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The three naiturally-occurrin isotopes o hydrogen. The fact that each isotope haes ane proton maks them aw variants o hydrogen: the identity o the isotope is gien bi the nummer o neutrons. Frae left tae richt, the isotopes are protium (1H) wi zero neutrons, deuterium (2H) wi ane neutron, an tritium (3H) wi twa neutrons.

Isotopes are variants o a pairteecular chemical element which differ in neutron nummer, althou aw isotopes o a gien element hae the same nummer o protons in each atom.