Thorium

Category: actinide Atomic number: 90 Colour: silver Melting point: 1,750°C (3,182°F) Boiling point: 4,788°C (8,650°F) First identified: 1829

The streets of many cities used to be lit up with a radioactive element: thorium oxide has the highest melting point of all oxides, and this led to it being used in the gaslights of the late nineteenth and early twentieth century. In the heat of the burning gas, it didn’t melt but instead gave off a bright white light. Fortunately, thorium is not as radioactive as some of its fellow actinides, and emits alpha particles, which would not penetrate through glass or human skin, so this was a safer method of lighting than it sounds. Indeed, it is still used in some camping equipment, although you will generally find versions that are specifically labelled as ‘thorium-free’!

Thorium is relatively abundant – there is three times as much of it in the Earth’s crust as uranium. This is because, while it is part of various radioactive decay chains, its half-life in the naturally occurring isotope thorium-232 is longer than the age of our planet.

Discovered by Jöns Jacob Berzelius in 1828, thorium was named after the Viking thunder god, Thor. Of course, he didn’t realize it was radioactive – that concept was unknown at the time. It is sometimes used in nuclear reactors instead of uranium. As thorium and uranium are not always found in the same places, some countries are working to build up their thorium reactors. For instance, India, whose east coast is rich in monazite (a thorium source), has been developing new technology that will enable it to use thorium more efficiently in the future.