- Protactinium (Pa) is a dense, silvery-gray, and highly radioactive metal with atomic number 91, positioned in the actinide series of the periodic table.
- Its electron configuration is [Rn] 5f² 6d¹ 7s², and it most commonly forms Pa⁵⁺ ions in compounds, though Pa⁴⁺ states are also known. The atomic structure contains ninety-one protons, typically one hundred and forty protons in its most stable isotope, and ninety-one electrons arranged in seven shells.
- Naturally occurring protactinium consists primarily of the isotope protactinium-231 (²³¹Pa), which has a half-life of about 32,760 years and is part of the uranium-235 decay chain. Another isotope, protactinium-234m, occurs in trace amounts with a half-life of only 1.17 minutes.
- Protactinium is extremely rare in nature, with an abundance of roughly one part per trillion in the Earth’s crust. It is typically found in uranium ores such as pitchblende (uraninite), where it is present as a decay product of uranium-235. Significant quantities can be extracted through complex separation processes from spent nuclear fuel or by irradiating thorium-230 in nuclear reactors, though this is costly and labor-intensive.
- The element was first identified in 1913 by German scientists Kasimir Fajans and Oswald Helmuth Gohring, who discovered the short-lived isotope ²³⁴mPa and named it “brevium” (Latin for “short-lived”). In 1918, British chemists Frederick Soddy and John Cranston, and independently German chemist Otto Hahn and Austrian chemist Lise Meitner, discovered the long-lived isotope ²³¹Pa, renaming the element “protactinium” to indicate that it is a precursor (“proto”) to actinium in the decay chain.
- Due to its rarity and intense radioactivity, protactinium has no large-scale commercial uses. It is mainly of interest to scientists for nuclear research and as a reference in geochemical dating methods. The isotope ²³¹Pa is valuable in sediment dating in marine geology, helping to reconstruct historical ocean circulation patterns.
- Chemically, protactinium is one of the more reactive actinides, closely resembling tantalum (Ta) in its pentavalent state. It readily forms oxides, halides, and oxyhalides, and it is soluble in acids but resistant to alkalis. Metallic protactinium is dense (15.37 g/cm³), hard, and has a high melting point of 1568 °C.
- Biologically, protactinium has no essential role and is highly toxic due to its radioactivity. Inhalation or ingestion can lead to the accumulation of Pa in bones and liver, significantly increasing cancer risk.
- From an environmental perspective, protactinium occurs only in trace amounts naturally and poses minimal widespread risk. However, in nuclear waste or research facilities, it must be handled with strict radiological controls due to its long half-life and alpha radiation emissions.
