Protactinium is a transactinide element with the atomic number 91. As a synthetic, highly radioactive metalloid, its properties are dominated by its decay chains and complex isotopic structure. It belongs to the actinide series, exhibiting variable oxidation states in compounds, and its isotopes are known for emitting various forms of radiation, primarily alpha and beta particles. Studies of its decay products are important in understanding nuclear physics and the behavior of superheavy elements. Its chemical behavior places it within a group of elements known for forming stable oxides and exhibiting complex interactions with halogens, making it a subject of specialized research in geochemistry and materials science.