(N/A) Hydrogen has three isotopes: protium,$^1_1H$,deuterium,$^2_1H$ (or $D$),and tritium,$^3_1H$ (or $T$). These isotopes differ from one another in the number of neutrons present in the nucleus.
Ordinary hydrogen (protium) has no neutrons,deuterium (heavy hydrogen) has one neutron,and tritium has two neutrons.
In $1934$,Harold $C$. Urey received the Nobel Prize for separating the hydrogen isotope of mass number $2$ using physical methods.
Protium is the most abundant form. Terrestrial hydrogen contains $0.0156 \%$ of deuterium,primarily as $HD$.
Tritium is rare,with a concentration of about one atom per $10^{18}$ atoms of protium. Among these,only tritium is radioactive,emitting low-energy $\beta^-$ particles with a half-life of $t_{1/2} = 12.33 \text{ years}$.
Since these isotopes share the same electronic configuration,they exhibit nearly identical chemical properties. The primary difference lies in their reaction rates,which is attributed to their different bond dissociation enthalpies.