JEE Main 2026 — Nuclear Physics Question with Solution

Isobars are atoms of different chemical elements that have the same number of nucleons (mass number $A$), but different atomic numbers ($Z$).
In the given options:
(1) ${}_1^3\text{H}$ and ${}_2^3\text{He}$: Both have the same mass number $A=3$. Since they have different atomic numbers ($Z=1$ and $Z=2$), they are isobars.
(2) ${}_1^2\text{H}$ and ${}_1^3\text{H}$: These have the same atomic number ($Z=1$) but different mass numbers. They are isotopes.
(3) ${}_{80}^{198}\text{Hg}$ and ${}_{79}^{197}\text{Au}$: These have different mass numbers ($198$ and $197$).
(4) ${}_{92}^{236}\text{U}$ and ${}_{92}^{238}\text{U}$: These have the same atomic number ($Z=92$) but different mass numbers. They are isotopes.
Thus, the pair ${}_1^3\text{H}$ and ${}_2^3\text{He}$ are isobars.