NCERT Class 9 Science Chapter 8 Solutions – Structure of the Atom
Looking for accurate and complete ncert class 9 science chapter 8 solutions for the new CBSE curriculum (2026-27)? You are in the right place! In this guide, we provide step-by-step, direct, and easy-to-understand solutions for all exercise questions from NCERT Exploration Class 9 Science Chapter 8: Structure of the Atom.
These answers are crafted specifically to help students write clean, exam-ready answers in their notebooks without extra confusion or lengthy fluff.
Question 1
Choose the correct options and explain the reason for the correct and incorrect options in the context of Ernest Rutherford's gold foil experiment:
(i) The experiment clearly showed the existence of neutrons in the nucleus.
(ii) The results disproved the plum pudding model and led to the idea of a nucleus at the centre of the atom.
(iii) The large deflection of a few alpha particles indicated that most of the mass of the atom and positive charge are packed into a tiny centre.
(iv) The way alpha particles were deflected showed that electrons move around the nucleus.
Correct Options: (ii) and (iii)
- (i) Incorrect: Neutrons were discovered much later by James Chadwick in 1932, not by Rutherford's alpha-scattering experiment.
- (ii) Correct: Rebound of alpha particles proved that positive charge is concentrated in a tiny nucleus, disproving Thomson's uniform 'plum pudding' model.
- (iii) Correct: Since 1 in 12,000 alpha particles bounced straight back, it showed nearly all the atomic mass and positive charge reside in a tiny, dense core.
- (iv) Incorrect: Deflection of positive $\alpha$-particles occurred due to repulsion from the positively charged nucleus, not due to light electrons.
Question 2
Which of the following statements are correct or incorrect according to Bohr's atomic model? Give a reason for each statement.
(i) Electrons lose energy while moving in fixed orbits and slowly fall into the nucleus.
(ii) Electrons can exist anywhere around the nucleus with no fixed energy.
(iii) Electrons revolve around the nucleus in orbits of fixed energy without losing energy.
(iv) Electrons can be found between energy levels as they move around the nucleus.
- (i) Incorrect: Bohr postulated that electrons in discrete orbits (stationary states) do not radiate energy, keeping the atom stable.
- (ii) Incorrect: Electrons can only occupy specific discrete energy levels ($K, L, M, N$) with fixed energies.
- (iii) Correct: Revolving in permitted stationary orbits involves no energy loss by radiation.
- (iv) Incorrect: Energy levels are quantized; electrons cannot exist in the space between energy levels.
Question 3
The composition of the nuclei of three atomic species X, Y, and Z are given in the table. Explain the relation between: (i) Y and Z, (ii) Z and X.
| Species | Protons ($Z$) | Neutrons ($N$) | Mass Number ($A = Z + N$) |
|---|---|---|---|
| X | 18 | 19 | 37 |
| Y | 17 | 18 | 35 |
| Z | 17 | 20 | 37 |
- (i) Relation between Y and Z: They are Isotopes. Both have the same atomic number ($Z = 17$, Chlorine) but different mass numbers ($A_Y = 35$ and $A_Z = 37$).
- (ii) Relation between Z and X: They are Isobars. Both have the same mass number ($A = 37$) but different atomic numbers ($Z_X = 18$ and $Z_Z = 17$).
Question 4
What conclusion did Rutherford draw about the position and characteristics of the atom's positively charged part based on the few alpha particles that bounced back or were deflected at large angles?
Rutherford concluded that:
- Location & Volume: The positive charge and almost all the mass of the atom are concentrated in an extremely small volume at the centre, called the nucleus.
- Density & Magnitude: The nucleus is highly dense and carries a concentrated positive charge that exerts strong electrostatic repulsion on incoming positive $\alpha$-particles.
Question 5
Explain and arrange the following statements in the correct chronological order to show how atomic models have evolved over time.
Correct Chronological Sequence:
(iv) → (ii) → (iii) → (i)
- 1. Dalton's Model (iv): Described atoms as indivisible and indestructible hard spheres.
- 2. Thomson's Model (ii): Depicted the atom as a positive sphere with embedded negative electrons ('plum pudding').
- 3. Rutherford's Model (iii): Proposed a dense, positively charged central nucleus with orbiting electrons.
- 4. Bohr's Model (i): Refined the model by proposing quantized, non-radiating fixed energy orbits for electrons.
Question 6
Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.
Electrons do not fly away because of the strong electrostatic force of attraction between the positively charged protons in the nucleus and the negatively charged electrons.
This attractive force provides the necessary centripetal force required to keep the electrons continuously revolving in their discrete circular orbits around the nucleus.
Question 7
Assertion (A): The discovery of subatomic particles helped in understanding the atomic structure.
Reason (R): The number of electrons is equal to the number of protons in a neutral atom.
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.
Correct Option: (ii) Both A and R are true, but R is not the correct explanation of A.
Reasoning: Both statements are individually true. However, R describes electrical neutrality, whereas discovery of subatomic particles (electrons, protons, neutrons) helped understand internal distribution, orbits, and nuclear structure.
Question 8
For an atom of magnesium with a mass number of 24 and atomic number 12, determine the number of (i) protons, (ii) neutrons, (iii) electrons, and also illustrate the arrangement of electrons in a magnesium atom.
- (i) Protons: Equal to Atomic Number ($Z$) = 12
- (ii) Neutrons: $\text{Mass Number } (A) - \text{Atomic Number } (Z) = 24 - 12 = \mathbf{12}$
- (iii) Electrons: Equal to Protons in neutral atom = 12
- Electronic Configuration: $K = 2, L = 8, M = 2$ (Written as **2, 8, 2**)
Question 9
Find the required information for the elements shown in Fig. 8.17 (a, b, c, d):
| Property | (a) Lithium | (b) Carbon | (c) Chlorine | (d) Oxygen |
|---|---|---|---|---|
| (i) Name of Element | Lithium | Carbon | Chlorine | Oxygen |
| (ii) Symbol | $\text{Li}$ | $\text{C}$ | $\text{Cl}$ | $\text{O}$ |
| (iii) Total Electrons | 3 | 6 | 17 | 8 |
| (iv) Valence Electrons | 1 | 4 | 7 | 6 |
| (v) Valency | 1 | 4 | $8 - 7 = \mathbf{1}$ | $8 - 6 = \mathbf{2}$ |
| (vi) Number of Protons | 3 | 6 | 17 | 8 |
| (vii) Atomic Number | 3 | 6 | 17 | 8 |
Question 10
Both Rutherford's and Bohr's models have electrons orbiting the nucleus. Why did Rutherford's model fail to explain atomic stability, while Bohr's model succeeded?
- Failure of Rutherford's Model: According to classical electromagnetic theory, an accelerating charged particle (revolving electron) must continuously radiate energy. As a result, the orbiting electron would lose energy, spiral inwards, and collapse into the nucleus in $< 10^{-8}\text{ seconds}$, making atoms unstable.
- Success of Bohr's Model: Bohr overcome this by postulating that electrons revolve only in specific non-radiating discrete orbits (stationary states). While revolving in these permitted orbits, electrons do not radiate energy, explaining why atoms are stable.
Question 11
An atom $^{70}\text{X}$ has 31 electrons. How many neutrons are there in its nucleus?
- Mass number ($A$) = $70$
- Number of electrons = $31 \implies \text{Atomic Number } (Z) = 31$
- $$\text{Number of Neutrons } (N) = A - Z = 70 - 31 = \mathbf{39}$$
Question 12
An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.
- (i) Number of Neutrons: $$N = \text{Mass Number } (A) - \text{Protons } (Z) = 197 - 79 = \mathbf{118}$$
- (ii) Number of Electrons: In a neutral atom, electrons = protons = 79.
Question 13
Complete Table 8.5:
Completed Table 8.5:
| Atomic number | Mass number | Number of neutrons | Number of protons | Number of electrons | Name of the element |
|---|---|---|---|---|---|
| 5 | 11 | 6 | 5 | 5 | Boron |
| 7 | 14 | 7 | 7 | 7 | Nitrogen |
| 12 | 24 | 12 | 12 | 12 | Magnesium |
| 15 | 31 | 16 | 15 | 15 | Phosphorus |
| 1 | 1 | 0 | 1 | 1 | Hydrogen |
Question 14
An element X has a mass number of 35 and contains 18 neutrons. Answer the sub-questions:
- (i) Electrons & Protons: $\text{Protons} = A - N = 35 - 18 = \mathbf{17}$. Therefore, $\text{Electrons} = \mathbf{17}$.
- (ii) Atomic Number: Equal to number of protons = $\mathbf{17}$.
- (iii) Identify Element X: Chlorine ($\text{Cl}$).
- (iv) Electronic Configuration: $K=2, L=8, M=7$ (Written as **2, 8, 7**).
- (v) Valence Electrons: Outer shell ($M$) contains **7** electrons.
- (vi) New Mass Number (with 2 added neutrons): $35 + 2 = \mathbf{37}$.
- (vii) Relation with new atom: Element X ($^{35}\text{Cl}$) and the new atom ($^{37}\text{Cl}$) are Isotopes of Chlorine.
Question 15
In an atom, there are 12 protons and 12 neutrons in the nucleus. Imagine that all electrons are replaced with hypothetical particles having the same charge as electrons but 500 times heavier. What effect will this have on: (i) Atomic number, (ii) Atomic mass, (iii) Mass number, (iv) Overall charge?
- (i) Atomic number: Unchanged (12). Atomic number is strictly determined by the number of protons in the nucleus.
- (ii) Atomic mass: Slightly increases. Standard electron mass ($\approx \frac{1}{1836}\text{ u}$) is negligible, but 12 particles 500 times heavier add $\approx 3.26\text{ u}$ extra mass ($24\text{ u} \rightarrow \approx 27.26\text{ u}$).
- (iii) Mass number: Unchanged (24). Mass number is defined strictly as the integer sum of nuclear nucleons ($\text{Protons} + \text{Neutrons} = 12 + 12 = 24$).
- (iv) Overall charge: Unchanged (0 / Neutral). Since the hypothetical particles have the exact same negative charge magnitude as electrons, 12 positive charges cancel 12 negative charges.
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