Thorium Facts

Thorium Chemical & Physical Properties

Pure thorium is a lustrous silvery metal.
Pure thorium is a lustrous silvery metal. Over the course of several months in air, it tarnishes to gray and eventually black. Photo is thorium foil (~0.5 mm thick) sealed in a glass ampoule under an argon atmosphere. Justin Urgitis,

Thorium Basic Facts

Atomic Number: 90

Symbol: Th

Atomic Weight: 232.0381

Discovery: Jons Jacob Berzelius 1828 (Sweden)

Electron Configuration: [Rn] 6d2 7s2

Word Origin: named for Thor, the Norse god of war and thunder

Isotopes: All of the isotopes of thorium are unstable. The atomic masses range from 223 to 234. Th-232 occurs naturally, with a half-life of 1.41 x 1010 years. It is an alpha emitter that goes through six alpha and four beta decay steps to become the stable isotope Pb-208.

Properties: Thorium has a melting point of 1750°C, boiling point ~4790°C, specific gravity of 11.72, with a valence of +4 and sometimes +2 or +3. Pure thorium metal is an air-stable silvery white which can retain its luster for months. Pure thorium is soft, very ductile, and capable of being drawn, swaged, and cold-rolled. Thorium is dimorphic, going from a cubic structure to a body-centered cubic structure at 1400°C. The melting point of thorium oxide is 3300°C, which is the highest melting point of the oxides. Thorium is attacked slowly by water. It does not readily dissolve in most acids, except hydrochloric acid. Thorium contaminated by its oxide will slowly tarnish to gray and finally black. The physical properties of the metal are highly dependent on the amount of oxide that is present. Powdered thorium is pyrophoric and must be handled with care. Heating thorium turnings in air will cause them to ignite and burn with a brilliant white light.

Thorium disintegrates to produce radon gas, an alpha emitter and radiation hazard, so areas where thorium is stored or handled require good ventilation.

Uses: Thorium is used as a nuclear power source. The internal heat of the earth is largely attributed to the presence of thorium and uranium. Thorium is also used for portable gas lights.

Thorium is alloyed with magnesium to impart creep resistance and high strength at elevated temperatures. The low work function and high electron emission make thorium useful for coating tungsten wire used in electronic equipment. The oxide is used to make lab crucibles and glass with a low dispersion and high index of refraction. The oxide is also used as a catalyst in converting ammonia to nitric acid, in producing sulfuric acid, and in petroleum cracking.

Sources: Thorium is found in thorite (ThSiO4) and thorianite (ThO2 + UO2). Thorium may be recovered from monzanite, which contains 3-9% ThO2 associated with other rare earths. Thorium metal can be obtained by reducing thorium oxide with calcium, by reduction of thorium tetrachloride with an alkali metal, by electrolysis of anhydrous thorium choride in a fused mixture of potassium and sodium chlorides, or by reduction of thorium tetrachloride with anhydrous zinc chloride.

Element Classification: Radioactive Rare Earth (Actinide)

Name Origin: Named for Thor, Norse god of thunder.

Thorium Physical Data

Density (g/cc): 11.78

Melting Point (K): 2028

Boiling Point (K): 5060

Appearance: gray, soft, malleable, ductile, radioactive metal

Atomic Radius (pm): 180

Atomic Volume (cc/mol): 19.8

Covalent Radius (pm): 165

Ionic Radius: 102 (+4e)

Specific Heat (@20°C J/g mol): 0.113

Fusion Heat (kJ/mol): 16.11

Evaporation Heat (kJ/mol): 513.7

Debye Temperature (K): 100.00

Pauling Negativity Number: 1.3

First Ionizing Energy (kJ/mol): 670.4

Oxidation States: 4

Lattice Structure: Face-Centered Cubic

Lattice Constant (Å): 5.080

References: Los Alamos National Laboratory (2001), Crescent Chemical Company (2001), Lange's Handbook of Chemistry (1952), CRC Handbook of Chemistry & Physics (18th Ed.)


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Helmenstine, Anne Marie, Ph.D. "Thorium Facts." ThoughtCo, Mar. 11, 2017, Helmenstine, Anne Marie, Ph.D. (2017, March 11). Thorium Facts. Retrieved from Helmenstine, Anne Marie, Ph.D. "Thorium Facts." ThoughtCo. (accessed January 17, 2018).