Helium also has isotopes. The difference between superscript and subscript gives the number of neutrons in the nucleus, and this can be 1, 2, 3, or 4. An atom has m nuclear n particles, n protons, and m - n neutrons. Instead of special names, the isotopes of helium and heavier elements are distinguished by giving their name and mass number, for example, helium-3 for . All but a minute fraction of helium atoms found on Earth are helium-4. Only one atom per million is helium-3, and helium-5 and -6 do not exist naturally.

Some isotopes of an element are stable and show no tendency to break down; others decompose spontaneously and therefore are radioactive. Light hydrogen and deuterium are stable, but tritium is radioactive. The or T nucleus apparently has an overbalance of neutrons to protons. In time it decays spontaneously, effectively converting one of the neutrons into a proton and an electron:

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