🧪 Structure of the Atom – Class 9 Science MCQs (NCERT)

Section 1: Discovery of Subatomic Particles & Models of the Atom (Q1–Q25)

Q1. Who discovered the electron?

A. Goldstein
B. J.J. Thomson
C. Rutherford
D. Chadwick

✅ Correct Answer: B. J.J. Thomson

Explanation:

• A. Goldstein → Wrong, discovered canal rays (protons).
• B. J.J. Thomson → Correct, discovered electron using cathode ray tube.
• C. Rutherford → Wrong, discovered nucleus.
• D. Chadwick → Wrong, discovered neutron.

Q2. Who discovered the proton?

A. J.J. Thomson
B. Goldstein
C. Rutherford
D. Bohr

✅ Correct Answer: B. Goldstein

Explanation:

• A. Thomson → Electron, not proton.
• B. Goldstein → Correct, discovered protons (canal rays).
• C. Rutherford → Nucleus, not proton.
• D. Bohr → Atomic model, not discovery.

Q3. Who discovered the neutron?

A. Rutherford
B. J.J. Thomson
C. Chadwick
D. Bohr

✅ Correct Answer: C. Chadwick

Explanation:

• A. Rutherford → Nucleus, not neutron.
• B. Thomson → Electron.
• C. Chadwick → Correct, discovered neutron in 1932.
• D. Bohr → Atomic model.

Q4. The charge of an electron is:

A. +1 unit
B. –1 unit
C. 0
D. +2 units

✅ Correct Answer: B. –1 unit

Explanation:

• A. +1 → Wrong, this is proton charge.
• B. –1 → Correct, electron is negatively charged.
• C. 0 → Wrong, neutron has no charge.
• D. +2 → Wrong.

Q5. Mass of an electron is approximately:

A. Equal to proton
B. 1/1837 of proton
C. 1837 times proton
D. Negligible but not measurable

✅ Correct Answer: B. 1/1837 of proton

Explanation:

• A. Equal → Wrong, electron is much lighter.
• B. Correct, electron ≈ 1/1837 of proton mass.
• **C. Wrong, opposite.
• **D. Wrong, it is measurable.

Q6. J.J. Thomson proposed which model of atom?

A. Nuclear model
B. Plum pudding model
C. Planetary model
D. Modern model

✅ Correct Answer: B. Plum pudding model

Explanation:

• A. Nuclear model → Rutherford.
• **B. Correct, atom = positive sphere with electrons embedded.
• C. Planetary model → Bohr.
• D. Modern model → Quantum mechanical model.

Q7. According to J.J. Thomson’s model, an atom is:

A. Neutral with protons at center
B. Positive sphere with embedded electrons
C. Electrons revolving around nucleus
D. Neutrons and protons in center

✅ Correct Answer: B. Positive sphere with embedded electrons

Explanation:

• A. Protons at center → Wrong, Rutherford’s idea.
• **B. Correct, like a plum pudding.
• **C. Wrong, Bohr’s model.
• **D. Wrong, neutrons not discovered then.

Q8. Which experiment led to the discovery of the nucleus?

A. Cathode ray experiment
B. Canal ray experiment
C. Gold foil experiment
D. Oil drop experiment

✅ Correct Answer: C. Gold foil experiment

Explanation:

• A. Cathode ray → Electron.
• B. Canal ray → Proton.
• C. Gold foil experiment (Rutherford) → Correct, showed nucleus.
• **D. Oil drop (Millikan) → Measured charge of electron.

Q9. Which scientist performed the gold foil experiment?

A. J.J. Thomson
B. Rutherford
C. Bohr
D. Chadwick

✅ Correct Answer: B. Rutherford

Explanation:

• A. Thomson → Plum pudding model.
• B. Rutherford → Correct, gold foil α-particle scattering experiment.
• C. Bohr → Planetary model.
• D. Chadwick → Neutron.

Q10. Which observation of Rutherford’s experiment was most surprising?

A. Most α-particles passed through foil.
B. Some α-particles deflected.
C. A few α-particles bounced back.
D. Electrons moved in orbits.

✅ Correct Answer: C. A few α-particles bounced back.

Explanation:

• A. Most passed → Expected.
• B. Some deflected → Expected.
• **C. Correct, bounce-back proved dense positive nucleus.
• **D. Wrong, not part of Rutherford’s experiment.

Q11. Rutherford’s model failed because:

A. Could not explain isotopes
B. Could not explain stability of atom
C. Could not explain neutrons
D. Could not explain nucleus

✅ Correct Answer: B. Could not explain stability of atom

Explanation:

• A. Isotopes → Not the issue.
• **B. Correct, accelerating electrons should radiate energy → atom collapse.
• **C. Neutrons discovered later, not related.
• **D. Nucleus was explained well.

Q12. Which model explained stability of atom?

A. Thomson’s model
B. Rutherford’s model
C. Bohr’s model
D. Quantum mechanical model

✅ Correct Answer: C. Bohr’s model

Explanation:

• A. Thomson → Wrong.
• B. Rutherford → Failed stability.
• **C. Correct, Bohr’s postulates explained stability.
• **D. Quantum model came later.

Q13. Which scientist proposed that electrons revolve in discrete orbits?

A. Rutherford
B. J.J. Thomson
C. Bohr
D. Chadwick

✅ Correct Answer: C. Bohr

Explanation:

• A. Rutherford → Didn’t explain discrete orbits.
• B. Thomson → Plum pudding.
• C. Bohr → Correct, planetary model with fixed energy levels.
• D. Chadwick → Neutron discovery.

Q14. According to Bohr, electrons in an atom:

A. Revolve in fixed orbits without radiating energy
B. Continuously lose energy
C. Randomly move inside nucleus
D. Do not exist

✅ Correct Answer: A. Revolve in fixed orbits without radiating energy

Explanation:

• A. Correct, main postulate of Bohr.
• **B. Wrong, contradicts stability.
• **C. Wrong, electrons outside nucleus.
• **D. Wrong, electrons exist.

Q15. The maximum number of electrons in shell = 2n², where n =:

A. Shell number
B. Atomic number
C. Mass number
D. Neutron number

✅ Correct Answer: A. Shell number

Explanation:

• A. Correct, formula 2n² (n = shell number).
• **B. Atomic number → Wrong.
• **C. Mass number → Wrong.
• **D. Neutron number → Wrong.

Q16. Maximum electrons in L shell (n=2) are:

A. 2
B. 8
C. 18
D. 32

✅ Correct Answer: B. 8

Explanation:

• Formula = 2n² = 2×2² = 8.
• A. 2 → Wrong, K shell.
• **B. Correct.
• C. 18 → Wrong, M shell.
• D. 32 → Wrong, N shell.

Q17. The K shell (n=1) can hold maximum:

A. 1 electron
B. 2 electrons
C. 4 electrons
D. 8 electrons

✅ Correct Answer: B. 2 electrons

Explanation:

• Formula 2n² = 2×1² = 2.

Q18. Which is NOT a postulate of Bohr’s model?

A. Electrons revolve in fixed orbits.
B. Energy is emitted when electrons jump orbits.
C. Electrons lose energy continuously while revolving.
D. Orbits are associated with fixed energy.

✅ Correct Answer: C. Electrons lose energy continuously while revolving.

Explanation:

• A, B, D → Correct postulates.
• C. Wrong, Bohr rejected this idea.

Q19. Who gave the modern definition of atomic number?

A. Dalton
B. Rutherford
C. Moseley
D. Bohr

✅ Correct Answer: C. Moseley

Explanation:

• A. Dalton → Atomic theory.
• B. Rutherford → Nucleus.
• C. Moseley → Correct, defined Z = number of protons.
• D. Bohr → Energy levels.

Q20. Which subatomic particle determines the identity of an element?

A. Neutron
B. Proton
C. Electron
D. Nucleus

✅ Correct Answer: B. Proton

Explanation:

• A. Neutron → Isotopes differ in neutrons.
• B. Proton → Correct, Z determines element.
• C. Electron → Can vary in ions.
• D. Nucleus → Contains protons + neutrons, but protons fix identity.

Q21. An atom is neutral because:

A. Equal protons and electrons
B. Equal protons and neutrons
C. Equal neutrons and electrons
D. Equal nucleons and electrons

✅ Correct Answer: A. Equal protons and electrons

Explanation:

• A. Correct, positive = negative charges.
• **B. Protons ≠ neutrons always.
• **C. Wrong, no relation.
• **D. Wrong.

Q22. Mass of proton is approximately:

A. 1 u
B. 0 u
C. 1837 u
D. 0.0005 u

✅ Correct Answer: A. 1 u

Explanation:

• A. Correct, proton mass ≈ 1 u.
• **B. Wrong, not zero.
• **C. Wrong, 1837 relates to electron.
• **D. Wrong.

Q23. Which particle has no charge?

A. Proton
B. Electron
C. Neutron
D. Positron

✅ Correct Answer: C. Neutron

Explanation:

• A. Proton → + charge.
• B. Electron → – charge.
• C. Neutron → Correct, neutral.
• D. Positron → + charge.

Q24. Which statement is true about isotopes?

A. Same protons, different neutrons
B. Same neutrons, different protons
C. Same electrons, different protons
D. Same nucleons, different protons

✅ Correct Answer: A. Same protons, different neutrons

Explanation:

• **A. Correct, isotopes = same Z, different mass numbers.
• **B. Wrong, protons differ = different elements.
• **C. Wrong, identity changes.
• **D. Wrong, nucleons differ.

Q25. Which statement is true about isobars?

A. Same atomic number, different mass number
B. Same mass number, different atomic number
C. Same number of protons and neutrons
D. Same isotopes but different charges

✅ Correct Answer: B. Same mass number, different atomic number

Explanation:

• A. Same atomic number → Isotopes.
• **B. Correct, isobars have same A, different Z.
• **C. Wrong, nucleon distribution differs.
• **D. Wrong, ions are different.

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