Protactinium
actinide · period 7 · group 3 · f-block
Protactinium (formerly protoactinium) is a chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray actinide metal which readily reacts with oxygen, water vapor and inorganic acids.
What the numbers say
At standard temperature and pressure Protactinium is a solid, sitting in period 7, group 3 of the f-block.
Its melting point of 1841 K (1568 °C) is the 23rd highest of the 110 elements with a measured value — mid-range among the elements.
At 15.37 g/cm³ it is the 11th densest of 96 elements with a quoted density.
Pauling electronegativity is 1.5. Paired with oxygen (3.44) that is a difference of 1.94, which puts the bond predominantly ionic — expect a stable oxide ceramic.
In the solid state it takes a centered tetragonal structure.
It is reported in 4 oxidation states (2, 3, 4, 5), which sets how many distinct compounds and phases it can form.
First identified in 1913.
Crystal structure
Solid Protactinium adopts a Centered Tetragonal arrangement, in the Tetragonal system — axes a = b ≠ c, angles α = β = γ = 90°. Symmetry controls the anisotropy of stiffness and thermal expansion, and the available slip systems set how far the metal deforms before it fractures.
Check it against computed structures
The structure search queries the Materials Project, OQMD and the Open Materials Database for computed crystal structures containing Pa — with DFT formation energy, hull stability and band gap where the provider publishes them.
K 2 · L 8 · M 18 · N 32 · O 20 · P 9 · Q 2
Properties
| Atomic number | 91 |
|---|---|
| Atomic mass | 231.035882 u |
| Series | actinide |
| Group | 3 |
| Period | 7 |
| Block | f-block |
| Phase at STP | Solid |
| Density | 15.37 g/cm³ |
| Melting point | 1841 K (1568 °C) |
| Boiling point | 4300 K (4027 °C) |
| Electronegativity (Pauling) | 1.5 |
| Electron configuration | [Rn] 5f² 6d¹ 7s² |
| Crystal structure | Centered Tetragonal |
| Oxidation states | 2,3,4,5 |
| Discovered | 1913 |
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See Pa in context with all 118 elements.
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