Protactinium is a dense, silvery, radioactive actinide metal. It occurs in trace amounts in uranium ores and is valued mainly for research. Its most stable isotope, Pa-231, has a half-life of about 32,760 years and the element commonly exhibits the +5 oxidation state.
Protactinium (Pa) is an actinide with atomic number 91, located in period 7 of the f-block, between thorium (Th) and uranium (U). The name means “before actinium,” reflecting its place in certain decay chains that form actinium from protactinium.
All known isotopes of Pa are unstable. The most stable, \(^{231}\mathrm{Pa}\), undergoes alpha decay with a half-life of ~3.276×104 years:
\(^{231}\mathrm{Pa} \;\to\; ^{227}\mathrm{Ac} + \alpha\)
Short-lived isotopes such as \(^{233}\mathrm{Pa}\) decay by beta emission:
\(^{233}\mathrm{Pa} \xrightarrow{\beta^-} \, ^{233}\mathrm{U}\)
These isotopes are central to research in geochronology, nuclear chemistry, and fuel cycles.
The ground-state configuration is often written as [Rn] 5f2 6d1 7s2. Protactinium most commonly exhibits the +5 oxidation state (Pa(V)), though +4 and +3 states also occur in suitable chemical environments.
Representative compounds include protactinium(V) oxide (Pa2O5), protactinium(V) fluoride (PaF5), and protactinium(V) chloride (PaCl5). In aqueous media, Pa(V) tends to form oxo and fluoro complexes and can show high coordination numbers due to its large ionic radius and participation of 5f/6d orbitals in bonding.
Protactinium occurs at trace levels in uranium ores (e.g., pitchblende). Isolation involves multi-step radiochemical separations from large quantities of uranium, thorium, and fission-product elements, often using solvent extraction and ion-exchange methods under strict radiological controls.
Because of scarcity, cost, and radioactivity, Pa is used mainly in research. Important applications include:
Yes. Pa is a radiotoxic heavy metal. It emits ionizing radiation (notably \(\alpha\) from \(^{231}\mathrm{Pa}\)) and concentrates in the body if inhaled/ingested. Handling requires licensed facilities, glove boxes/fume hoods, appropriate PPE, contamination monitoring, and compliant waste management.
In the thorium cycle, neutron capture by \(^{232}\mathrm{Th}\) forms \(^{233}\mathrm{Th}\), which beta decays to \(^{233}\mathrm{Pa}\) and then to fissile \(^{233}\mathrm{U}\):
\(^{232}\mathrm{Th}(n,\gamma)\,^{233}\mathrm{Th} \xrightarrow{\beta^-} \, ^{233}\mathrm{Pa} \xrightarrow{\beta^-} \, ^{233}\mathrm{U}\)
Managing the chemistry of \(^{233}\mathrm{Pa}\) is important to optimize conversion to \(^{233}\mathrm{U}\) and limit parasitic captures.
A key step in the actinium (\(^{235}\mathrm{U}\)) series is:
\(^{231}\mathrm{Pa} \;\to\; ^{227}\mathrm{Ac} + \alpha \;\to\; \cdots \to\; ^{207}\mathrm{Pb}\;\text{(stable)}\)
Chains proceed via successive \(\alpha\) and \(\beta\) decays until a stable lead isotope is reached.