Kazimierz Fajans was a Polish American physical chemist of Polish-Jewish origin, a pioneer in the science of radioactivity and the co-discoverer of chemical element protactinium.
Kazimierz Fajans
Fajans at the Bunsen Congress, Munich, 1928
Protactinium is a chemical element; it has symbol Pa and atomic number 91. It is a dense, radioactive, silvery-gray actinide metal which readily reacts with oxygen, water vapor, and inorganic acids. It forms various chemical compounds, in which protactinium is usually present in the oxidation state +5, but it can also assume +4 and even +3 or +2 states. Concentrations of protactinium in the Earth's crust are typically a few parts per trillion, but may reach up to a few parts per million in some uraninite ore deposits. Because of its scarcity, high radioactivity, and high toxicity, there are currently no uses for protactinium outside scientific research, and for this purpose, protactinium is mostly extracted from spent nuclear fuel.
Microscope image of a sample of protactinium-233
Dmitri Mendeleev's 1871 periodic table with a gap for protactinium on the bottom row of the chart, between thorium and uranium
Protactinium occurs in uraninite ores.