Thorium

Thorium, ₉₀Th

A thin square of thorium metal in a glass ampoule
Thorium
Pronunciation	/ˈθɔːriəm/ ⓘ ​(THOR-ee-əm)
Appearance	silvery
Standard atomic weight Ar°(Th)
	232.0377±0.0004 232.04±0.01 (abridged)
Thorium in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Ce ↑ Th ↓ — actinium ← thorium → protactinium
Atomic number (Z)	90
Group	f-block groups (no number)
Period	period 7
Block	f-block
Electron configuration	[Rn] 6d2 7s2
Electrons per shell	2, 8, 18, 32, 18, 10, 2
Physical properties
Phase at STP	solid
Melting point	2023 K ​(1750 °C, ​3182 °F)
Boiling point	5061 K ​(4788 °C, ​8650 °F)
Density (at 20° C)	11.725 g/cm3
Heat of fusion	13.81 kJ/mol
Heat of vaporisation	514 kJ/mol
Molar heat capacity	26.230 J/(mol·K)
Specific heat capacity	113.041 J/(kg·K)
Vapour pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 2633 2907 3248 3683 4259 5055
Atomic properties
Oxidation states	common: +4 −1, +1, +2, +3
Electronegativity	Pauling scale: 1.3
Ionisation energies	1st: 587 kJ/mol 2nd: 1110 kJ/mol 3rd: 1930 kJ/mol
Atomic radius	empirical: 179.8 pm
Covalent radius	206±6 pm
Spectral lines of thorium
Other properties
Natural occurrence	primordial
Crystal structure	​face-centred cubic (fcc) (cF4)
Lattice constant	a = 508.45 pm (at 20 °C)
Thermal expansion	11.54×10−6/K (at 20 °C)
Thermal conductivity	54.0 W/(m⋅K)
Electrical resistivity	157 nΩ⋅m (at 0 °C)
Magnetic ordering	paramagnetic
Molar magnetic susceptibility	132.0×10−6 cm3/mol (293 K)
Young's modulus	79 GPa
Shear modulus	31 GPa
Bulk modulus	54 GPa
Speed of sound thin rod	2490 m/s (at 20 °C)
Poisson ratio	0.27
Mohs hardness	3.0
Vickers hardness	295–685 MPa
Brinell hardness	390–1500 MPa
CAS Number	7440-29-1
History
Naming	after Thor, the Norse god of thunder
Discovery	Jöns Jakob Berzelius (1829)
Isotopes of thorium
Main isotopes Decay Isotope abun­dance half-life (t1/2) mode pro­duct 227Th trace 18.693 d α 223Ra 228Th trace 1.9125 y α 224Ra 229Th trace 7916 y α 225Ra 230Th 0.02% 75400 y α 226Ra 231Th trace 25.52 h β− 231Pa 232Th 100.0% 1.40×1010 y α 228Ra 233Th trace 21.83 min β− 233Pa 234Th trace 24.11 d β− 234Pa

Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and has a high melting point. Thorium is an electropositive actinide whose chemistry is dominated by the +4 oxidation state; it is quite reactive and can ignite in air when finely divided.

All known thorium isotopes are unstable. The most stable isotope, ²³²Th, has a half-life of 14.0 billion years, or about the age of the universe; it decays very slowly via alpha decay, starting a decay chain named the thorium series that ends at stable ²⁰⁸Pb. On Earth, thorium and uranium are the only elements with no stable or nearly-stable isotopes that still occur naturally in large quantities as primordial elements. Thorium is estimated to be over three times as abundant as uranium in the Earth's crust, and is chiefly refined from monazite sands as a by-product of extracting rare-earth elements.

Thorium was discovered in 1828 by the Swedish chemist