Structure of Atom PYQs – Chemistry, SSC & Railway

Correct Answer: Option 2 — In the nucleus
Ernest Rutherford (1911) bombarded a thin gold foil with fast-moving alpha (α) particles and observed that (i) most particles passed straight through, (ii) a few were deflected through small angles, and (iii) about 1 in 12,000 bounced straight back.
From these observations he concluded that an atom is mostly empty space, that all of its positive charge and almost its entire mass are packed into an extremely small central region which he named the nucleus, and that the electrons revolve around the nucleus in circular paths called orbits.
The nucleus is about 10⁻¹⁵ m across while the atom is about 10⁻¹⁰ m across — the atom is roughly 100,000 times bigger than its nucleus, and yet the nucleus carries nearly all the mass, because protons and neutrons are about 1836 times heavier than an electron.
❌ Why the other options are incorrect
✗ In the shells of electrons — an electron's mass is only about 1/1836 u, so all the shells together add almost nothing to the atom's mass.
✗ In the electron cloud — the "electron cloud" is a feature of the later quantum mechanical model, and electrons carry negligible mass in any case.
✗ It is equally distributed throughout — this was J. J. Thomson's plum-pudding (watermelon) model, which Rutherford's α-scattering experiment disproved.
📌 SSC & Railway Exam Facts
• Rutherford's gold foil (α-scattering) experiment discovered the nucleus of the atom.
• Rutherford is called the Father of Nuclear Physics; he received the Nobel Prize in Chemistry, 1908.
• Electron — J. J. Thomson, Proton — Goldstein/Rutherford, Neutron — James Chadwick (1932).
• Main defect of Rutherford's model: a revolving electron should radiate energy and spiral into the nucleus — corrected by Niels Bohr.
• Mass of a proton ≈ mass of a neutron ≈ 1 u; mass number = protons + neutrons.

Correct Answer: Option 2 — 3
Isotopes are atoms of the same element having the same atomic number (Z) but different mass numbers (A) — they differ only in the number of neutrons. Hydrogen is the classic example and has three naturally occurring isotopes:
• Protium (₁H¹) — 1 proton, 0 neutrons; abundance about 99.98%; the only atom in nature with no neutron.
• Deuterium (₁H², D) — 1 proton, 1 neutron; it forms heavy water, D₂O, used as a moderator in nuclear reactors.
• Tritium (₁H³, T) — 1 proton, 2 neutrons; it is radioactive and occurs only in traces.
All three show the same chemical properties because each has the same electronic configuration (1 electron); only their physical properties (mass, density, boiling point) differ.
❌ Why the other options are incorrect
✗ 5 — hydrogen isotopes heavier than tritium (H-4 to H-7) have been made artificially and live for less than a fraction of a second; they do not exist in nature.
✗ 2 — this counts only protium and deuterium and leaves out tritium, which does occur naturally in trace amounts.
✗ 1 — 1 is the atomic number of hydrogen (its number of protons and electrons), not the number of its isotopes.
📌 SSC & Railway Exam Facts
• Isotopes — same Z, different A (₆C¹² and ₆C¹⁴; ₁₇Cl³⁵ and ₁₇Cl³⁷).
• Isobars — same mass number, different atomic number (₂₀Ca⁴⁰ and ₁₈Ar⁴⁰).
• Heavy water (D₂O) is a moderator in nuclear reactors; C-14 is used for carbon dating.
• U-235 is used in nuclear reactors, Co-60 in cancer therapy, I-131 for thyroid/goitre.
• Average atomic mass of chlorine is 35.5 u because Cl-35 and Cl-37 occur in a 3 : 1 ratio.

Correct Answer: Option 1 — Niels Bohr
Niels Bohr (1913) proposed the planetary model of the atom, in which electrons revolve around the nucleus much as the planets revolve around the Sun, but only in certain fixed circular paths called orbits, shells or energy levels — K, L, M, N…
His main postulates were:
• Electrons revolve only in discrete (quantised) energy levels, and while in such an orbit they do not radiate energy — this makes the atom stable.
• Each shell has a definite energy; energy is absorbed when an electron jumps to a higher shell and emitted as light when it falls to a lower one.
• The maximum number of electrons a shell can hold is 2n² — K = 2, L = 8, M = 18, N = 32.
This removed the biggest flaw of Rutherford's model, which could not explain why a revolving electron does not lose energy and spiral into the nucleus.
❌ Why the other options are incorrect
✗ Ernest Rutherford — he gave the nuclear model after the α-scattering experiment; his electrons had no fixed energy levels, which left his atom unstable.
✗ J.J. Thomson (Joseph John Thomson) — he discovered the electron and gave the plum-pudding (watermelon) model, which had no nucleus and no orbits at all.
✗ Dmitri Mendeleev — a periodic table chemist, not an atomic-model physicist; he arranged the elements in order of increasing atomic mass.
📌 SSC & Railway Exam Facts
• Bohr won the Nobel Prize in Physics, 1922; the Bohr–Bury scheme governs the filling of shells.
• Maximum electrons in a shell = 2n²; the outermost shell can never hold more than 8 electrons.
• Atomic models in order: Dalton → Thomson → Rutherford → Bohr → quantum (Schrödinger).
• Valence electrons are the electrons of the outermost shell; they decide valency and chemical behaviour.
• Electron — J.J. Thomson (1897); proton — Goldstein; neutron — Chadwick (1932).

Correct Answer: Option 3 — E Rutherford
Ernest Rutherford discovered the nucleus in 1911 through his famous alpha-particle scattering (gold foil) experiment. He directed a beam of fast α-particles (He²⁺) at a very thin gold foil and recorded where they landed on a zinc sulphide screen.
His observations and conclusions were:
• Most α-particles passed straight through → the atom is mostly empty space.
• A few were deflected through small angles → there is a concentrated positive charge inside the atom.
• About 1 in 12,000 rebounded → all the positive charge and nearly all the mass are packed into a tiny central body, which he named the nucleus.
He also concluded that the electrons revolve around the nucleus, and that the nucleus is about 10⁻¹⁵ m across — roughly one hundred-thousandth of the size of the whole atom.
❌ Why the other options are incorrect
✗ J Chadwick — James Chadwick discovered the neutron (1932), the neutral particle inside the nucleus, not the nucleus itself.
✗ JJ Thomson — he discovered the electron (1897) using a cathode ray tube and gave the plum-pudding model, which had no nucleus at all.
✗ Neils Bohr — Niels Bohr improved Rutherford's model by introducing fixed energy levels (orbits); he did not discover the nucleus.
📌 SSC & Railway Exam Facts
• Rutherford — nucleus (1911); Thomson — electron (1897); Chadwick — neutron (1932); Goldstein — proton (canal rays).
• Rutherford is called the Father of Nuclear Physics; Nobel Prize in Chemistry, 1908.
• The α-particle Rutherford used is a helium nucleus, He²⁺.
• Nucleus ≈ 10⁻¹⁵ m; atom ≈ 10⁻¹⁰ m — the atom is about 10⁵ times larger than its nucleus.
• Protons and neutrons together are called nucleons; mass number = number of nucleons.