Gallium

Category: post-transition metal Atomic number: 31 Colour: silvery white Melting point: 30°C (86°F) Boiling point: 2,229°C (4,044°F) First identified: 1875

Gallium was the first of Mendeleev’s ‘missing elements’ to be discovered – he had predicted there would be an element to fill the missing space under aluminium and called it ‘eka-aluminium’ as a result. Within five years, the space had been filled: the French scientist Paul-Émile Lecoq de Boisbaudran was unaware of Mendeleev’s prediction when he examined a piece of zincblende (or sphalerite) ore using spectroscopy and noticed two unusual violet lines in the spectrum it produced. He isolated this element and named it gallium (after the Latin name for France, although there is also a possible pun on his own name, Lecoq, as gallus is Latin for ‘rooster’). Gallium is also found in a variety of other minerals, including bauxite, but is generally produced as a by-product in the making of various metals (for instance, when aluminium is refined from bauxite).

You can melt a piece of solid gallium in your hand, so low is its melting point – some scientists have taken advantage of this by making gallium spoons, which can be used as a practical joke (since they will melt when used to stir tea or coffee). It is often used in preference to the more toxic mercury in thermometers. And it has extremely useful semiconductor properties: in the form of gallium arsenide, it gives a faster performance than the traditional silicon semiconductor in a chip. It can be alloyed with most metals and is useful if a low melting point is required for the resulting alloy. It also has a few medical applications – the radioactive isotope gallium-67 can be used to diagnose and locate cancerous growths, while there is ongoing work on the use of a gallium compound in a new generation of antimalaria medication.