Elements are the basic building blocks of the chemist’s world. The first and simplest of these elements is the gaseous element hydrogen, H. Hydrogen ordinarily occurs in molecular form as two joined atoms. However, we speak of hydrogen here as if it consisted of one lone atom.
Now the element hydrogen has an atomic number of 1. This means it’s central core—or nucleus—contains 1 proton. All hydrogen atoms have 1 electron in an orbital outside the nucleus. Generally, the nucleus contains no other particle than that lone proton. However, it may also contain either one or two neutrons as well.
Hydrogen with no neutron in the nucleus is sometimes called protium. Hydrogen with one neutron in the nucleus is deuterium. Hydrogen with two neutrons in the nucleus is tritium. Tritium, alone of the three, is less than completely stable and is radioactive. The ratios of these three forms of hydrogen in the vicinity of the earth is approximately:
99.98% H-1 (protium)
0.016% H-2 (deuterium)
>0.01% H-3 (tritium)
As may be imagined, for most practical purposes, it is protium or H-1 that is of interest. However, there are specific, important uses for deuterium, which must be separated out to be of practical interest.
Deuterium was discovered in 1931. Since a neutron weighs just a bit more than a proton, deuterium is just slightly more than twice as heavy as protium is. Thus two atoms of deuterium combined with one atom of oxygen is called “heavy water.” Since the oxygen atom is the same in both regular water and heavy water, and it contains the bulk of the mass, heavy water is only approximately 10% heavier than “regular” water. Chemically heavy water bears resemblance to regular water, but there are important differences.
Percentage-wise, the difference in mass and other properties between protium and deuterium is much greater than the difference between isotopes of other elements, say for instance, between carbon-12 and carbon-13. Mass affects bond lengths and bond lengths affect other characteristics. Bond lengths affect chemistry, and since water is so important to life, deuterium can be extremely useful in biological research.
Nuclear magnetic resonance (NMR) and infrared (IR) spectroscopic properties also vary considerably between protium and deuterium. Thus labeling organic compounds with one or more deuterium atoms in specific locations in molecules can help in the study of organic reactions and processes. And in the case of the NMR, various deuterated solvents prove quite useful in structure elucidation.
Tritium is often times a waste product—a nuisance—in nuclear reactions. It presents health risks if it reaches an underground water supply, perhaps through leakage. For instance, in the US there was an important tritium leak. Reference to it can be seen in this document from the Brookhaven National Laboratory (BNL). Sometimes, however, tritium is deliberately employed in the armaments business in fission bombs and in the fission component of fusion bombs.
Afterword: If you are particularly interested in these isotopes of hydrogen, and perhaps isotopes in general, you may find this article particularly interesting: Isotopic Varieties of H2O: How Many Kinds of Water are In Your Glass?