Protactinium is the third member of the actinides located on the second row below the periodic table. It is a silvery, radioactive metal with a melting point of 1568 degrees Celsius. It was discovered in 1913 by Kasimir Fajans. It is a reactive metal that quickly forms oxides on exposure to air. Protactinium 231 has a half life of 33000 years.

Protactinium does not have many applications outside of research. However, it can be used to model the movements of water in the ocean from the decay of uranium.