NCERT Competency Based Questions for Class 9 Science Exploration Chapter 8 “Journey Inside the Atom” are designed to help students develop conceptual understanding, scientific reasoning and problem-solving skills. These questions cover important topics such as subatomic particles, atomic models, Rutherford’s alpha-particle scattering experiment, Bohr’s model, electronic configuration, valency, isotopes, isobars, atomic number, mass number and electron distribution. This collection includes exam-oriented competency-based questions with clear answers and explanations, making it useful for CBSE Class 9 Science students preparing for school exams and the 2026-27 academic session.
Table of Contents (Quick Links):
4. Correct and Incorrect Questions
For Chapter 8 Journey Inside the Atom – Main Exercise Question Answers
Competency-Based Questions and Answers
1. During Rutherford’s alpha-particle scattering experiment, approximately 100,000 alpha particles were directed at an extremely thin gold foil. It was observed that about 99,900 particles passed straight through without deflection, a small fraction deflected through small angles and only 1 particle rebounded through 180°.
(i) What conclusion about atomic structure is drawn from the fact that most alpha particles passed straight through?
(ii) What does the rebound of a minuscule fraction at 180° prove regarding the size and mass distribution of the nucleus?
Answer:
(i) Most of the space inside an atom is empty.
(ii) The entire positive charge and almost all the mass of the atom are concentrated in an extremely small central region called the nucleus.
Explanation:
(i) Particles passing straight through:
Since the overwhelming majority traversed the gold foil undeflected, it showed that the atom consists primarily of empty space.
(ii) Particles rebounding at 180°:
To reverse the trajectory of a high-energy alpha particle, a strong electrostatic repulsive force concentrated around a massive center is necessary. The rarity of this occurrence (1 in 100,000) indicates that the dense, positively charged mass occupies an extraordinarily small volume relative to the whole atom.
2. Two elements, A and B, have atomic numbers 12 and 8 respectively.
(i) Write the electronic configurations of both elements using the Bohr-Bury scheme.
(ii) Determine the valency of both elements.
(iii) Describe how electron transfer occurs between A and B to achieve stable octets when they react chemically.
Answer:
(i) Electronic configuration of A is 2, 8, 2 and B is 2, 6.
(ii) The valency of A is 2 and the valency of B is 2.
(iii) Element A transfers its 2 valence electrons to element B, forming A²⁺ and B²⁻ ions.
Explanation:
(i) Electronic configuration:
Element A (Z = 12): K = 2, L = 8, M = 2 → 2, 8, 2
Element B (Z = 8): K = 2, L = 6 → 2, 6
(ii) Valency determination:
Element A has 2 valence electrons. Losing these 2 electrons gives it a stable octet in the L-shell, so its valency is 2.
Element B has 6 valence electrons. It requires 8 – 6 = 2 electrons to complete its octet, so its valency is 2.
(iii) Electron transfer:
Element A transfers 2 electrons from its M-shell to the outer shell of element B. Element A becomes an A²⁺ cation with configuration 2, 8, while element B becomes a B²⁻ anion with configuration 2, 8. Both ions attain stable noble gas configurations.
3. Naturally occurring chlorine consists of two isotopes: ³⁵₁₇Cl (atomic mass 35 u, natural abundance 75%) and ³⁷₁₇Cl (atomic mass 37 u, natural abundance 25%).
(i) Calculate the average atomic mass of chlorine.
(ii) Explain why the atomic mass of an element can be fractional, whereas the mass number of an individual atom is always an integer.
Answer:
(i) The average atomic mass of chlorine is 35.5 u.
(ii) An individual atom contains an integer count of nucleons (p⁺ + n⁰), making its mass number an integer. Atomic mass represents the weighted average across all natural isotopes, which often yields a fractional value.
Explanation:
(i) Calculation:
Average Atomic Mass = (35 × 75/100) + (37 × 25/100)
Average Atomic Mass = (35 × 3/4) + (37 × 1/4)
Average Atomic Mass = (105 + 37)/4
Average Atomic Mass = 142/4 = 35.5 u
(ii) Reason for fractional mass:
An individual atom contains a whole-number count of protons and neutrons, so its mass number (A = p⁺ + n⁰) must be a whole number. However, the atomic mass of an element represents the weighted average of its naturally occurring isotopes. Therefore, it can have a decimal or fractional value.
4. The subatomic compositions of three unknown species P, Q and R are given below:
| Species | Protons | Neutrons | Electrons |
| P | 18 | 22 | 18 |
| Q | 20 | 20 | 20 |
| R | 18 | 20 | 18 |
(i) Determine the mass numbers (A) of P, Q and R.
(ii) Identify which pair represents isotopes and which pair represents isobars.
(iii) Compare the chemical properties of P and R.
Answer:
(i) Aₚ = 40, Aq= 40, Aᵣ = 38.
(ii) P and R are isotopes; P and Q are isobars.
(iii) P and R exhibit identical chemical properties due to having the same number of valence electrons.
Explanation:
(i) Mass numbers (A = p⁺ + n⁰):
Aₚ = 18 + 22 = 40 (Atomic number Z = 18)
Aq = 20 + 20 = 40 (Atomic number Z = 20)
Aᵣ = 18 + 20 = 38 (Atomic number Z = 18)
(ii) Identification:
P and R: Both contain 18 protons (Z = 18) but have different mass numbers (40 and 38). Therefore, P and R are isotopes of argon (⁴⁰₁₈Ar and ³⁸₁₈Ar).
P and Q: Both have the same mass number (40) but different atomic numbers (18 and 20). Therefore, P and Q are isobars (⁴⁰₁₈Ar and ⁴⁰₂₀Ca).
(iii) Chemical properties:
Both P and R have 18 electrons, resulting in the same electronic configuration (2, 8, 8). Because chemical properties are governed by valence electron configurations, both isotopes have essentially identical chemical properties.
Hypothetical Questions and Answers
1. Suppose J.J. Thomson’s Plum Pudding model had been completely accurate, with positive charge spread out uniformly throughout a spherical atom. If Rutherford had performed his alpha-particle scattering experiment on such atoms, what experimental observations would have been recorded?
Answer:
Almost all alpha particles would have passed straight through the foil with only negligible, microscopic deflections and essentially no particles would have bounced back at large angles or 180°.
Explanation:
In Thomson’s model, the positive charge is not concentrated at a central core. Consequently, the electric field would be too weak to cause large deflections of fast-moving, massive alpha particles (⁴₂He²⁺).
Without a dense, highly charged nucleus, no alpha particle would experience the concentrated repulsion required for wide-angle scattering > 90° or a 180° rebound. The particles would pass through the foil while experiencing only slight deflections caused by weak, diffuse interactions.
2. Imagine that Niels Bohr had never introduced the concept of discrete non-radiating orbits and classical electromagnetic theory (Maxwell’s theory) applied to orbiting electrons in an atom. What would happen to the stability and existence of atoms?
Answer:
The electrons would continuously radiate energy, spiral into the nucleus within roughly 10⁻⁸ s, causing all atoms and all matter in the universe to collapse instantly.
Explanation:
According to classical physics, an electron in circular orbit around the nucleus experiences centripetal acceleration. As an accelerated charged particle, it would continuously radiate energy in the form of electromagnetic radiation.
As energy is lost, the electron’s orbital radius would continuously decrease. The electron would follow an inward spiral path toward the positively charged nucleus. This would make stable atoms impossible and would prevent the existence of stable matter as we know it.
Bohr resolved this problem by postulating that electrons occupy discrete, non-radiating energy states.
3. Consider a hypothetical universe where neutrons are not neutral, but instead carry a +1 positive elementary charge while retaining their original mass. For a helium nucleus containing 2 protons and 2 such hypothetical neutrons:
(i) What would be the net nuclear charge?
(ii) How many electrons would be required to form an electrically neutral helium atom?
(iii) What would be the consequence on nuclear stability?
[Concept: Net Nuclear Charge: The sum of the charges of all constituent nucleons.
Atomic Neutrality: Net Positive Charge = Net Negative Charge.
Electrostatic Repulsion vs. Strong Force: Identical positive charges exert mutually repulsive electrostatic forces within the nucleus. ]
Answer:
(i) The net nuclear charge would be +4.
(ii) 4 electrons would be required to neutralize the atom.
(iii) Nuclear stability would decrease drastically due to extreme electrostatic repulsion among all four positively charged nucleons.
Explanation:
(i) Net nuclear charge:
Nuclear charge = (2 protons × +1) + (2 charged neutrons × +1) = +4
(ii) Number of electrons required:
To balance the +4 nuclear charge, the atom would require 4 negatively charged electrons.
Electronic configuration: K = 2, L = 2
(iii) Nuclear stability:
In real physics, neutral neutrons contribute to the strong nuclear force without adding electrostatic repulsion. If neutrons carried a positive charge, all four nucleons would mutually repel through Coulombic forces.
This additional electrostatic repulsion would make the nucleus much less stable and could lead to nuclear disintegration.
Assertion Reason Question Answer
1. Assertion (A): An atom as a whole is electrically neutral.
Reason (R): The total positive charge of protons inside the nucleus is exactly equal to the total negative charge of electrons outside the nucleus.
Choose the correct option:
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
[Concept: Electrical Neutrality: An electrically neutral atom has no net electric charge because its positive and negative charges balance each other.
Subatomic Charge Balance: The positive charge on a proton (+1.6 × 10⁻¹⁹ C) is equal in magnitude to the negative charge on an electron (-1.6 × 10⁻¹⁹ C).]
Answer: (i) Both A and R are true and R is the correct explanation of A.
Explanation:
In a neutral atom, the number of positively charged protons in the nucleus is exactly equal to the number of negatively charged electrons outside the nucleus. Since their charges are equal in magnitude and opposite in sign, they cancel each other, resulting in a net electric charge of zero. Therefore, both A and R are true and R correctly explains A.
2. Assertion (A): Isotopes of an element exhibit identical chemical properties.
Reason (R): Isotopes have the same number of neutrons in their nuclei.
Choose the correct option:
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
[Concept: Isotopes Definition: Isotopes are atoms of the same element that have the same atomic number (Z) but different mass numbers (A).
Chemical Reactivity: The chemical properties of an atom are determined mainly by its electronic configuration, especially the valence electrons, rather than by the number of neutrons.]
Answer: (iii) A is true, but R is false.
Explanation:
Assertion (A) is true because isotopes of the same element have the same number of protons and, in neutral atoms, the same number of electrons. Therefore, they have the same electronic configuration and similar chemical properties.
Reason (R) is false because isotopes have different numbers of neutrons. For example, ³⁵₁₇Cl has 18 neutrons, while ³⁷₁₇Cl has 20 neutrons.
3. Assertion (A): Rutherford’s alpha-particle scattering experiment led to the discovery of the atomic nucleus.
Reason (R): Almost all alpha particles were deflected by 180° when they struck the gold foil.
Choose the correct option:
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
[Concept: Rutherford’s Scattering Experiment: High-energy, positively charged alpha particles were directed at a very thin gold foil.
Experimental Observations: Most alpha particles passed straight through, a few were deflected through small angles and a very small fraction were deflected through large angles or rebounded.]
Answer: (iii) A is true, but R is false.
Explanation:
Assertion (A) is true because the scattering of alpha particles showed that the positive charge and most of the mass of an atom are concentrated in a very small central region called the nucleus.
Reason (R) is false because almost all alpha particles passed straight through the gold foil without deflection. Only a very small fraction were deflected through large angles, with an extremely small number rebounding at approximately 180°.
4. Assertion (A): Calcium (⁴⁰₂₀Ca) and Argon (⁴⁰₁₈Ar) are classified as isobars.
Reason (R): Calcium and Argon have different chemical properties and different atomic numbers, but share the same mass number.
Choose the correct option:
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
[Concept: Isobars Definition: Isobars are atoms of different elements that have different atomic numbers (Z) but the same mass number (A). Nucleon Count: Isobars have the same total number of nucleons (protons + neutrons) in their nuclei.]
Answer: (i) Both A and R are true and R is the correct explanation of A.
Explanation:
Calcium has Z = 20 and A = 40, while Argon has Z = 18 and A = 40. They are different elements because they have different atomic numbers. They also have different electronic configurations and chemical properties. However, both have the same mass number of 40. Therefore, they satisfy the definition of isobars and Reason (R) correctly explains Assertion (A).
5. Assertion (A): Electrons revolving in discrete orbits proposed by Bohr do not continuously radiate energy.
Reason (R): Bohr postulated that electrons occupy stationary, non-radiating energy shells around the nucleus.
Choose the correct option:
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
[Concept: Bohr’s Model of the Atom: Bohr improved Rutherford’s model by introducing discrete or quantized energy levels for electrons.
Stationary States: An electron in a permitted stationary orbit does not continuously emit or absorb electromagnetic radiation.]
Answer: (i) Both A and R are true and R is the correct explanation of A.
Explanation:
According to classical electromagnetic theory, an accelerating charged particle should continuously radiate energy. This would cause an orbiting electron to lose energy and eventually fall into the nucleus.
To explain atomic stability, Niels Bohr proposed that electrons occupy specific discrete orbits called stationary orbits. Electrons do not radiate energy while remaining in these permitted orbits. Therefore, both A and R are true and Reason (R) correctly explains Assertion (A).
6. Assertion (A): Noble gases like Helium, Neon and Argon have a valency of zero.
Reason (R): The outermost shell of noble gas atoms contains a completely filled, stable electron configuration.
Choose the correct option:
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
[Concept: Valency: Valency is the combining capacity of an atom, determined by the number of electrons it needs to lose, gain or share to attain a stable electron configuration.
Chemical Inertness: Atoms with a complete outermost shell have maximum stability and very little tendency to react.]
Answer: (i) Both A and R are true and R is the correct explanation of A.
Explanation:
Helium has a completely filled K-shell with 2 electrons (duplet), while Neon (2, 8) and Argon (2, 8, 8) have 8 electrons in their outermost shells (octet). Since their outermost shells are completely filled, they do not normally need to gain, lose or share electrons. Therefore, their valency is zero and Reason (R) correctly explains Assertion (A).
7. Assertion (A): The mass of an atom is practically equal to the sum of the masses of its protons and neutrons alone.
Reason (R): The mass of an electron is extremely small and considered negligible compared to the mass of a proton or neutron.
Choose the correct option:
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
[Concept: Relative Subatomic Masses: A proton and a neutron each have an approximate relative mass of 1 u.
Electron Mass: The mass of an electron is approximately 1/1836 of the mass of a proton.]
Answer: (i) Both A and R are true and R is the correct explanation of A.
Explanation:
An atom consists of protons, neutrons and electrons. However, an electron is much lighter than a proton or neutron, so its contribution to the total atomic mass is negligible for ordinary calculations.
Therefore, the mass of an atom is practically concentrated in its nucleus, which contains protons and neutrons. Thus, Reason (R) correctly explains Assertion (A).
8. Assertion (A): The M-shell of an atom can accommodate a maximum of 18 electrons, but Potassium (Z = 19) fills its fourth shell (N-shell) before completing the M-shell.
Reason (R): According to the Bohr-Bury scheme, the outermost shell of an atom cannot accommodate more than 8 electrons, regardless of its maximum capacity.
Choose the correct option:
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
[Concept: Maximum Shell Capacity (2n²): For n = 3 (M-shell), the maximum theoretical capacity is:
2n² = 2(3)² = 18 electrons
Outermost Shell Constraint: According to the simplified Bohr-Bury scheme used at this level, the outermost shell cannot contain more than 8 electrons.]
Answer: (i) Both A and R are true and R is the correct explanation of A.
Explanation:
Although the M-shell can theoretically accommodate 18 electrons, the outermost shell is limited to 8 electrons under the Bohr-Bury rules.
For Potassium (Z = 19), the electronic configuration is 2, 8, 8, 1. After 8 electrons occupy the M-shell, the 19th electron enters the N-shell instead of making the M-shell contain 9 electrons.
Therefore, Reason (R) correctly explains the electronic configuration stated in Assertion (A).
9. Assertion (A): The ordinary hydrogen atom (Protium, ¹₁H) has an atomic number equal to its mass number.
Reason (R): The nucleus of a Protium atom contains one proton and one neutron.
Choose the correct option:
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
[Concept: Atomic Number (Z): The atomic number is the number of protons in the nucleus. (Z = p⁺)
Mass Number (A): The mass number is the total number of protons and neutrons in the nucleus. (A = p⁺ + n⁰)
Protium Structure: Protium is the common hydrogen isotope whose nucleus contains one proton and no neutrons.]
Answer: (iii) A is true, but R is false.
Explanation:
For Protium (¹₁H), the atomic number is Z = 1 because it contains 1 proton. Its mass number is A = 1 because its nucleus contains 1 proton and 0 neutrons.
Therefore:
Z = 1
A = 1
Thus, Assertion (A) is true because the atomic number and mass number are both 1.
However, Reason (R) is false because the nucleus of Protium contains only 1 proton and 0 neutrons.
n⁰ = A – Z = 1 – 1 = 0
Therefore, the correct option is (iii).
Correct and Incorrect Question Answer
1. Which of the following statements are correct or incorrect regarding J.J. Thomson’s model of the atom and the discovery of subatomic particles? Give a reason for each statement.
(i) Thomson proposed that an atom consists of a positively charged sphere with electrons embedded in it.
(ii) The discovery of canal rays by E. Goldstein led directly to the identification of the neutron.
(iii) Thomson’s model successfully explained why alpha particles bounce back from a thin metal foil.
(iv) According to Thomson’s model, the negative and positive charges are equal in magnitude, making the atom electrically neutral.
[Concept: Thomson’s Plum Pudding Model: Thomson proposed that the atom was a uniformly positively charged sphere with negatively charged electrons embedded in it, making the atom electrically neutral.
Historical Timeline of Discoveries: Canal rays were associated with positively charged particles and contributed to the discovery of the proton. The neutron was discovered much later by James Chadwick in 1932.]
Answer: Statements (i) and (iv) are correct; statements (ii) and (iii) are incorrect.
Explanation:
(i) Correct: Thomson proposed that an atom is a sphere of positive charge in which negatively charged electrons are embedded.
(ii) Incorrect: Canal rays or anode rays, discovered by E. Goldstein consisted of positively charged particles and contributed to the discovery of the proton, not the neutron. The neutron was discovered by James Chadwick in 1932.
(iii) Incorrect: Thomson’s model could not explain the large-angle scattering or backward deflection of alpha particles observed in Rutherford’s experiment because it assumed that positive charge was spread uniformly throughout the atom.
(iv) Correct: According to Thomson’s model, the total positive charge was equal in magnitude to the total negative charge of the electrons. Therefore, the atom as a whole was electrically neutral.
2. Choose the correct and incorrect statements regarding the Bohr-Bury scheme of electron distribution and explain the reason for each.
(i) The maximum number of electrons that can be accommodated in the nth energy shell is given by the formula 2n².
(ii) The outermost valence shell of an atom can accommodate up to 18 electrons if space is available.
(iii) Electrons are accommodated in a given shell only after the inner shells are completely filled step-wise.
(iv) The maximum capacity of the M-shell is 8 electrons.
[Concept: Maximum Capacity Rule (2n²): The maximum theoretical capacity of a shell depends on its principal quantum number n.
Outermost Shell Rule: In the Bohr-Bury scheme, the outermost shell cannot accommodate more than 8 electrons.
Stepwise Filling: Electrons are distributed progressively from inner shells to outer shells.]
Answer: Statements (i) and (iii) are correct; statements (ii) and (iv) are incorrect.
Explanation:
(i) Correct: According to the Bohr-Bury scheme, the maximum number of electrons that a shell can accommodate is given by 2n².
For example:
K-shell (n = 1) = 2 electrons
L-shell (n = 2) = 8 electrons
M-shell (n = 3) = 18 electrons
N-shell (n = 4) = 32 electrons
(ii) Incorrect: The outermost shell cannot accommodate more than 8 electrons according to the Bohr-Bury scheme, even if its theoretical capacity calculated by 2n² is higher.
(iii) Correct: Electrons are filled progressively from the inner shells to the outer shells. A higher shell is occupied after the lower shell has been filled according to the applicable rules.
(iv) Incorrect: The M-shell has a theoretical maximum capacity of 18 electrons.
2n² = 2(3)² = 18 electrons
It can contain up to 8 electrons when it is the outermost shell under the simplified Bohr-Bury rule.
3. Which of the following statements are correct or incorrect regarding Valency and Chemical Reactivity? Provide a scientific reason for each.
(i) The valency of an element is always equal to the number of valence electrons in its outermost shell.
(ii) Noble gases have zero valency because their outermost shells already possess a stable octet or duplet in the case of Helium.
(iii) Non-metallic elements like Fluorine (Z = 9) and Oxygen (Z = 8) tend to lose electrons to satisfy their valency.
(iv) An atom with 7 valence electrons has a valency of 1.
[Concept: Valency Calculation: For atoms with 1 to 4 valence electrons, valency is generally equal to the number of valence electrons. For atoms with 5 to 7 valence electrons, valency is generally calculated as 8 minus the number of valence electrons.
Chemical Reactivity: Atoms tend to gain, lose or share electrons to achieve a stable electronic configuration.]
Answer: Statements (ii) and (iv) are correct; statements (i) and (iii) are incorrect.
Explanation:
(i) Incorrect: Valency is not always equal to the number of valence electrons. For atoms having more than 4 valence electrons, valency is generally calculated by subtracting the number of valence electrons from 8.
(ii) Correct: Noble gases have completely filled outermost shells. Helium has a stable duplet with 2 electrons, while Neon and Argon have stable octets. Therefore, they have valency zero.
(iii) Incorrect: Non-metals such as Fluorine and Oxygen generally tend to gain or share electrons rather than lose them. Fluorine needs 1 electron and Oxygen needs 2 electrons to complete their octets.
(iv) Correct: An atom with 7 valence electrons needs only 1 electron to complete its octet. Therefore, its valency is:
Valency = 8 – 7 = 1
4. Choose the correct and incorrect statements in the context of Isotopes and Isobars and explain the reason for each.
(i) Isotopes of an element have different atomic numbers but identical mass numbers.
(ii) ³⁵₁₇Cl and ³⁷₁₇Cl exhibit identical chemical properties because they have the same electronic configuration.
(iii) Isobars like ⁴⁰₁₈Ar and ⁴⁰₂₀Ca have identical chemical properties because their mass numbers are the same.
(iv) Radioactive isotopes such as Cobalt-60 and Iodine-131 find important applications in medicine and industry.
[Concept: Isotopes: Isotopes are atoms of the same element having the same atomic number (Z) but different mass numbers (A).
Isobars: Isobars are atoms of different elements having different atomic numbers (Z) but the same mass number (A).
Isotopic Applications: Some radioactive isotopes have important applications in medicine, industry and scientific research.]
Answer: Statements (ii) and (iv) are correct; statements (i) and (iii) are incorrect.
Explanation:
(i) Incorrect: Isotopes have the same atomic number but different mass numbers because they contain different numbers of neutrons.
(ii) Correct: ³⁵₁₇Cl and ³⁷₁₇Cl both have 17 electrons in their neutral state and the same electronic configuration, 2, 8, 7. Therefore, they have essentially the same chemical properties.
(iii) Incorrect: Chemical properties depend mainly on the electronic configuration and valence electrons, not on the mass number. Argon and Calcium have different atomic numbers and different electronic configurations, so their chemical properties are different.
(iv) Correct: Radioactive isotopes have several useful applications. Cobalt-60 is used in radiation therapy and industrial applications, while Iodine-131 is used mainly for diagnosis and treatment of thyroid disorders.
5. Which of the following statements are correct or incorrect regarding Subatomic Particles (Protons, Neutrons, Electrons)? Give a reason for each statement.
(i) The mass of a neutron is nearly equal to the mass of a proton and it carries a neutral (zero) charge.
(ii) A standard hydrogen atom (¹₁H) contains one proton, one electron and one neutron.
(iii) The total mass of an atom is concentrated inside the nucleus because the mass of electrons is negligible.
(iv) Protons and electrons are held together in the nucleus by the strong nuclear force.
[Concept: Subatomic Properties:
Protons: Positive charge (+1) and relative mass approximately 1 u.
Neutrons: No charge (0) and relative mass approximately 1 u.
Electrons: Negative charge (-1) and mass approximately 1/1840 of a proton.
Nuclear Composition: The nucleus contains protons and neutrons, while electrons are present outside the nucleus in shells or energy levels.]
Answer: Statements (i) and (iii) are correct; statements (ii) and (iv) are incorrect.
Explanation:
(i) Correct: A neutron has a mass nearly equal to that of a proton and carries no electric charge.
(ii) Incorrect: The ordinary hydrogen atom, Protium (¹₁H), contains 1 proton and 1 electron but no neutron.
Number of neutrons = A – Z = 1 – 1 = 0
(iii) Correct: The mass of an electron is extremely small compared with that of a proton or neutron. Therefore, almost all of the atom’s mass is concentrated in the nucleus, which contains protons and neutrons.
(iv) Incorrect: Electrons are not located inside the nucleus. They are present outside the nucleus in shells or energy levels. The strong nuclear force acts between nucleons, that is, protons and neutrons, within the nucleus.
Multiple Choice Questions (MCQs)
1. Which subatomic particle was discovered by J. J. Thomson?
A. Proton
B. Neutron
C. Electron
D. Nucleus
Correct Option: C. Electron
Answer: J. J. Thomson identified the electron through his studies of cathode rays.
2. Which model of the atom was proposed by Rutherford?
A. Plum pudding model
B. Nuclear model
C. Solid sphere model
D. Quantum mechanical model
Correct Option: B. Nuclear model
Answer: Rutherford proposed that the atom has a tiny, dense, positively charged nucleus at its centre, with electrons around it.
3. What did most alpha particles do when passed through the gold foil?
A. They bounced back.
B. They were completely absorbed.
C. They passed through without deflection.
D. They stopped at the nucleus.
Correct Option: C. They passed through without deflection.
Answer: This showed that most of the atom is empty space.
4. According to Bohr and Bury, what is the maximum number of electrons that can be accommodated in the M-shell?
A. 2
B. 8
C. 18
D. 32
Correct Option: C. 18
Answer: The maximum number of electrons in a shell is given by 2n². For the M-shell, n = 3, so the capacity is 18 electrons.
5. What is the atomic number of an element equal to?
A. Number of neutrons
B. Number of protons
C. Number of protons + neutrons
D. Number of electrons + neutrons
Correct Option: B. Number of protons
Answer: Atomic number is equal to the number of protons present in the nucleus.
6. Which of the following represents isotopes?
A. Atoms having the same mass number but different atomic numbers
B. Atoms having the same atomic number but different mass numbers
C. Atoms having different numbers of protons and neutrons
D. Atoms having the same number of neutrons
Correct Option: B. Atoms having the same atomic number but different mass numbers
Answer: Isotopes are atoms of the same element with the same atomic number but different mass numbers.
7. Which pair represents isobars?
A. ¹²C and ¹⁴C
B. ³⁵Cl and ³⁷Cl
C. ⁴⁰Ar and ⁴⁰Ca
D. ¹H and ²H
Correct Option: C. ⁴⁰Ar and ⁴⁰Ca
Answer: Isobars are atoms of different elements having the same mass number but different atomic numbers.
8. What is the valency of an atom with the electronic configuration 2, 8, 7?
A. 7
B. 8
C. 2
D. 1
Correct Option: D. 1
Answer: The atom needs one electron to complete its outer shell, so its valency is 1. The chapter defines valency as the combining capacity of an atom.
9. Which isotope is used in radiation treatment for cancer?
A. Uranium-235
B. Cobalt-60
C. Carbon-14
D. Iodine-131
Correct Option: B. Cobalt-60
Answer: Cobalt-60 is a radioactive isotope used in radiation treatment for cancer.
10. Which statement correctly describes the mass number of an atom?
A. It is the number of electrons in the atom.
B. It is the number of protons only.
C. It is the total number of protons and neutrons.
D. It is the total number of protons and electrons.
Correct Option: C. It is the total number of protons and neutrons.
Answer: Mass number is the total number of nucleons, that is, protons and neutrons, present in the nucleus.
Very Short Answer Questions
1. Why did Rutherford conclude that most of the atom is empty space?
Answer: Most alpha particles passed through the gold foil without any deflection. Rutherford concluded that most of the space inside an atom is empty.
2. Why could Rutherford’s atomic model not explain the stability of an atom?
Answer: According to the model, revolving electrons would continuously lose energy and spiral inward towards the nucleus. Therefore, Rutherford’s model could not explain why atoms remain stable.
3. What is the difference between atomic number and mass number?
Answer: Atomic number is the number of protons in the nucleus, whereas mass number is the total number of protons and neutrons in the nucleus.
4. Why do isotopes of an element have the same chemical properties?
Answer: Isotopes have the same atomic number and therefore the same number of electrons and similar electronic configuration. Since chemical properties mainly depend on valence electrons, their chemical properties are the same.
5. What is valency? How is it related to the electronic configuration of an atom?
Answer: Valency is the combining capacity of an atom. It depends on the number of electrons that an atom can gain, lose or share to achieve a stable electronic configuration.
6. Write the electronic configurations of magnesium, sulphur and argon.
Answer: Magnesium (Z = 12): 2, 8, 2; Sulphur (Z = 16): 2, 8, 6; Argon (Z = 18): 2, 8, 8.
7. What is valency? What is the valency of oxygen?
Answer: Valency is the combining capacity of an atom. Oxygen has the electronic configuration 2, 6 and needs two electrons to complete its octet. Therefore, its valency is 2.
8. Why are isotopes of the same element called isotopes?
Answer: Isotopes are atoms of the same element having the same atomic number but different mass numbers because they contain different numbers of neutrons.
9. What is the valency of carbon and hydrogen? How is methane (CH₄) formed?
Answer: Carbon has valency 4 and hydrogen has valency 1. Therefore, one carbon atom combines with four hydrogen atoms to form methane (CH₄).
10. Why is an atom electrically neutral as a whole?
Answer: An atom contains positively charged protons and negatively charged electrons in equal numbers. Since their charges are equal in magnitude and opposite in sign, they balance each other, making the atom electrically neutral.
11. What was the major drawback of Rutherford’s model of the atom?
Answer: According to classical electromagnetic theory, a revolving electron should continuously radiate energy. It would lose energy, spiral towards the nucleus and eventually collapse into it. Thus, Rutherford’s model could not explain the stability of atoms.
12. An atom of an element has Atomic Number (Z = 17) and Mass Number (A = 35). Write its electronic configuration and find its valency.
Answer: Electronic configuration: 2, 8, 7
Valency = 8 − 7 = 1
Therefore, the valency of the element is 1.
Frequently Asked Questions (FAQs)
1. Which naturally occurring isotope has no neutrons in its nucleus?
Answer: Protium (¹₁H), the standard isotope of hydrogen, contains 1 proton, 1 electron and 0 neutrons. It is the only known atomic species in the universe that lacks neutrons (n⁰ = A − Z = 1 − 1 = 0).
2. How do you differentiate between Isotopes and Isobars?
Answer: Isotopes: Atoms of the same element having identical atomic numbers (Z) but different mass numbers (A) due to differing neutron counts (e.g., ¹²₆C and ¹⁴₆C).
Isobars: Atoms of different chemical elements having different atomic numbers (Z) but the same mass number (A) (e.g., ⁴⁰₁₈Ar and ⁴⁰₂₀Ca).
3. How is the valency of an element determined from its electronic configuration?
Answer: Valency represents the combining capacity of an atom:
For atoms with 1 to 4 valence electrons: Valency = Number of valence electrons (e.g., Na: 2, 8, 1 → Valency 1).
For atoms with 5 to 8 valence electrons: Valency = 8 − Number of valence electrons (e.g., Cl: 2, 8, 7 → 8 − 7 = Valency 1).
4. Why do noble gases (Helium, Neon, Argon) have a valency of zero?
Answer: Noble gases possess completely filled outermost electron shells (a stable duplet of 2 electrons for Helium and a stable octet of 8 electrons for Neon and Argon). Because their valence shells are fully saturated, they have zero tendency to gain, lose or share electrons.
5. What are the key applications of radioactive isotopes in medicine and industry?
Answer: Cobalt-60 (⁶⁰Co): Used in radiotherapy for cancer treatment.
Iodine-131 (¹³¹I): Used in the diagnosis and treatment of goitre/thyroid gland disorders.
Uranium-235 (²³⁵U): Used as nuclear fuel in power reactors for electricity generation.
Carbon-14 (¹⁴C): Used in carbon dating to estimate the age of ancient fossils.
6. What is the 2n² rule in the Bohr-Bury scheme?
Answer: The 2n² rule defines the maximum number of electrons a particular energy level (shell n) can hold:
K-shell (n = 1): 2(1)² = 2 electrons
L-shell (n = 2): 2(2)² = 8 electrons
M-shell (n = 3): 2(3)² = 18 electrons
N-shell (n = 4): 2(4)² = 32 electrons
7. What are canal rays and who discovered them?
Answer: Canal rays (anode rays) are streams of positively charged subatomic particles that travel from the anode toward the cathode in a modified gas discharge tube. They were discovered by E. Goldstein in 1886, which subsequently led to the discovery of the proton.
8. Why is the entire mass of an atom concentrated in its nucleus?
Answer: The mass of an electron (≈ 1/1840 u) is negligible compared to the mass of a proton (≈ 1 u) or neutron (≈ 1 u). Because all protons and neutrons (nucleons) are packed tightly within the nucleus, practically the entire mass of the atom resides in the nucleus.
9. What was J.J. Thomson’s Plum Pudding model of the atom?
Answer: J.J. Thomson proposed that an atom is a sphere of positively charged matter in which negatively charged electrons are embedded like seeds in a watermelon or raisins (plums) in a pudding, with total positive charge balancing total negative charge.
10. If an atom has Z = 16 and A = 32, how many protons, neutrons and electrons are present?
Answer: Protons (p⁺): Z = 16
Electrons (e⁻): Z = 16 (in a neutral atom)
Neutrons (n⁰): A − Z = 32 − 16 = 16
The element is Sulphur (³²₁₆S), with an electronic configuration of 2, 8, 6 and a valency of 2 (8 − 6 = 2).
