Lead

Lead is a dense (11.3 g/cm3), soft, bluish-grey metal in group 14, below carbon, silicon, germanium and tin. Like thallium beside it, lead shows a strong inert pair effect: its 6s2 electrons are relativistically stabilised, so the +2 oxidation state dominates its chemistry and lead(IV) compounds such as PbO2 are strong oxidising agents. Its low melting point (327 °C), malleability and resistance to corrosion, thanks to a protective surface layer of oxide, carbonate or sulfate, made it one of the first metals people worked.

Lead's nucleus matters as much as its electrons. Lead-208 has 82 protons and 126 neutrons, both 'magic numbers' that fill nuclear shells, making it the heaviest stable nuclide. The uranium and thorium decay chains all end at stable lead isotopes, which lets geologists date rocks by measuring lead. Because lead is so dense, it absorbs X-rays and gamma rays efficiently. In the body, Pb2+ imitates calcium and zinc, binding to enzymes and nerve proteins, which is why lead exposure harms children's brain development even at low levels.

Discovery

Lead has been smelted from galena for at least 8,000 years and was used by Egyptians, Babylonians and especially Romans, who made water pipes, roofing and cooking vessels from it. No single discoverer exists. The English word comes from Old English lead, while the symbol Pb comes from the Latin plumbum, the root of 'plumber'. In 1956 Clair Patterson used lead isotopes in meteorites to date Earth at about 4.55 billion years.