UPSC CDS 2 2020 GK Paper

Concept:Isotopes of hydrogen differ in neutron count; only one is radioactive.Explanation:Hydrogen has three isotopes: protium (${}^1_1\mathrm{H}$), deuterium (${}^2_1\mathrm{H}$), and tritium (${}^3_1\mathrm{H}$).Protium has zero neutrons, deuterium has one neutron, and tritium has two neutrons.Only tritium is unstable and undergoes radioactive decay (beta decay) with a half-life of $12.32$ years.It emits low-energy $\beta^{-}$ particles and transforms into helium-3 (${}^3_2\mathrm{He}$).Protium and deuterium are stable and non-radioactive.Answer:Tritium is the radioactive isotope of hydrogen.