Protactinium is a rare, silvery-gray metallic element with the symbol Pa and atomic number 91. It belongs to the actinide series and is found in trace amounts in uranium ores. Protactinium is radioactive and has no stable isotopes, with Protactinium-231 being the most common isotope. Due to its scarcity and radioactivity, protactinium has limited practical applications. It is primarily used in scientific research and has potential uses in nuclear reactors. The element was first isolated in 1913 by Otto Hahn and Lise Meitner, contributing to the understanding of radioactive elements.