Atomic number 90

Thorium

Th
+ phase
− phase

Shells Orbitals 6d 7s

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90 electrons in 7 shells: 2, 8, 18, 32, 18, 10, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Actinide
Period: 7
Block: f
Standard atomic weight: 232.04(1) Da
Electron configuration: [Rn] 6d² 7s²
Phase at 25 °C: solid
Melting point: 1750 °C
Boiling point: 4788 °C
Density: 11.72 g/cm³
Electronegativity (Pauling): 1.3
Discovered: 1828, Jöns Jacob Berzelius

Neighbours in the table

Ac Pa

A black mineral from Løvøya, an island in Norway, brought thorium into chemistry. Los Alamos tells how Morten Esmark found it and his father Jens passed a sample in 1828 to Jöns Jacob Berzelius, a Swedish chemist. By Los Alamos’s dating, Berzelius established in 1829 that it held a new element. Jefferson Lab and CIAAW, the atomic-weights commission of IUPAC, the International Union of Pure and Applied Chemistry, date the discovery to 1828, in thorite, ThSiO₄. Berzelius named the element after Thor, the Norse god of thunder.

Thorium stands among the actinides of the f block. None of its 90 electrons, though, is in a 5f orbital: shells of 2, 8, 18, 32, 18, 10 and 2, [Rn] 6d² 7s². Its oxidation state, the charge each atom would carry if every bond were ionic, is +4 in Jefferson Lab’s listing. Los Alamos adds +3, +2 and +1.

Thorium, a silvery-white metal, tarnishes in air to grey and then black, Los Alamos reports. Los Alamos calls the physical properties of thorium strongly dependent on how much oxide it holds. The pure metal is soft and very ductile, easily cold-rolled or drawn. At 1400 °C its cubic crystal structure gives way to a body-centred cubic one.

Where it is found

Jefferson Lab’s estimates on PubChem’s record give 9.6 milligrams of thorium per kilogram of crust and 1 × 10⁻⁶ milligrams per litre of seawater. Los Alamos rates it about three times as abundant as uranium. Thorium-232, 99.98 per cent of natural thorium, formed in the cores of dying stars and has survived for over 4.5 billion years. Los Alamos attributes much of the Earth’s internal heat to thorium and uranium.

Thorite also carries the element, and thorianite, a rare mixture of ThO₂ and UO₂, may hold up to about 12 per cent thorium oxide. Monazite, which carries rare-earth minerals too, is the commercial source. Los Alamos puts its thorium content at 2.5 per cent, or 3 to 9 per cent as ThO₂.

Identified world resources of thorium come to an estimated 6.4 million tons, the US Geological Survey (USGS) reports. India has the largest share on its count, 850,000 tons, ahead of Brazil. Monazite is mined chiefly for its rare earths, and little of the byproduct thorium is recovered. Without rare-earth demand, the survey judges, monazite would probably go unmined under present market conditions.

Why it matters

Thorium metal found no practical use before Carl Auer von Welsbach’s gas mantle, invented in 1885. A mantle made mostly of thorium oxide gives a dazzling light in a gas flame. Because radium and actinium form among thorium’s decay products, the safety of such mantles came into question. Los Alamos names rare-earth elements as their replacement, the USGS yttrium compounds.

Thorium oxide melts at 3300 °C, the highest melting point of any oxide on Los Alamos’s figures. It is used for high-temperature laboratory crucibles and controls the grain size of tungsten for lamps. The metal has a low work function, the energy needed to pull an electron from its surface, and emits electrons readily, so it coats tungsten wire in electronic equipment. Glass holding the oxide bends light strongly while spreading colours little, and it goes into high-quality lenses.

In a reactor, thorium-232 captures a neutron and becomes thorium-233. Two beta decays then lead through protactinium-233 to uranium-233, a fissionable material, in Jefferson Lab’s account on PubChem. Thorium-232 also dates rocks by decaying to lead-208. Because the steps between are short-lived, the lead that builds up depends mainly on the thorium present and the time elapsed, IUPAC’s notes explain. Thorium-234, with a half-life of 24 days, helps estimate how much organic carbon moves through the ocean.

Properties

Properties of thorium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	90
Standard atomic weight	232.04(1) Da	Da	IUPAC/CIAAW 2024
Melting point	1750 °C	°C	Los Alamos: 1750 °C
Boiling point	4788 °C	°C	Los Alamos: 4788 °C
Density	11.72 g/cm³	g/cm³
Electronegativity (Pauling)	1.3
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 6d²
Phase at 25 °C	solid

Safety

Thorium in every form is radioactive, giving off alpha particles and weak gamma rays, the US Environmental Protection Agency (EPA) states. Haz-Map, an occupational health database in PubChem, names bone as the critical organ and rates internal toxicity very high. Inhaled thorium dust may raise the risk of lung or bone cancer, EPA says, and the International Labour Organization’s safety card in PubChem calls thorium carcinogenic to humans. Everyday traces pose little hazard, EPA adds; powdered metal can ignite by itself in air, and the PubChem Laboratory Chemical Safety Summary covers handling.

Questions

What is the half-life of thorium-232?

Los Alamos gives 14.05 billion years, and the IAEA's evaluation 1.40 × 10¹⁰ years. In the IAEA data every decay emits an alpha particle, apart from a spontaneous-fission branch, the nucleus splitting on its own, of 1.1 × 10⁻⁹ per cent. The Atomic Mass Data Center (AMDC) also lists neon-nucleus emission, below 2.78 × 10⁻¹⁰ per cent, and a possible double beta decay. Ten steps later the chain ends at stable lead-208.

How is thorium metal made?

D. Lely, Jr. and L. Hamburger first isolated the metal in 1914, CIAAW records. Los Alamos lists several routes: reducing thorium oxide with calcium, reducing the tetrachloride with an alkali metal, or electrolysing anhydrous thorium chloride in molten sodium and potassium chlorides. For very pure metal, van Arkel and de Boer devised the crystal bar or iodide process in 1925.

Is thorium dangerous to have at home?

Traces of thorium occur in soil, water, rocks, plants and animals, EPA says, and in some older ceramics whose glazes used uranium and thorium; such pieces generally carry no serious health risk. Swallowed thorium mostly leaves the body within several days, EPA adds, and the small remainder settles in bone. Inhaled dust can stay in the lungs for a long time, depending on its chemical form.

Could thorium replace uranium as a nuclear fuel?

A neutron captured by thorium-232 turns it into thorium-233, which beta decay carries through protactinium-233 to fissionable uranium-233. Los Alamos estimates that the thorium in the Earth's crust holds more usable energy than uranium and fossil fuels together. Test reactors have run on it, and the US Geological Survey reports firms and countries working on thorium-fuelled designs.