Protactinium is a rare and highly radioactive metal with the atomic number 91 and the symbol Pa. It is a silvery-gray metal that readily reacts with oxygen, water vapor, and inorganic acids. Protactinium is found naturally in small amounts in uranium ores and is created through the decay of thorium. It has a number of unique properties, including its ability to be used as a fuel in some types of nuclear reactors and its use as a target for scientific research. However, protactinium is also highly toxic and poses a significant health risk to humans if not handled properly.