Q90. Which subatomic particle was discovered by James Chadwick in 1932?
[SSC MTS 2025 paper (held 11 February 2026, Shift 1)]
(A) Proton
(B) Electron
(C) Neutron
(D) Alpha particle
Answer: (C) Neutron
Explanation: James Chadwick established the existence of the electrically neutral neutron through experiments involving radiation from beryllium. Neutrons have masses close to protons and, together with protons, constitute most of the atomic nucleus.
Q89. If chlorine-35 and chlorine-37 occur in approximately a 3:1 ratio, what is chlorine’s relative atomic mass?
[RRB Group D (18 August 2026, Shift 1)]
(A) 37 u
(B) 36 u
(C) 35.5 u
(D) 34.5 u
Answer: (C) 35.5 u
Explanation: The average is weighted by isotopic abundance: (35 × 0.75) + (37 × 0.25) = 35.5 u. Individual isotopes have nearly integral mass numbers, but a naturally occurring elemental sample contains a mixture.
Q88. Which nuclear decay increases atomic number by one without changing mass number?
[SSC CGL (24 September 2025, Shift 3)]
(A) Alpha emission
(B) Beta-minus emission
(C) Beta-plus emission
(D) Gamma emission
Answer: (B) Beta-minus emission
Explanation: In beta-minus decay, one neutron becomes a proton while an electron and an antineutrino are emitted. Proton count therefore rises by one, but the total number of nucleons and the mass number remain unchanged.
Q87. What happens to proton number during beta-plus decay?
[SSC CGL (22 September 2025, Shift 3)]
(A) It falls by one without a change in mass number
(B) It rises by one without a mass change
(C) Both proton and mass numbers decline
(D) Proton number falls by two and mass number by four
Answer: (A) It falls by one without a change in mass number
Explanation: A proton converts into a neutron, releasing a positron and a neutrino during beta-plus decay. Therefore atomic number decreases by one while mass number remains constant. The weak interaction mediates this nuclear transformation.
Q86. How does the total mass of fission products compare with that of the original reactants?
[SSC CGL (22 September 2025, Shift 2)]
(A) Greater
(B) Exactly equal
(C) Slightly smaller
(D) Only temperature dependent
Answer: (C) Slightly smaller
Explanation: In energy-releasing fission, the total rest mass of the final products is slightly less than the initial rest mass. The missing mass appears as kinetic energy and radiation according to Einstein’s relationship E = mc².
Q85. Which statement is NOT generally a property of the nuclear force?
[RRB JE CBT-II CMA (22 April 2025, Shift 2)]
(A) It is attractive at ordinary nuclear distances
(B) It is much stronger than Coulomb repulsion at short range
(C) It shows saturation
(D) Its strength depends strongly on a nucleon’s electric charge
Answer: (D) Its strength depends strongly on a nucleon’s electric charge
Explanation: The residual strong force between nucleons is approximately charge-independent: proton–proton, neutron–neutron and proton–neutron interactions share similar strong components. It is powerful over femtometre distances and demonstrates saturation because each nucleon interacts mainly with nearby nucleons.
Q84. What is the splitting of a heavy nucleus into smaller nuclei with energy release called?
[RRB JE CBT-II CMA (22 April 2025, Shift 1)]
(A) Fusion
(B) Ordinary radioactive decay only
(C) Fission
(D) Ionization
Answer: (C) Fission
Explanation: Nuclear fission divides a heavy atomic nucleus into smaller fragments, commonly releasing neutrons and substantial energy. Its energy derives from increased binding energy per nucleon in the products. Controlled fission provides reactor heat.
Q83. A radionuclide has a 10-day half-life. When will 75% of the original amount have decayed?
[SSC Selection Post Graduate (29 August 2025, Shift 1)]
(A) 5 days
(B) 10 days
(C) 20 days
(D) 30 days
Answer: (C) 20 days
Explanation: After the first ten days, 50% remains. After a second ten-day half-life, only 25% remains; therefore 75% has decayed. The calculation uses exponential decay and assumes an unchanged decay probability.
Q82. Which listed carbon nuclide is radioactive?
[SSC JE Mechanical (6 December 2025, Shift 2)]
(A) Carbon-12
(B) Carbon-13
(C) Carbon-14
(D) Carbon-6
Answer: (C) Carbon-14
Explanation: Carbon-14 is unstable and undergoes beta-minus decay to nitrogen-14 with a half-life of about 5,730 years. It is produced in the atmosphere and used for dating once-living material through measurement of its residual activity.
Q81. Dubnium, an artificial radioactive element with atomic number 105, belongs to which group?
[SSC GD Constable (12 February 2025, Shift 2)]
(A) Group 3
(B) Group 7
(C) Group 5
(D) Group 11
Answer: (C) Group 5
Explanation: Dubnium, symbol Db, is a synthetic transactinide element with proton number 105. On the periodic table, it occupies Group 5, below tantalum. Its isotopes are radioactive and are usually produced in particle-accelerator experiments.
