For heavier elements, the addition of one or two neutrons has a less important effect on properties; thus isotopes are not given special names.

Only for hydrogen, in which additional neutrons double or triple the atomic mass, have special names and symbols been developed:


= H = light hydrogen or "ordinary" hydrogen, with one proton and no neutrons in the nucleus.
= D =deuterium (from "deutero-" or two), with one proton and one neutron in the nucleus.
= T =tritium (from "tri-" or three), with one proton and two neutrons in the nucleus

Ordinary water has the chemical formula . Heavy water, , has become familiar because of its use as a moderator or neutron absorber in certain types of nuclear reactors. About 150 hydrogen atoms per million on our planet are D atoms. Tritium is radioactive and must be produced artificially.