Identify isotone pairs of nuclei among the following pairs of nuclei.
(A) \( {}^{3}_{2}He \) and \( {}^{3}_{1}H \)
(B) \( {}^{13}_{6}C \) and \( {}^{14}_{7}N \)
(C) \( {}^{2}_{1}H \) and \( {}^{4}_{2}He \)
(D) \( {}^{198}_{80}Hg \) and \( {}^{197}_{79}Au \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B) and (D) only
Isotones are nuclei with the same number of neutrons, N = A − Z (mass number minus atomic number).
(A) \( {}^{3}_{2}\mathrm{He} \) and \( {}^{3}_{1}\mathrm{H} \) — Incorrect. N = 3 − 2 = 1 and N = 3 − 1 = 2. They have the same mass number, so they are isobars, not isotones.
(B) \( {}^{13}_{6}\mathrm{C} \) and \( {}^{14}_{7}\mathrm{N} \) — Correct. N = 13 − 6 = 7 and N = 14 − 7 = 7.
(C) \( {}^{2}_{1}\mathrm{H} \) and \( {}^{4}_{2}\mathrm{He} \) — Incorrect. N = 2 − 1 = 1 and N = 4 − 2 = 2.
(D) \( {}^{198}_{80}\mathrm{Hg} \) and \( {}^{197}_{79}\mathrm{Au} \) — Correct. N = 198 − 80 = 118 and N = 197 − 79 = 118.
NCERT (2026-27):
Atomic Masses and Composition of Nucleus – “… All nuclides with same mass number A are called isobars. For example, the nuclides \( {}^{3}_{1}\mathrm{H} \) and \( {}^{3}_{2}\mathrm{He} \) are isobars. Nuclides with same neutron number N but different atomic number Z, for example \( {}^{198}_{80}\mathrm{Hg} \) and \( {}^{197}_{79}\mathrm{Au} \), are called isotones.”