About

Thorium is a weakly radioactive, silvery metal that is stable and tarnishes black when exposed to air. It is much more abundant than uranium and is being seriously investigated as a safer and more efficient alternative fuel for thorium-based nuclear reactors. It was previously used in high-quality optics and gas mantles for lanterns due to its high refractive index and incandescence.

Atomic Properties

Atomic Number90

Atomic Weight232.038 u

Neutrons (common)142

Electron Configuration[Rn] 6d² 7s²

Full Configuration1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 6d² 7s²

Electrons per Shell2, 8, 18, 32, 18, 10, 2

Electronegativity1.3

Electron Affinity112.5 kJ/mol

Oxidation States+4, +3, +2

Covalent Radius180 pm

Ionic Radius94 pm

Periodic Table Position

Group3

Period7

BlockF

Chemical SeriesActinoids

Physical Properties

State at STPsolid

Appearancesilvery-white radioactive metal that tarnishes to black

Crystal Structurefcc

Magnetic Orderingparamagnetic

Speed of Sound2,490 m/s

Poisson Ratio0.27

Thermal Properties

Melting Point2,023 K

Boiling Point5,061 K

Thermal Expansion0 1/K

Thermal Conductivity54 W/(m·K)

Specific Heat0.113 J/(g·K)

Molar Heat26.22 J/(mol·K)

Heat of Fusion13.81 kJ/mol

Heat of Vaporization514 kJ/mol

Abundance

In Earth's Crust9.6 ppm

In Ocean0 ppm

In Solar System0 ppm

In Universe0 ppm

Discovery

Discovered byJöns Jacob Berzelius

Discovery Year1,828

Occurrenceprimordial

CAS Number7440-29-1

Electron Configuration

Energy

↿⇂

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Isotopes of Thorium

Symbol ▲	Mass (u) ▲	Abundance ▲	Half-life ▲	Decay Mode ▲
²²⁹Th	229.031760	—	7916 y	Alpha Decay
²³⁰Th	230.033130	Trace	75.4 ky	Alpha Decay
²³²Th	232.038050	100%	14.0 Gy	Alpha Decay

Interactive Visualization

Explore the 3D Bohr model of Thorium in our interactive viewer.

View 3D Model of Thorium