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Why is tritium radioactive while hydrogen-1 and hydrogen-2 are stable?

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radioactivity Figure 1: Radioactive decay of beryllium-7 to lithium-7 by electron capture (EC; see text).

Tritium is radioactive because its nucleus is unstable, whereas hydrogen-1 and hydrogen-2 (deuterium) are stable isotopes.1

In physics, the stability of a nucleus is determined by the balance of its constituent protons and neutrons. For light elements, isotopes with an intermediate number of neutrons are generally stable. Hydrogen-1 consists of a single proton, and hydrogen-2 consists of one proton and one neutron; both configurations are stable.2345

Tritium (hydrogen-3), however, contains one proton and two neutrons. This specific neutron-to-proton ratio results in an unstable nucleus that seeks a more stable state. To achieve this, tritium undergoes beta decay, a process where one of its neutrons converts into a proton, emitting an electron and an antineutrino in the process. This transformation changes the nucleus into helium-3, which is a stable isotope. Because tritium spontaneously dissipates excess energy through this radioactive disintegration, it is classified as a radioactive isotope.42361