Isotopes of Hydrogen

Hydrogen has three naturally-occurring isotopes -- atoms with the same one proton (and hence the same atomic number, 1) but different numbers of neutrons:

• Protium ($_1^1H$, usually just written H) -- the ordinary form, with no neutron at all. It is overwhelmingly the most common isotope, making up 99.985% of natural hydrogen, and it is the only one of the three that contains no neutron.
• Deuterium ($_1^2H$, symbol D), also called heavy hydrogen, has one proton and one neutron, and constitutes about 0.015% of natural hydrogen.
• Tritium ($_1^3H$, symbol T) has one proton and two neutrons. It is radioactive and occurs only in extremely small traces -- roughly one atom per 10¹⁸ hydrogen atoms.

Since natural hydrogen is a mixture of all three isotopes, and hydrogen exists as the diatomic molecule, combining two atoms at random from this mixture gives rise to six possible molecular species: H₂, HD, D₂, HT, T₂ and DT. …