Atoms and Nuclei Part 3

Concept:Isobars are atoms of different elements having the same mass number ($A$) but different atomic numbers ($Z$).Explanation:Option A: ${}_1^3\mathrm{H}$ and ${}_2^3\mathrm{He}$Mass numbers: 3 and 3 (same).Atomic numbers: 1 and 2 (different).Hence they are isobars.Option B: ${}_{80}^{198}\mathrm{Hg}$ and ${}_{79}^{197}\mathrm{Au}$Mass numbers: 198 and 197 (different).Atomic numbers: 80 and 79 (different).They are neither isobars nor isotopes.Option C: ${}_1^2\mathrm{H}$ and ${}_1^3\mathrm{H}$Mass numbers: 2 and 3 (different).Atomic numbers: both 1 (same).They are isotopes of hydrogen, not isobars.Option D: ${}_{92}^{236}\mathrm{U}$ and ${}_{92}^{238}\mathrm{U}$Mass numbers: 236 and 238 (different).Atomic numbers: both 92 (same).They are isotopes of uranium, not isobars.Only option A satisfies the isobar definition.Answer:Option A: ${}_1^3\mathrm{H}$ and ${}_2^3\mathrm{He}$ are isobars.