Protactinium (Pa)

Silvery-gray metal

Protactinium is a rare, dense, and highly radioactive actinide that bridges thorium and uranium, and with [Rn] 5f² 6d¹ 7s², protactinium-231 (half-life 32,760 years) is an alpha emitter. It is one of the rarest natural elements (~10⁻¹⁰ by mass in Earth's crust) and has no practical applications beyond research.

Key metrics

Density15.4g/cm³

Melting point1,841K

Boiling point4,300K

Electronegativity1.5Pauling

Discovery

1913discovered

Discovered by: Kasimir Fajans
Reactivity profile: Highly radioactive
Reactivity: High
Bonding: Metallic

Electron configuration

\textcolor{#6b7280}{[Rn]}\; \textcolor{#a855f7}{5_{f}^{2}}\; \textcolor{#22c55e}{6_{d}^{1}}\; \textcolor{#ef4444}{7_{s}^{2}}

[Rn] 5f2 6d1 7s2

Wave function

\Psi_{\text{Pa}} \approx \frac{1}{\sqrt{91!}} \det \left| \chi_1(\mathbf{r}_1) \dots \chi_{91}(\mathbf{r}_{91}) \right|

Probability density $|\Psi|^2$ across the Pa valence shell.

Crystal structure

Face-Centered Cubic

Lattice constantsa = 1.00

Cell anglesα = β = γ = 90°

Coordination number12

Atoms per cell4

Atomic packing factor0.74

Physical properties

Atomic radius: 161 pm
Ionization energy: 568 kJ/mol
Electron affinity: 50 kJ/mol
Crystal structure: Face-centered cubic

Oxidation states

+4+5

Acid/base

Basic

Strongly acidicStrongly basic

State at 25 °CSolid

1841 K4300 K298 K

Emission spectrum

3 transitions

380450500550600650700780

VioletBlueGreenYellowRed

409.3nmPa I (violet)Violet60%

431.6nmPa I (blue)Violet100%

526.5nmPa I (green)Green40%

Dominant line 431.6 nmRange 409–527 nmTransitions 3