Thorium (Th)

Thorium is about three times more abundant than uranium and every atom of it can in principle be bred into fissile uranium-233, which is why thorium reactors keep being proposed; the practical obstacle is that the fuel cycle produces uranium-232, whose decay chain includes a hard gamma emitter that makes the fuel awkward to handle. Thoriated gas mantles were the standard for outdoor lighting for a century and old ones are still measurably radioactive. Its configuration is anomalous — [Rn] 7s² 6d², with no 5f electrons, so thorium behaves chemically much more like hafnium than like a rare earth.

Properties

Atomic number

90

Symbol

Th

Standard atomic weight

232.04 u

Category

Actinide

Period

7

Block

d-block

Electron configuration

[Rn] 7s² 6d²

Electrons per shell

2, 8, 18, 32, 18, 10, 2

Melting point

2,023 K — 1749.8 °C — 3181.7 °F

Boiling point

5,061 K — 4787.9 °C — 8650.1 °F

Density

11.72 g/cm³

Electronegativity

1.3 (Pauling)

Oxidation states

+4

Discovered

1829 by Jöns Jacob Berzelius

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 6d²

Noble gas shorthand

[Rn] 7s² 6d²

Shells

K: 2 · L: 8 · M: 18 · N: 32 · O: 18 · P: 10 · Q: 2

What it is used for

• Thoriated tungsten electrodes for TIG welding
• High-refractive-index optical glass, in older lenses
• Gas mantles for camping and street lanterns, historically
• Proposed fuel for molten salt and thorium-cycle reactors
• Thorium-lead dating of rocks and meteorites

Hazards

• Radioactive alpha emitter, though thorium-232's 14-billion-year half-life makes it weakly so
• Thorium dust is the real hazard: inhaled, it lodges in the lungs
• Old thoriated lenses yellow with age from their own radiation

Anomalous electron configuration: [Rn] 7s² 6d² rather than the predicted 7s² 5f². Thorium is one of only three actinides — with protactinium and uranium — that has a characteristic terrestrial isotopic composition and therefore a real standard atomic weight rather than a bracketed mass number.

Neighbours in the table