "no energy radiation" in these orbits as the key solutions.

Atomic Structure Long Answer Type Questions

 

Question 1. Find the % composition of the elements in the following compounds:
(1) Water
(2) Sodium Sulphate
Answer:
(1) For Water (\( H_2O \)):
Atomic mass of Hydrogen (H) \( = 1 \text{ u} \)
Atomic mass of Oxygen (O) \( = 16 \text{ u} \)
Molar mass of Water (\( H_2O \)) \( = (2 \times 1) + 16 = 2 + 16 = 18 \text{ g/mol} \)

Percentage of Hydrogen \( = \frac{\text{Mass of Hydrogen in water}}{\text{Molar mass of Water}} \times 100 \)
\( \implies = \frac{2 \text{ g}}{18 \text{ g}} \times 100 \)
\( \implies = \frac{100}{9} = 11.11\% \)

Percentage of Oxygen \( = \frac{\text{Mass of Oxygen in water}}{\text{Molar mass of Water}} \times 100 \)
\( \implies = \frac{16 \text{ g}}{18 \text{ g}} \times 100 \)
\( \implies = \frac{800}{9} = 88.89\% \)

(2) For Sodium Sulphate (\( Na_2SO_4 \)):
Atomic mass of Sodium (Na) \( = 23 \text{ u} \)
Atomic mass of Sulphur (S) \( = 32 \text{ u} \)
Atomic mass of Oxygen (O) \( = 16 \text{ u} \)
Molar mass of Sodium Sulphate (\( Na_2SO_4 \)) \( = (2 \times \text{Na}) + (\text{S}) + (4 \times \text{O}) \)
\( \implies = (2 \times 23) + 32 + (4 \times 16) \)
\( \implies = 46 + 32 + 64 = 142 \text{ g/mol} \)

In 142g of sodium sulphate, there are 46g of sodium, 32g of sulphur, and 64g of oxygen.

Percentage of Sodium \( = \frac{\text{Mass of Sodium}}{\text{Molar mass of } Na_2SO_4} \times 100 \)
\( \implies = \frac{46 \text{ g}}{142 \text{ g}} \times 100 \)
\( \implies = \frac{2300}{71} \approx 32.39\% \)

Percentage of Sulphur \( = \frac{\text{Mass of Sulphur}}{\text{Molar mass of } Na_2SO_4} \times 100 \)
\( \implies = \frac{32 \text{ g}}{142 \text{ g}} \times 100 \)
\( \implies = \frac{1600}{71} \approx 22.54\% \)

Percentage of Oxygen \( = \frac{\text{Mass of Oxygen}}{\text{Molar mass of } Na_2SO_4} \times 100 \)
\( \implies = \frac{64 \text{ g}}{142 \text{ g}} \times 100 \)
\( \implies = \frac{3200}{71} \approx 45.07\% \)
Calculating percentage composition is crucial for understanding the stoichiometry of reactions and the purity of compounds.
In simple words: To find the percentage of each element in a compound, first calculate the total weight (molar mass) of the compound. Then, for each element, divide its total weight in the compound by the molar mass and multiply by 100. For water, hydrogen is 11.11% and oxygen is 88.89%. For sodium sulphate, sodium is about 32.39%, sulphur is about 22.54%, and oxygen is about 45.07%.

🎯 Exam Tip: Always show your calculations clearly, including the molar mass determination. Ensure you sum the percentages at the end to check if they add up to approximately 100% (allowing for rounding). Pay attention to subscripts in chemical formulas.

 

Question 2.
(1) Define one mole, illustrate its relationship with Avogadro constant,
(2) Calculate the number of moles in:
• \( 12.044 \times 10^{23} \) atoms of carbon
• 64g of oxygen atoms
• 66g of carbon dioxide molecules.
Answer:
(1) A mole is defined as the amount of a substance that contains exactly \( 6.022 \times 10^{23} \) elementary entities (like atoms, molecules, or ions). This number, \( 6.022 \times 10^{23} \), is known as Avogadro's constant. The relationship is direct: one mole of any substance always contains Avogadro's constant number of particles. For instance, 1 mole of carbon atoms contains \( 6.022 \times 10^{23} \) carbon atoms, and 1 mole of water molecules contains \( 6.022 \times 10^{23} \) water molecules. This allows us to work with macroscopic amounts of substances while dealing with microscopic particles.

(2) Calculation of the number of moles:
• For \( 12.044 \times 10^{23} \) atoms of carbon:
We know that 1 mole of carbon \( = 6.022 \times 10^{23} \) atoms.
So, \( 1 \) atom of carbon \( = \frac{1}{6.022 \times 10^{23}} \) mole of carbon.
Therefore, \( 12.044 \times 10^{23} \) atoms of carbon \( = \frac{1}{6.022 \times 10^{23}} \times 12.044 \times 10^{23} = 2 \) moles of carbon.

• For 64g of oxygen atoms:
The molar mass of oxygen atoms (O) is 16 g/mol.
Number of moles \( = \frac{\text{Given mass}}{\text{Molar mass}} \)
\( \implies = \frac{64 \text{ g}}{16 \text{ g/mol}} = 4 \) moles of oxygen atoms.

• For 66g of carbon dioxide molecules:
The molar mass of \( CO_2 = \text{Atomic mass of C} + (2 \times \text{Atomic mass of O}) = 12 + (2 \times 16) = 12 + 32 = 44 \text{ g/mol} \).
Number of moles \( = \frac{\text{Given mass}}{\text{Molar mass}} \)
\( \implies = \frac{66 \text{ g}}{44 \text{ g/mol}} = 1.5 \) moles of carbon dioxide molecules.
In simple words: (1) A mole is a specific quantity of anything, which is always \( 6.022 \times 10^{23} \) particles (Avogadro's constant). It's a way to count huge numbers of tiny things. (2) To find the number of moles: for carbon atoms, since \( 12.044 \times 10^{23} \) is double Avogadro's number, it's 2 moles. For 64g of oxygen atoms, divide 64 by oxygen's molar mass (16), giving 4 moles. For 66g of carbon dioxide, divide 66 by its molar mass (44), giving 1.5 moles.

🎯 Exam Tip: Clearly define "mole" and Avogadro's constant. When calculating moles, always ensure you use the correct molar mass (atomic for atoms, molecular for molecules) and pay attention to whether "oxygen" refers to atoms (O) or molecules (\( O_2 \)).

 

Question 3.
(a) Define: (1) Molecular mass, (2) Avogadro constant.
(b) Calculate the number of molecules in 50g of \( CaCO_3 \) (Atomic mass of Ca = 40u, C = 12u and O=16u)
(c) If one mole of sodium atom weighs 23g, what is the mass (in g) of one atom of sodium?
Answer:
(a) Definitions:
(1) Molecular mass: It is the sum of the atomic masses of all the atoms present in one molecule of a substance. It is typically expressed in atomic mass units (u). For example, the molecular mass of water (\( H_2O \)) is 18u. Molecular mass provides a relative measure of the mass of a molecule.
(2) Avogadro constant: The Avogadro constant is the number of constituent particles (atoms, molecules, ions, etc.) that are contained in one mole of a substance. Its value is approximately \( 6.022 \times 10^{23} \). This constant is essential for relating macroscopic quantities of matter to the number of atoms or molecules.

(b) Calculate the number of molecules in 50g of \( CaCO_3 \):
Atomic mass of Ca \( = 40 \text{ u} \)
Atomic mass of C \( = 12 \text{ u} \)
Atomic mass of O \( = 16 \text{ u} \)
Molar mass of \( CaCO_3 = \text{Ca} + \text{C} + (3 \times \text{O}) = 40 + 12 + (3 \times 16) = 40 + 12 + 48 = 100 \text{ g/mol} \)

We know that 1 mole of \( CaCO_3 \) (100g) contains \( 6.022 \times 10^{23} \) molecules.
So, 100g of \( CaCO_3 \) has \( 6.022 \times 10^{23} \) molecules.
Therefore, 1g of \( CaCO_3 \) has \( = \frac{6.022 \times 10^{23}}{100} \) molecules.
And 50g of \( CaCO_3 \) has \( = \frac{6.022 \times 10^{23}}{100} \times 50 \)
\( \implies = 3.011 \times 10^{23} \) molecules.

(c) Mass (in g) of one atom of sodium:
Given that one mole of sodium atoms weighs 23g.
We know that 1 mole of sodium atoms contains Avogadro's number of atoms, i.e., \( 6.022 \times 10^{23} \) atoms.
So, mass of \( 6.022 \times 10^{23} \) atoms of Na \( = 23 \text{ g} \).
Therefore, mass of 1 atom of Na \( = \frac{23 \text{ g}}{6.022 \times 10^{23}} \)
\( \implies \approx 3.819 \times 10^{-23} \text{ g} \).
In simple words: (a) Molecular mass is the weight of one molecule, and Avogadro's constant is the number of particles in one mole (\( 6.022 \times 10^{23} \)). (b) First, find the total weight of one mole of calcium carbonate (which is 100g). Since 100g has Avogadro's number of molecules, 50g will have half of that, which is \( 3.011 \times 10^{23} \) molecules. (c) If 23g is the weight of Avogadro's number of sodium atoms, then the weight of just one sodium atom is 23 divided by Avogadro's number, which is a tiny amount (\( 3.819 \times 10^{-23} \) grams).

🎯 Exam Tip: Be precise with definitions for (a). For calculations (b) and (c), always clearly state the molar mass or Avogadro's number used. Ensure units are consistent throughout the calculation.

 

Question 4. A number of electrons, protons, and neutrons in chemical species A, B, C, and D is given below.

Now answer the following questions:
1. What is the mass number of A and B?
2. What is the atomic number of B?
3. Which two elements represent a pair of isotopes and why?
4. What is the valency of element C? Also, justify your answers.
Answer:
1. Mass number is the sum of protons and neutrons.
For Element A: Mass number \( = \text{Protons} + \text{Neutrons} = 3 + 4 = 7 \).
For Element B: Mass number \( = \text{Protons} + \text{Neutrons} = 9 + 8 = 17 \).
2. The atomic number is equal to the number of protons.
For Element B: Atomic number \( = \text{Protons} = 9 \).
3. Elements C and D represent a pair of isotopes. This is because isotopes are atoms of the same element, meaning they have the same atomic number (number of protons) but different mass numbers (due to different numbers of neutrons). Both C and D have 8 protons, making them the same element (Oxygen). However, C has 8 neutrons (Mass number \( = 8 + 8 = 16 \)), while D has 10 neutrons (Mass number \( = 8 + 10 = 18 \)).
4. For Element C:
It has 8 electrons. Its electronic configuration is 2, 6 (K-shell has 2, L-shell has 6).
Valency is the combining capacity of an element. Element C has 6 electrons in its outermost shell. To achieve a stable octet (8 electrons), it needs to gain 2 more electrons. Therefore, its valency is 2.
In simple words: 1. Element A has a mass number of 7 (3 protons + 4 neutrons), and Element B has a mass number of 17 (9 protons + 8 neutrons). 2. Element B has an atomic number of 9 because it has 9 protons. 3. Elements C and D are isotopes because they both have 8 protons (same element) but different numbers of neutrons (C has 8, D has 10). 4. Element C has 8 electrons, so its outer shell has 6 electrons. It needs 2 more to be full, so its valency is 2.

🎯 Exam Tip: Systematically use the definitions: atomic number = protons; mass number = protons + neutrons; for a neutral atom, electrons = protons. To justify isotopes, explicitly state the same protons and different neutrons. For valency, write the electron configuration and explain the tendency to gain or lose electrons to achieve an octet.

 

Question 5. Give reasons for the following:
(a) The nucleus of an atom is heavy.
(b) The nucleus of an atom is positively charged.
(c) An atom is electrically neutral.
Answer:
(a) The nucleus of an atom is heavy because it contains almost all of the atom's mass. Protons and neutrons, which are much heavier than electrons (about 1836 times heavier), reside in the nucleus. Electrons, despite their high speed, have very little mass and orbit far from the nucleus. Thus, the tiny nucleus holds the majority of the atom's weight.
(b) The nucleus of an atom is positively charged because it contains all the protons, which are subatomic particles carrying a positive electrical charge. Neutrons, also in the nucleus, have no charge. Electrons, which are negatively charged, orbit outside the nucleus. Therefore, the net charge of the nucleus comes solely from its protons.
(c) An atom is electrically neutral because in a neutral atom, the number of positively charged protons in the nucleus is exactly equal to the number of negatively charged electrons orbiting outside the nucleus. The positive and negative charges cancel each other out perfectly, resulting in an overall neutral charge for the atom. This balance makes most atoms non-reactive in their elemental state.
In simple words: (a) The nucleus is heavy because it holds all the heavy particles (protons and neutrons) of an atom. (b) The nucleus is positive because it only contains positive protons and neutral neutrons. (c) An atom is neutral because it has an equal number of positive protons and negative electrons, so their charges cancel each other out.

🎯 Exam Tip: For each reason, clearly identify the subatomic particles involved and their respective masses and charges. Ensure your explanation directly addresses "why" based on these particle properties.