Which of the following pair of nuclides are Isobars?

The correct answer is Option (2) → \( {}^{3}_{1}\mathrm{H},\ {}^{3}_{2}\mathrm{He} \)

Isobars are nuclides with the same mass number \( A \) but different atomic numbers \( Z \). Nuclides with the same \( Z \) and different \( A \) are isotopes, and nuclides with the same number of neutrons \( N = A - Z \) are isotones.

Option (1) \( {}^{2}_{1}\mathrm{H},\ {}^{3}_{1}\mathrm{H} \) — Incorrect. Both have \( Z = 1 \) and different \( A \) (2 and 3); they are isotopes of hydrogen, not isobars.

Option (2) \( {}^{3}_{1}\mathrm{H},\ {}^{3}_{2}\mathrm{He} \) — Correct. Both have \( A = 3 \), while \( Z = 1 \) for tritium and \( Z = 2 \) for helium-3, so they are isobars.

Option (3) \( {}^{198}_{80}\mathrm{Hg},\ {}^{197}_{79}\mathrm{Au} \) — Incorrect. The mass numbers are 198 and 197, so they are not isobars; both have \( N = 118 \), which makes them isotones.

Option (4) \( {}^{1}_{1}\mathrm{H},\ {}^{3}_{1}\mathrm{H} \) — Incorrect. Both have \( Z = 1 \) and different \( A \) (1 and 3); they are isotopes of hydrogen.

NCERT (2026-27):
Atomic Masses and Composition of Nucleus – “… The nuclei of isotopes of a given element contain the same number of protons, but differ from each other in their number of neutrons.”

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.”