Uranium (U)

Uranium-lead dating of a meteorite gave Clair Patterson the age of the Earth in 1956 — 4.55 billion years, a figure that has barely moved since. Only about 0.72% of natural uranium is the fissile uranium-235, and enriching that fraction is the single hardest step in building either a reactor or a bomb, which is why the whole non-proliferation regime is organised around centrifuges. At Oklo in Gabon, roughly 1.7 billion years ago, groundwater and a rich ore body produced natural nuclear reactors that ran intermittently for hundreds of thousands of years; the isotope ratios in the rock still show it.

Properties

Atomic number

92

Symbol

U

Standard atomic weight

238.03 u

Category

Actinide

Period

7

Block

f-block

Electron configuration

[Rn] 7s² 5f³ 6d¹

Electrons per shell

2, 8, 18, 32, 21, 9, 2

Melting point

1,408 K — 1134.8 °C — 2074.7 °F

Boiling point

4,404 K — 4130.9 °C — 7467.5 °F

Density

18.95 g/cm³

Electronegativity

1.38 (Pauling)

Oxidation states

+6, +5, +4, +3

Discovered

1789 by Martin Heinrich Klaproth

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f³ 6d¹

Noble gas shorthand

[Rn] 7s² 5f³ 6d¹

Shells

K: 2 · L: 8 · M: 18 · N: 32 · O: 21 · P: 9 · Q: 2

What it is used for

• Fuel for nuclear power reactors, as enriched uranium dioxide
• Depleted uranium in armour-piercing rounds and tank armour
• Counterweights and ballast in aircraft control surfaces, historically
• Uranium glass and ceramic glazes, which fluoresce green under UV
• Uranium-lead dating, which is how the age of the Earth was established

Hazards

• Radioactive, though uranium-238's 4.5-billion-year half-life makes it a weak emitter
• Chemically a heavy-metal kidney poison, which is the more immediate danger
• Finely divided uranium metal is pyrophoric

Anomalous electron configuration: [Rn] 7s² 5f³ 6d¹ rather than the predicted 7s² 5f⁴. Uranium is the heaviest element that occurs on Earth in more than trace amounts.

Neighbours in the table