protactinium

Silver-grey, radioactive, metallic element of the actinide series, atomic number 91, relative atomic mass 231.036. It occurs in nature in very small quantities in pitchblende and other uranium ores. It has 14 known isotopes; the longest-lived, Pa-231, has a half-life of 32,480 years.

The element was discovered in 1913 (Pa-234, with a half-life of only 1.2 minutes) as a product of uranium decay by Kasimir Fajans and O Göhring; other isotopes were found in later years and the name was officially adopted in 1949, although it had been in use since 1918.