Science MCQ, Class 9th, Chapter-4, Structure of The Atom

This article covers NCERT based MCQ of Class 9th, Chapter-4, Structure of The Atom with brief explanation for PRT, TGT, PGT exams like HTET, KVS, NVS, SSC and all other state competition exams. 

Along with MCQ, Chapter Key-Points for quick learning NCERT related facts are also discussed below. 

Right Answer for MCQ will be highlighted on Hover / Click on the option. 

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Summary of the Chapter-4-Structure of The Atom with Key-Points:

1. J.J. Thomson is credited with the discovery of the electron, while E. Goldstein is credited with the discovery of the proton.  

2. Thomson suggested that electrons are embedded within a positively charged sphere.  

3. Rutherford's alpha-particle scattering experiment led to the identification of the atomic nucleus.  

4. In Rutherford's atomic model, electrons orbit a small nucleus, but this model could not explain atomic stability.  

5. Niels Bohr's atomic model improved on this by suggesting that electrons occupy distinct energy levels or shells around the nucleus. Atoms are more stable and less reactive when these shells are filled.  

6. J. Chadwick discovered neutrons, which, along with electrons and protons, make up the three subatomic particles. Electrons carry a negative charge, protons a positive charge, and neutrons have no charge. An electron's mass is roughly 1/2000th that of a hydrogen atom, while protons and neutrons each have a mass of one unit.  

7. Atomic shells are named K, L, M, N, and so on.  

8. Valency refers to an atom's ability to combine with other atoms.  

9. The atomic number of an element equals the number of protons in its nucleus.  

10. An atom's mass number is the total count of nucleons (protons and neutrons) in its nucleus.  

11. Isotopes are variants of the same element with different mass numbers.  

12. Isobars are atoms with the same mass number but different atomic numbers.  

13. An element's identity is determined by the number of protons in its nucleus.

NCERT Based MCQ With Explanation:

1. Who discovered the electron?

(a) E. Goldstein

(b) J.J. Thomson

(c) Ernest Rutherford

(d) Neils Bohr

Explanation: J.J. Thomson discovered the electron through experiments with cathode rays in 1897. His discovery showed that atoms are not indivisible as previously thought, leading to the understanding that atoms contain subatomic particles like electrons.

2. Which experiment led to the discovery of the atomic nucleus?

(a) Cathode ray tube experiment

(b) Oil drop experiment

(c) Alpha particle scattering experiment

(d) Rutherford’s gold foil experiment

Explanation: Rutherford's gold foil experiment in 1909 led to the discovery of the nucleus. Alpha particles were directed at a thin sheet of gold foil, and while most passed through, some were deflected, indicating the presence of a dense, positively charged nucleus at the center of the atom.

3. According to Bohr's model, why do electrons not radiate energy while revolving in discrete orbits?

(a) They are stationary

(b) They are not charged

(c) They occupy specific energy levels

(d) They are in constant motion

Explanation: Bohr's model introduced the idea that electrons revolve in discrete orbits or energy levels and do not radiate energy while in these orbits. Electrons only emit or absorb energy when they jump between these levels.

4. What was the major limitation of Rutherford's model of the atom?

(a) It could not explain the stability of the atom

(b) It did not account for protons

(c) It did not include neutrons

(d) It incorrectly identified electrons as stationary

Explanation: Rutherford's model suggested that electrons orbit the nucleus, but it failed to explain why the atom does not collapse. According to classical physics, the revolving electrons should radiate energy, spiral inward, and collapse into the nucleus.

5. Which of the following particles has no charge?

(a) Proton

(b) Electron

(c) Neutron

(d) Alpha particle

Explanation: Neutrons are subatomic particles with no charge, located in the nucleus alongside protons. J. Chadwick discovered the neutron in 1932, which helped explain atomic mass.

6. How many electrons can the K-shell hold?

(a) 2

(b) 8

(c) 18

(d) 32

Explanation: According to Bohr-Bury rules, the maximum number of electrons that can occupy a given shell is determined by the formula  2n² , where  n  is the shell number. For the K-shell (n = 1), the maximum number of electrons is  2 ^* 1² = 2 .

7. Which of the following is a correct pair of isotopes?

(a)  ₆C¹²  and  ₆C¹⁴

(b)  ₈O¹⁶  and  ₇N¹⁴

(c)  ₁₈Ar⁴⁰  and  ₂₀Ca⁴⁰

(d)  ₁H¹  and  ₂He²

Explanation: Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Both ₆C¹² and  ₆C¹⁴ are isotopes of carbon, differing in the number of neutrons.

8. Which scientist proposed the plum pudding model of the atom?

(a) Ernest Rutherford

(b) Neils Bohr

(c) J.J. Thomson

(d) E. Goldstein

Explanation: J.J. Thomson proposed the "plum pudding" model in which electrons were scattered like plums within a positively charged "pudding." This model was later disproved by Rutherford’s experiment.

9. In an atom of oxygen, if the atomic number is 8 and the mass number is 16, how many neutrons are present?

(a) 6

(b) 8

(c) 10

(d) 16

Explanation: The number of neutrons in an atom is calculated by subtracting the atomic number from the mass number. For oxygen, neutrons = 16 (mass number) – 8 (atomic number) = 8.

10. What is the valency of sodium (Na) with atomic number 11?

(a) 1

(b) 2

(c) 7

(d) 8

Explanation: Sodium has one electron in its outermost shell (3rd shell). To achieve a stable electronic configuration, it tends to lose this electron, giving it a valency of 1.

11. Who discovered the proton?

(a) J.J. Thomson

(b) Ernest Rutherford

(c) Niels Bohr

(d) James Chadwick

Explanation: Rutherford discovered the proton in 1917. He found that the positive charge in an atom is concentrated in the nucleus and is due to the presence of protons.

12. What did Rutherford’s gold foil experiment show about the structure of the atom?

(a) The atom is solid

(b) The atom has a dense nucleus

(c) Electrons are embedded in a positive sphere

(d) Neutrons are present in the nucleus

Explanation: Rutherford's experiment showed that most of the atom is empty space, with a small, dense, positively charged nucleus at the center, where most of the atom’s mass is concentrated.

13. What is the charge of a proton?

(a) -1

(b) +1

(c) 0

(d) +2

Explanation: Protons carry a positive charge (+1), and they reside in the nucleus of an atom.

14. Which subatomic particle was discovered by J.J. Thomson?

(a) Neutron

(b) Proton

(c) Electron

(d) Photon

Explanation: J.J. Thomson discovered the electron, a negatively charged subatomic particle, using cathode rays.

15. Which element has an atomic number of 6?

(a) Oxygen

(b) Nitrogen

(c) Carbon

(d) Helium

Explanation: The atomic number of an element is equal to the number of protons in its nucleus. Carbon has 6 protons, giving it an atomic number of 6.

16. What is the mass of a neutron?

(a) Same as a proton

(b) Twice the mass of a proton

(c) Half the mass of a proton

(d) Negligible

Explanation: Neutrons and protons have nearly the same mass. Both are approximately 1 atomic mass unit (amu).

17. Which of the following is NOT a fundamental particle in an atom?

(a) Proton

(b) Neutron

(c) Electron

(d) Positron

Explanation: The fundamental particles of an atom are protons, neutrons, and electrons. A positron is the antiparticle of an electron, not a fundamental component of atoms.

18. Which of the following models suggested that atoms consist of electrons embedded in a positively charged sphere?

(a) Bohr’s model

(b) Rutherford’s model

(c) Thomson’s model

(d) Quantum model

Explanation: J.J. Thomson's plum pudding model suggested that electrons were scattered within a positively charged sphere, similar to seeds in a watermelon.

19. What charge does an electron carry?

(a) Neutral

(b) Positive

(c) Negative

(d) Variable

Explanation: Electrons are negatively charged particles found outside the nucleus in various energy levels.

20. According to Bohr’s model, what prevents the electrons from spiraling into the nucleus?

(a) Their positive charge

(b) The gravitational force

(c) They occupy specific energy levels

(d) The strong nuclear force

Explanation: In Bohr’s model, electrons revolve in fixed orbits or energy levels without losing energy, preventing them from spiraling into the nucleus.