40. Which one of the following is a radioactive isotope of carbon?
[SSC JE Mechanical — 6 Dec 2025, Shift 2]
(A) Carbon-12
(B) Carbon-13
(C) Carbon-14
(D) Carbon-6
Answer: (C) Carbon-14
Explanation: Carbon-14 contains six protons and eight neutrons. Unlike stable carbon-12 and carbon-13, it undergoes radioactive beta decay with a half-life of roughly 5,730 years. Its predictable decay underpins radiocarbon dating of once-living materials.
39. Which radioactive decay increases the atomic number of the parent nucleus by one without changing its mass number?
[SSC CGL — 24 Sept 2025, Shift 3]
(A) Alpha decay
(B) Beta-minus decay
(C) Beta-plus decay
(D) Gamma decay
Answer: (B) Beta-minus decay
Explanation: During beta-minus decay, a neutron converts into a proton, emitting an electron and an antineutrino. Proton count therefore rises by one, while the combined number of protons and neutrons stays constant. The daughter nucleus belongs to a different element.
38. In Bohr’s model of hydrogen, as electrons move to higher energy levels (larger n), how does the difference between successive energy levels change?
[SSC CGL — 22 Sept 2025, Shift 1]
(A) It increases continuously.
(B) It decreases continuously.
(C) It remains constant.
(D) It first increases, then decreases.
Answer: (B) It decreases continuously.
Explanation: The allowed hydrogen energies follow Eₙ = −13.6/n² electronvolts. Increasing n moves each level closer to zero, but the separation between consecutive levels becomes smaller. Thus transitions between neighbouring high-n states release lower-energy photons than transitions between nearby low-n states.
37. A major limitation of Rutherford’s atomic model was its inability to explain which of the following?
[SSC Selection Post (Higher Secondary) — 29 Aug 2025, Shift 2]
(A) The existence of isotopes
(B) The deflection of alpha particles
(C) The stability of the atom and the atomic spectrum
(D) The indivisible nature of atoms
Answer: (C) The stability of the atom and the atomic spectrum
Explanation: Under classical electromagnetic theory, an orbiting electron accelerates and should continuously radiate energy, eventually collapsing into the nucleus. Rutherford’s model also could not explain discrete atomic spectral lines. Bohr later introduced quantized stationary orbits to address these problems.
36. According to Bohr’s model, what happens when an electron goes from a higher-energy orbit to a lower-energy orbit?
[SSC Stenographer — 8 Aug 2025, Shift 1]
(A) Its angular momentum first becomes quantized.
(B) The electron gains energy.
(C) A photon equal to the energy difference is absorbed.
(D) A photon equal to the energy difference is emitted.
Answer: (D) A photon equal to the energy difference is emitted.
Explanation: A downward electronic transition releases the energy difference as a photon, whose frequency satisfies ΔE = hν. An upward transition instead requires photon absorption. Discrete energy gaps create the characteristic line spectrum of an atom rather than a continuous distribution.
35. Which observation in Rutherford’s experiment showed that an atom’s positive charge is concentrated in a tiny region?
[SSC Stenographer — 7 Aug 2025, Shift 1]
(A) Electrons occupied fixed energy levels.
(B) A very small fraction of alpha particles rebounded almost backwards.
(C) Most alpha particles passed straight through the foil.
(D) A small fraction of alpha particles deflected through small angles.
Answer: (B) A very small fraction of alpha particles rebounded almost backwards.
Explanation: A rare alpha particle rebounded nearly backwards because it approached a very small, massive, positively charged nucleus and experienced intense electrostatic repulsion. The more common straight-through trajectories showed that most of the atom’s volume is empty space.
34. Which one of the following statements is correct?
[UPSC NDA & NA (I) — 2025, General Ability Test]
(A) A neutron forms by combining an electron and a proton and is therefore neutral.
(B) An electron has a mass about 1/2000 that of a proton.
(C) An isotope of cobalt is used to treat goitre.
(D) J. J. Thomson proposed a nucleus containing only neutrons.
Answer: (B) An electron has a mass about 1/2000 that of a proton.
Explanation: An electron’s mass is approximately one eighteen-hundred-and-thirty-sixth that of a proton, commonly rounded to one two-thousandth. Cobalt-60 is used in radiotherapy, while iodine-131 treats certain thyroid conditions. Thomson proposed no atomic nucleus in his plum-pudding model.
33. In which year did Henry Moseley use X-rays to connect elemental spectra with atomic numbers?
[SSC GD Constable — 20 Feb 2025, Shift 3]
(A) 1925
(B) 1931
(C) 1913
(D) 1902
Answer: (C) 1913
Explanation: Moseley established a systematic relationship between characteristic X-ray frequencies and atomic number in 1913. This demonstrated that proton number, rather than average atomic mass, is fundamental to elemental identity and helped resolve anomalies in earlier periodic arrangements.
32. What was discovered through Rutherford’s gold foil experiment?
[RRB Technician Grade III — 29 Dec 2024, Shift 3]
(A) Neutron
(B) Nucleus
(C) Electron
(D) Proton
Answer: (B) Nucleus
Explanation: The unexpectedly large deflections of a few alpha particles indicated a tiny, dense, positively charged centre within an atom. Rutherford therefore established the nuclear model. Most particles passed through the foil, implying that much of the atomic volume is empty space.
31. Which of the following is NOT a subatomic particle?
[RRB Technician Grade III — 26 Dec 2024, Shift 1]
(A) Neutron
(B) Electron
(C) Proton
(D) Molecule
Answer: (D) Molecule
Explanation: Protons, neutrons and electrons are constituents of atoms. A molecule, however, is formed from two or more bonded atoms and is therefore a larger chemical entity, not a subatomic particle. Water and oxygen molecules provide familiar examples.
