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
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.