Uranium

Category: actinide Atomic number: 92 Colour: silvery grey Melting point: 1,132°C (2,070°F) Boiling point: 4,131°C (7,468°F) First identified: 1789

Pitchblende (or uranite), the ore from which several other radioactive elements would be extracted, was known to medieval silver miners, who occasionally unearthed the black or brown mineral. Martin Klaproth studied it in 1789 and managed to produce a yellow compound that he correctly believed to contain a new element (which he named after the planet Uranus). Uranium was later isolated by the French chemist Eugène Péligot. It was not known that it was radioactive until 1896, when Henri Becquerel left a sample on an unexposed photographic plate, which became cloudy, suggesting that it must be giving off some kind of rays.

Most of the uranium in the planet is uranium-238 (about 99 per cent) while a small proportion is uranium-235, and there are tiny quantities of a few other isotopes. Uranium has a long half-life, which is why there is quite a large amount of it still available – the radioactive decay of the mineral is a significant source of heat inside the planet (and drives phenomena such as volcanoes). It has also played a significant role in allowing us to estimate the planet’s age – it is formed in supernovae in a ratio of uranium-235 to uranium-238 of about 8:5. Comparing the original amounts of each to their current ratio (and taking their half-lives into account) gives us an estimate of the Earth’s age.

Uranium is the only naturally occurring element that can be used as fuel in nuclear reactors. It is also used to power nuclear submarines and (along with plutonium) to build nuclear weapons, which work by releasing huge amounts of energy and radiation as atoms are either torn apart (nuclear fission) or fused together (nuclear fusion) in a detonation.