Page "Chemical element" Paragraph 53

As a special case, the three naturally occurring isotopes of the element hydrogen are often specified as H for < sup > 1 </ sup > H ( protium ), D for < sup > 2 </ sup > H ( deuterium ), and T for < sup > 3 </ sup > H ( tritium ).

This convention is easier to use in chemical equations, replacing the need to write out the mass number for each atom.

For example, the formula for heavy water may be written D < sub > 2 </ sub > O instead of < sup > 2 </ sup > H < sub > 2 </ sub > O.