Protactinium (Pa)
Protactinium (Pa) is an actinide with atomic number 91 and a standard atomic weight of 231.04. At room temperature it is a solid that melts at 1,572 °C.
Protactinium key facts
Protactinium at a glance
Identity
| Symbol | Pa |
|---|---|
| Atomic number (Z) | 91 |
| Atomic weight | 231.04 ± 0.01 (CIAAW 2024) |
| Category | Actinide |
| Group | — (f-block: actinide series) |
| Period | 7 |
| Block | f-block |
| CAS number | 7440-13-3 |
Atomic properties
| Electron configuration | [Rn] 5f2 6d1 7s2 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f2 6s2 6p6 6d1 7s2 |
|---|---|
| Electrons per shell | 2, 8, 18, 32, 20, 9, 2 |
| Oxidation states | +5 Less common: +2, +3, +4 |
| Electronegativity (Pauling) | 1.5 |
| First ionisation energy | 5.89 eV (568.3 kJ/mol) 2nd: 11.9 eV · 3rd: 18.6 eV |
| Atomic radius (empirical) | 180 pm |
| Covalent radius | 169 pm |
| Van der Waals radius | 243 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 1,572 °C · 2,861.6 °F · 1,845.15 K |
| Density | 15.4 g/cm³ |
| Crystal structure | Tetragonal |
Isotopes & abundance
| Natural occurrence | From radioactive decay |
|---|---|
| Radioactive | Yes |
| Natural isotopes (abundance) | 231Pa 100% (radioactive) |
| Known isotopes | 31 |
| Abundance in Earth's crust | 0.000001 mg/kg |
| Abundance in seawater | 0 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Protactinium electron configuration
Protactinium has 91 electrons. Written in short form, its ground-state configuration is [Rn] 5f2 6d1 7s2 — the electrons of the noble gas radon, plus 5f2 6d1 7s2.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f2 6s2 6p6 6d1 7s2.
An exception to the filling rule. The simple aufbau (Madelung) order predicts 5f3, 6d0, but protactinium actually has 5f2, 6d1. Half-filled and filled subshells, and the close energies of the outer orbitals, make this arrangement more stable.
Discovery and name
1913 / 1917 — Kasimir Fajans and Oswald Helmuth Göhring; Otto Hahn and Lise Meitner · Germany.
Greek prōtos (first) + actinium — it decays into actinium.
Known for
One of the rarest and most expensive naturally occurring elements; mainly of research interest.
Isotopes of protactinium
Naturally occurring protactinium is a mix of 1 isotope. Each has 91 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 231Pa | 91 | 140 | 100% | radioactive · half-life 32.65 thousand years |
31 isotopes of protactinium are known in our data set (half-lives as published in nuclear data tables).
Protactinium and its neighbours in the actinide series
Neighbouring elements in the same f-block series:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Actinium | 89 | [227] | 1,050 °C | 3,200 °C | 10 g/cm³ | 1.1 |
| Thorium | 90 | 232.04 | 1,750 °C | 4,785 °C | 11.7 g/cm³ | 1.3 |
| Protactinium | 91 | 231.04 | 1,572 °C | — | 15.4 g/cm³ | 1.5 |
| Uranium | 92 | 238.03 | 1,135 °C | 4,131 °C | 19.1 g/cm³ | 1.38 |
| Neptunium | 93 | [237] | 644 °C | — | 20.2 g/cm³ | 1.36 |
| Plutonium | 94 | [244] | 640 °C | 3,228 °C | 19.7 g/cm³ | 1.28 |