Atomic Structure – NDA Chemistry PYQs

Q1. Which one of the following particles in the nucleus of an atom was discovered by J. Chadwick?
(a) Electron (b) Proton (c) Positron (d) Neutron

Answer: (d) Neutron
James Chadwick discovered the neutron in 1932 by bombarding beryllium with alpha particles. The neutron has no charge and a mass of approximately 1 u. It sits inside the nucleus alongside protons.
[NDA 2025-I]

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Q2. Which one of the following findings is NOT observed in Rutherford’s Nuclear Model of Atom?
(a) There is a neutral centre in an atom called nucleus
(b) Nearly all the mass of an atom resides in the nucleus
(c) Electrons revolve around the nucleus in a circular path of fixed energy called orbits
(d) The size of a nucleus is very small as compared to the size of atom

Answer: (a) and (c) — but the key answer is (a) and (c)
Option (a) is wrong because the nucleus is POSITIVELY charged — not neutral. Option (c) is also wrong because fixed energy orbits were proposed by BOHR — not Rutherford. NDA tests option (c) most often as the NOT-observed finding from Rutherford’s model.
[NDA 2021-I, NDA 2025-I]

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Q3. Which one of the following statements is correct?
(a) A neutron is formed by combination of an electron and a proton — therefore it is neutral
(b) The mass of an electron is about 1/2000 times that of a proton
(c) An isotope of cobalt is used in the treatment of goiter
(d) J. J. Thomson proposed that the nucleus of an atom contains only neutrons

Answer: None of the above are correct — all four are false statements. This is a NOT-correct type question.
(a) False — neutron is NOT formed by proton + electron. It is a separate particle.
(b) False — electron mass is 1/1836 of proton, NOT 1/2000.
(c) False — Co-60 treats cancer, NOT goitre. Goitre is iodine deficiency, treated by iodised salt.
(d) False — Thomson proposed the plum pudding model. He said nothing about neutrons. The nucleus containing neutrons was Chadwick’s discovery.
[NDA 2025-I]

Q4. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of?
(a) Electron (b) Proton (c) Nucleus (d) Neutron

Answer: (c) Nucleus
Rutherford fired alpha particles at a thin gold foil. Most passed through, but a few bounced back at large angles. This showed that most of the atom is empty space and almost all the mass is concentrated in a tiny, dense, positively charged centre — the nucleus.
[NDA 2017-I, NDA 2021-II]

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Q5. Which one of the following conclusions could NOT be derived from Rutherford’s α-particle scattering experiment?
(a) Most of the space in the atom is empty
(b) The radius of the atom is about 10⁵ times the radius of the nucleus
(c) Electrons move in a circular path of fixed energy called orbits
(d) Nearly all the mass of the atom resides in the nucleus

Answer: (c) Electrons move in a circular path of fixed energy called orbits
This was Bohr’s conclusion — not Rutherford’s. Rutherford’s experiment told us about the nucleus. It did not say anything about how electrons are arranged or how they move. Bohr added that idea in 1913.
[NDA 2021-I]

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Q6. What is the maximum number of electrons in the M-Shell?
(a) 6 (b) 8 (c) 18 (d) 32

Answer: (c) 18
The M shell is the third shell (n=3). Maximum electrons = 2n² = 2×9 = 18. K shell = 2, L shell = 8, M shell = 18, N shell = 32.
[NDA 2021-II]

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Q7. The number of neutrons in ₁₃Al²⁷ is?
(a) 40 (b) 27 (c) 14 (d) 13

Answer: (c) 14
Neutrons = Mass number − Atomic number = 27 − 13 = 14. The atomic number 13 tells us there are 13 protons. The mass number 27 tells us there are 27 nucleons (protons + neutrons) in total. So neutrons = 27 − 13 = 14.
[NDA 2010-I]

Q8. Which of the statements with regard to Isotopes and Isobars is/are correct?

1. Isotopes have same mass number.
2. Isobars have same atomic number.
   (a) 1 only (b) 2 only (c) Both 1 and 2 (d) Neither 1 nor 2

Answer: (d) Neither 1 nor 2
Both statements are false. Isotopes have the SAME ATOMIC NUMBER but different mass numbers. Isobars have the SAME MASS NUMBER but different atomic numbers. The question swaps the definitions — both are therefore wrong.
[NDA 2008-II]

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Q9. Which of the following pairs is/are correctly matched?

1. Isotopes: Atoms with same atomic number but different atomic mass
2. Isobars: Atoms with same number of neutrons but different atomic number
3. Isotones: Atoms with same mass number but different atomic number

Answer: (b) 1 only
Statement 1 is correct — isotopes have same atomic number (Z) but different mass number (A).
Statement 2 is wrong — that is the definition of isotones, not isobars. Isobars have same mass number.
Statement 3 is wrong — that is the definition of isobars, not isotones. Isotones have same number of neutrons.
[NDA 2012-I]

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Q10. The number of protons in a negatively charged atom (anion) is?
(a) More than the atomic number of the element
(b) Less than the atomic number of the element
(c) More than the number of electrons in the atom
(d) Less than the number of electrons in the atom

Answer: (d) Less than the number of electrons in the atom
An anion has gained extra electrons. So it has more electrons than protons. The number of protons equals the atomic number and does NOT change when an ion forms. Only electrons change.
[NDA 2011-II]

Q11. Atomic mass of an element is equal to the sum of number of?
(a) Electrons and protons only
(b) Protons and neutrons only
(c) Electrons, protons and neutrons
(d) Electrons and neutrons only

Answer: (b) Protons and neutrons only
Atomic mass = mass number = number of protons + number of neutrons. Electrons have negligible mass — they contribute almost nothing to the atomic mass. So electrons are not counted.
[NDA 2020]

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Q12. An element A has valencies equal to 3 and 5. It combines with another element B having valency equal to 2. What are the formulae of the compounds thus formed?
(a) A₃B₃ and A₂B₅ (b) A₂B₃ and A₂B₂ (c) A₂B₃ and A₂B₅ (d) A₂B₃ and A₃B₅

Answer: (c) A₂B₃ and A₂B₅
When A has valency 3 and B has valency 2: cross multiply → A₂B₃.
When A has valency 5 and B has valency 2: cross multiply → A₂B₅.
The rule is: valency of A becomes subscript of B, and valency of B becomes subscript of A.
[NDA 2007-I]

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Q13. In which one of the following is the valence electronic configuration ns²np³ found?
(a) Carbon (b) Oxygen (c) Nitrogen (d) Argon

Answer: (c) Nitrogen
ns²np³ means 2 electrons in s-orbital and 3 electrons in p-orbital of the outermost shell. Total valence electrons = 2 + 3 = 5. Nitrogen has atomic number 7. Electronic configuration: 2, 5 — so 5 valence electrons. Carbon has 4 valence electrons. Oxygen has 6. Argon has 8.
[NDA 2008-I]

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Q14. An element has electronic configuration 2, 8, 2. It will most likely be in the same group of the periodic table as?
(a) Na (b) Al (c) Mg (d) K

Answer: (c) Mg
An element with configuration 2, 8, 2 has 2 valence electrons. Elements in the same group have the same number of valence electrons. Magnesium (Mg) has atomic number 12 and configuration 2, 8, 2 — also 2 valence electrons. So they are in the same group.
[NDA 2025-I]