Thorium

In 1828, Norwegian amateur mineralogist Morten Thrane Esmark found an interesting black mineral on an island in Norway. In 1829, Jöns Jakob Berzelius of the Royal Karolinska Institute, Stockholm extracted thorium from the rock specimen sent to him by Esmark and realized that it had not previously been reported. The mineral turned out to be thorium silicate, now known as thorite. Berzelius produced a sample of metallic thorium by heating thorium fluoride with potassium, and confirmed it as a new metal. 

The radioactivity of thorium was first demonstrated in 1898 by Gerhard Schmidt and confirmed by Marie Curie. Thorium, like uranium, survives on Earth because it has isotopes with long half-lives. 

Thorium is a bright silvery metal that tarnishes olive gray when it is exposed to air; it is moderately soft and malleable and has a high melting point. It is nearly half as dense as uranium and plutonium, but is harder than both. It is about as hard as soft steel and can be rolled into sheets or pulled into wire. Its properties vary widely with the amount of impurities in the sample.

Thorium is highly reactive and tarnishes quite easily, becoming gray and finally black at the surface. Small bits of thorium can spontaneously ignite in air. It burns with a brilliant white light. 

Thorium is used as an alloying agent with other metals such as magnesium to improve high-temperature strength. 

Thorium was used to make ceramics, welding rods, camera and telescope lenses, and fire brick. It was also used in commercial photoelectric cells, added to glass for a high-refractive index, formerly a component of mantles for gas and kerosene lamps, and has been used in the manufacture of tungsten filaments for lightbulbs and vacuum tubes.

It is used in heat resistant paint and metals for the space industry.  

Thorium can be used heavy water reactors, high-temperature gas-cooled reactors, light water reactors, and molten salt reactors.  It is also used in scientific research. 

Thorium can form alloys with many other metals, such as magnesium, chromium, and uranium. Thorium forms hydrides, dioxides, acids, fluorides, nitrides, halides, iodides, borides, carbides, silicides, and salts. The salts are quite acidic and are highly soluble in water. Thorium can also form perchlorates, sulfates, stultifies, nitrates, carbonates, phosphates, vanadates, molybdates, and chromates, and their hydrated forms. 

There are 32 characterized isotopes of thorium. All known thorium isotopes occur in nature and are unstable. They range in mass number from thorium-207 to thorium-238. The most stable isotope, thorium-232, has a half-life of 14.05 billion years—three times the age of the earth, and just slightly longer than the age of the universe. Thorium-232 is the only isotope that occurs in quantity in nature. The rest of the half-lives range from 75,380 years to less than 10 minutes.

Thorium is the 41st most abundant element in the Earth’s crust. It is found in trace quantities in virtually all rock, soil, water, plants, and animals. 

Thorium is found as the minerals thorite, uranothorite, and thorianite. It is also found in monazite, a phosphate mineral, which is the most common source of thorium. It occurs in igneous rocks and other large deposits worldwide—principally India, South Africa, Brazil, Australis, and Malaysia. Thorium also occurs as a minor part of most minerals. It can be mined and refined from naturally occurring ores in rock or sands, and can be found in waste products such a mill tailings. 

Man-made thorium is rare.