 Hydrogen is the only element that has different names for its isotopes in common use today . During the early study of radioactivity , various heavy radioactive isotopes were given their own names , but such names are no longer used , except for deuterium and tritium . The symbols D and T ( instead of 2H and 3H ) are sometimes used for deuterium and tritium , but the corresponding symbol for protium , P , is already in use for phosphorus and thus is not available for protium . In its nomenclatural guidelines , the International Union of Pure and Applied Chemistry allows any of D , T , 2H , and 3H to be used , although 2H and 3H are preferred . 
