Actinium
actinide · period 7 · group 3 · f-block
Actinium is a chemical element with the symbol Ac and atomic number 89. It was first isolated by Friedrich Oskar Giesel in 1902, who gave it the name emanium; the element got its name by being wrongly identified with a substance André-Louis Debierne found in 1899 and called actinium.
What the numbers say
At standard temperature and pressure Actinium is a solid, sitting in period 7, group 3 of the f-block.
Its melting point of 1500 K (1227 °C) is the 41st highest of the 110 elements with a measured value — mid-range among the elements.
At 10.07 g/cm³ it is the 27th densest of 96 elements with a quoted density.
Pauling electronegativity is 1.1. Paired with oxygen (3.44) that is a difference of 2.34, which puts the bond predominantly ionic — expect a stable oxide ceramic.
In the solid state it takes a face-centered cubic structure — FCC packing gives 12 independent slip systems, which is why FCC metals are the ductile, formable ones.
It is reported in 2 oxidation states (2, 3), which sets how many distinct compounds and phases it can form.
First identified in 1899.
Crystal structure
Solid Actinium adopts a Face-centered Cubic arrangement, in the Cubic (isometric) 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 Ac — with DFT formation energy, hull stability and band gap where the provider publishes them.
K 2 · L 8 · M 18 · N 32 · O 18 · P 9 · Q 2
Properties
| Atomic number | 89 |
|---|---|
| Atomic mass | 227 u |
| Series | actinide |
| Group | 3 |
| Period | 7 |
| Block | f-block |
| Phase at STP | Solid |
| Density | 10.07 g/cm³ |
| Melting point | 1500 K (1227 °C) |
| Boiling point | 3500 K (3227 °C) |
| Electronegativity (Pauling) | 1.1 |
| Electron configuration | [Rn] 6d¹ 7s² |
| Crystal structure | Face-centered Cubic |
| Oxidation states | 2,3 |
| Discovered | 1899 |
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See Ac in context with all 118 elements.
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