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
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: