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Thorium is a chemical element with the symbol Th and atomic number 90. As a naturally occurring, slightly radioactive metal, it has been considered as an alternative nuclear fuel to uranium. When pure, thorium is a silvery white metal that retains its luster for several months. However, when it is exposed to oxygen, thorium slowly tarnishes in air, becoming grey and eventually black. Thorium dioxide (ThO2), also called thoria, has the highest melting point of any oxide (3300°C). When heated in air, thorium metal turnings ignite and burn brilliantly with a white light.

Thorium has the largest liquid range of any element: 2946 K between the melting point and boiling point.


Name, Symbol, Number thorium, Th, 90
Chemical series Actinides
Group, Period, Block n/a, 7, f
Appearance silvery white
Standard atomic weight 232.0381(2) g·mol-1
Electrons per shell 2, 8, 18, 32, 18, 10, 2
Physical properties
Phase solid
Density (near r.t.) 11.7 g·cm−3
Melting point 2115K
(1842°C, 3348°F)
Boiling point 5061K
(4788°C, 8650°F)
Heat of fusion 13.81 kJ·mol-1
Heat of vaporization 514 kJ·mol-1
Heat capacity (25°C) 26.230 J·mol-1·K-1
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
at T(K) 2633 2907 3248 3683 4259 5055
Atomic properties
Crystal structure cubic face centered
Oxidation states 4
(weakly basic oxide)
Electronegativity 1.3 (Pauling scale)
Ionization energies
1st: 587 kJ·mol-1
2nd: 1110 kJ·mol-1
3rd: 1930 kJ·mol-1
Atomic radius 180 pm
Magnetic ordering no data
Electrical resistivity (0 °C) 147 nΩ·m
Thermal conductivity (300K) 54.0 W·m-1·K-1
Thermal expansion (25°C) 11.0 µm·m-1·K-1
Speed of sound (thin rod) (20 °C) 2490 m/s
Young's modulus 79 GPa
Shear modulus 31 GPa
Bulk modulus 54 GPa
Poisson ratio 0.27
Mohs hardness 3.0
Vickers hardness 350 MPa
Brinell hardness 400 MPa
CAS registry number 7440-29-1


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