Calcium

Calcium ([Ar]4s2) readily gives up its two outer electrons to form Ca2+. As an alkaline earth metal, it is less reactive than potassium next door, because it has a higher nuclear charge and must lose two electrons, but it still reacts steadily with water. The Ca2+ ion forms poorly soluble salts with carbonate, phosphate and sulfate, so calcium readily precipitates as solid minerals. Those minerals, limestone, marble, gypsum and apatite, make calcium the fifth most abundant element in Earth's crust, at about 4% by mass.

Much of the world's limestone was made by living things: corals, shellfish and microscopic plankton extract calcium and carbonate from seawater to build shells and skeletons. Heating limestone produces lime, the basis of mortar and cement, and cement production alone accounts for roughly 8% of global CO2 emissions. The human body contains about 1 kg of calcium, 99% of it in bones and teeth as hydroxyapatite. The remaining 1% is vital: brief rises in Ca2+ concentration inside cells trigger muscle contraction, the release of neurotransmitters and blood clotting.

Discovery

Lime (calcium oxide) was used in mortar and plaster by ancient Egyptians and Romans, but calcium metal was not isolated until 1808. That year Humphry Davy electrolysed a mixture of moist lime and mercuric oxide, producing a calcium amalgam, and then distilled off the mercury to leave the metal. He named it from the Latin calx (genitive calcis), meaning lime.