Material science - Lead

Lead is a chemical element with symbol Pb (from the Latin plumbum) and atomic number 82. It is a heavy metal with a density exceeding that of most common materials; it is soft, malleable, and melts at a relatively low temperature. When freshly cut, it has a bluish-white tint; it tarnishes to a dull gray upon exposure to air. Lead has the second-highest atomic number of the classically stable elements and lies at the end of three major decay chains of heavier elements. Lead is a relatively unreactive post-transition metal. Its weak metallic character is illustrated by its amphoteric nature (lead and lead oxides react with both acids and bases) and tendency to form covalent bonds. Compounds of lead are usually found in the +2 oxidation state, rather than the +4 common with lighter members of the carbon group. Exceptions are mostly limited to organolead compounds. Like the lighter members of the group, lead exhibits a tendency to bond to itself; it can form chains, rings, and polyhedral structures. Lead is easily extracted from its ores and was known to prehistoric people in Western Asia. A principal ore of lead, galena, often bears silver, and interest in silver helped initiate widespread lead extraction and use in ancient Rome. Lead production declined after the fall of Rome and did not reach comparable levels again until the Industrial Revolution. Nowadays, global production of lead is about ten million tonnes annually; secondary production from recycling accounts for more than half of that figure. Lead has several properties that make it useful: high density, low melting point, ductility, and relative inertness to oxidation. Combined with relative abundance and low cost, these factors resulted in the extensive use of lead in construction, plumbing, batteries, bullets and shot, weights, solders, pewters, fusible alloys, and radiation shielding. In the late 19th century, lead was recognized as highly toxic, and since then it has been phased out for many applications. Lead is a neurotoxin that accumulates in soft tissues and bones, damaging the nervous system and causing brain disorders and, in mammals, blood disorders.