Protactinium, formerly protoactinium, symbol Pa, radioactive metallic element with an atomic number of 91. Protactinium is a member of the actinide series of the periodic table (see Periodic Law). It was discovered in 1913 by Kasimir Fajans and Otto H. Göhring.

Protactinium is a member of the uranium-actinium radioactive-decay series and trace amounts are found in uranium ores such as pitchblende. Isotopes of protactinium ranging in mass number from 212 to 239 are known; the element’s atomic weight is 231.04. Protactinium-233 has a half-life of 27 days. Protactinium-231, the most stable isotope, has a half-life of more than 32,500 years; it decays to actinium by emission of an alpha particle. This decay chain gave protactinium its name, which is a shortened form of proto-actinium. Protactinium melts at about 1572°C (about 2862°F), boils at about 4227°C (about 7641°F), and has a specific gravity of about 15.37.

See also Radioactivity.