\( {}^{3}_{1}\mathrm{H} \) and \( {}^{3}_{2}\mathrm{He} \) belong to which of the following categories?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Isobars
Option (1) Isomers — Incorrect. Isomers have the same composition, that is, the same numbers of protons and neutrons. These two nuclides have different atomic numbers (1 and 2), so they are not isomers.
Option (2) Isobars — Correct. Both nuclides have the same mass number, \( A=3 \), while their atomic numbers (1 and 2) differ. Nuclides with the same mass number are isobars.
Option (3) Isotopes — Incorrect. Isotopes have the same atomic number. Here \( Z=1 \) for hydrogen and \( Z=2 \) for helium, so these are nuclei of two different elements.
Option (4) Isotones — Incorrect. Isotones have the same neutron number. Hydrogen-3 has \( 3-1=2 \) neutrons and helium-3 has \( 3-2=1 \) neutron, so the neutron numbers differ.
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.”