Lithium is the lightest metal there is. Its density is 0.534 grams per cubic centimetre, just over half that of water, and under ordinary conditions it is the least dense of all the solid elements. That is light enough for a piece of lithium to float on the lightest hydrocarbon oils, and it is one of only three metals that can float on water, with sodium and potassium. It is also soft enough to cut with a knife.
Floating is a simple contest of densities: a solid floats on a liquid that is denser than it is. Water is 1 gram per cubic centimetre, and at 0.534 lithium is just over half of that, which is why it is one of only three metals that can float on water at all.
A metal that will not sink
Being light causes a practical problem. Lithium is an alkali metal, and like the others in its column it is highly reactive and flammable: left in air it tarnishes, first to a dull grey and then black, and it reacts with water. So it has to be kept away from both.
Its heavier relatives, sodium and potassium, are usually stored under mineral oil, which keeps the air off them. That trick fails for lithium. It is not dense enough to sink in the oil, so it floats to the top, where the air can reach it. Lithium metal is therefore usually stored in a hydrocarbon sealant instead, often petroleum jelly, which holds it in place and keeps the air out.
The stone element
Lithium was discovered in 1817 by Johan August Arfwedson, who was working in the laboratory of the Swedish chemist Jöns Jakob Berzelius. While analysing a mineral called petalite, Arfwedson found a new element whose compounds resembled those of sodium and potassium.
Berzelius chose the name. Sodium was known partly from its abundance in animal blood, and potassium had been found in plant ashes, but this new alkali came out of a solid mineral. So he named it after the Greek word lithos, stone.
A year later, in 1818, Christian Gmelin noticed that lithium salts colour a flame bright red. Neither he nor Arfwedson managed to isolate the pure metal; that was first done in 1821 by William Thomas Brande.
From a stone to a battery
Lithium and its compounds have many uses, from heat-resistant glass and ceramics to lubricating greases, but one use now dwarfs the rest. Batteries alone, lithium-metal and lithium-ion, use more than three quarters of all the lithium produced.
The Short opens on a small silver cube bobbing on oil and ends on the element tile. Element 3, lithium: a metal from a stone, light enough to float.
