protactinium

Protactinium (Pa) is a chemical element with atomic number 91. It is a dense, radioactive metal in the actinide series. The element was identified in 1917 as an intermediate in the decay chain of uranium and was originally named protoactinium; the name Protactinium was later adopted to reflect its position before actinium in the periodic table.

Natural protactinium occurs only in trace amounts, produced by the radioactive decay of uranium in uranium-bearing ores. Because of its scarcity and radioactivity, it is not found in useful quantities in nature and is mainly encountered in laboratory and research settings, where it is produced or isolated by irradiation and chemical separation in nuclear facilities.

Protactinium has no stable isotopes. The most stable isotope is protactinium-231, with a half-life of about 32,760 years. Other isotopes have much shorter half-lives, ranging from minutes to days. In nature, the presence of Pa-231 is tied to the uranium-235 decay chain, but overall natural abundance is extremely low.

Chemically, protactinium commonly exhibits the +5 oxidation state in aqueous solution, with other accessible states such as +4 and +3. It forms oxides and fluoride compounds and shares properties typical of early actinides, while also displaying some unique behavior related to its electronic structure.

Applications of protactinium are largely limited to scientific research. It has been used to study actinide chemistry, nuclear reactions, and radiochemical separation methods. Due to its rarity and intense radioactivity, it has no significant commercial applications. Handling requires specialized radiological safety measures, including shielding and containment to protect against alpha radiation and other decay products.