Isotopes of Hydrogen

The Three Isotopes

Hydrogen has three naturally occurring isotopes, differing only in the number of neutrons in the nucleus:

Protium is overwhelmingly the dominant form on Earth. Deuterium (also called heavy hydrogen) makes up about 0.0156% of terrestrial hydrogen, mostly present as HD rather than pure $D_2$. American chemist Harold C. Urey won the 1934 Nobel Prize for isolating deuterium by physical separation methods. Tritium is far rarer still — about one atom per $10^{18}$ atoms of protium — and unlike the other two, it is radioactive, decaying by low-energy $\beta^-$ emission with a half-life of 12.33 years.

Because all three isotopes share the same electronic configuration, their chemical properties are almost identical; only their rates of reaction differ noticeably, a consequence of different bond dissociation enthalpies. Their physical properties diverge more sharply, since the relative mass differences between them are large (deuterium is roughly twice as heavy as protium, tritium roughly three times).

Table 9.1 — Atomic and Physical Properties of Hydrogen