Protactinium ($Pa$)
The radioactive parent of actinium—a rare and elusive actinide that occupies a critical place in the decay chain of uranium-235.
Protactinium is an element that existed in a state of scientific uncertainty for decades. It was first identified in 1913 by Kazimierz Fajans and Oswald Helmuth Göhring, who named it Brevium due to its short half-life. It was later confirmed and isolated in 1918 by two independent teams: Otto Hahn and Lise Meitner, and Frederick Soddy and John Cranston. The name was eventually changed to Protactinium, meaning "parent of actinium," because it is the radioactive progenitor that decays into actinium.
Occupying the third position in the Actinide series, protactinium is a dense, silvery-grey metal. It is highly radioactive and extremely rare. It serves as a vital marker in nuclear science, connecting the heavy uranium isotopes to the stable decay products of the actinide series.
Atomic & Physical Properties
Protactinium is a malleable, ductile metal. Its physical properties are dominated by its dense electronic structure and the high level of alpha and beta activity across its isotopes.
| Property | Value |
|---|---|
| Atomic Number | 91 |
| Standard Atomic Weight | [231.036] |
| Electron Configuration | $[Rn] 5f^2 6d^1 7s^2$ |
| Most Stable Isotope | 231Pa (Half-life: 32,760 years) |
| Common Oxidation States | +5 (Most stable), +4 |
| Melting Point | 1841 K (1568 °C) |
| Density | 15.37 g/cm³ |
The Parent of Actinium
Protactinium-231 is a critical milestone in the decay chain of Uranium-235. It is produced by the alpha decay of Actinium-227's ancestor, and it itself decays by alpha emission into Actinium-227:
This relationship makes protactinium vital for geological dating of deep-sea sediments, where the ratio of protactinium-231 to uranium isotopes can be used to determine the age of oceanic deposits.
Chemical Reactivity
Protactinium is chemically complex, with the $+5$ oxidation state being the most thermodynamically stable. It behaves similarly to tantalum but is more basic in its oxide form.
1. Reaction with Air
The metal reacts with oxygen when heated to form the very stable Protactinium(V) oxide ($Pa_2O_5$).
2. Reaction with Acids
It dissolves in mineral acids, particularly hydrofluoric acid, which forms stable anionic complexes of the form $[PaF_7]^{2-}$.
Radiotoxicity & Safety
Protactinium is exceptionally dangerous. As an alpha-emitter, it is a significant internal radiation hazard. If inhaled or ingested, the alpha radiation it emits is highly damaging to living tissue, leading to an increased risk of bone cancer and liver disease. Like many actinides, it is also chemically toxic, which compounds the risk of exposure.
This is the ninety-first part of our "Elements and Their Properties" series. We are deep into the actinide frontier! To master the mechanics of radioactive decay chains and heavy-element chemistry, visit our Success Blueprint.
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