2. Atoms, Molecules and Moles   Previous PageNext Page
     Isotopes of Helium

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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