Lead ( ) is a chemical element with the symbol Pb (from the Latin plumbum) and atomic number 82. It is a heavy metal, denser than most common materials. Lead is soft, malleable, and has a relatively low melting point. When freshly cut or melted, it appears shiny silvery with a bluish tint, but tarnishes to dull gray on exposure to air. Lead has the highest atomic number of any stable element, and three of its isotopes are endpoints of major nuclear decay chains of heavier elements. Lead is a relatively un-reactive post-transition metal. Its weak metallic character is shown by its amphoteric behavior: lead and lead oxides react with both acids and bases, and it tends to form covalent bonds. Lead compounds usually occur in the +2 oxidation state rather than the +4 state common in lighter members of the carbon group, with exceptions mostly limited to organolead compounds. Like the lighter members of the group, lead can bond with itself, forming chains and polyhedral structures. Since lead is easily extracted from its ores, prehistoric people in the Near East were aware of it. Galena is a principal ore of lead which often bears silver. Interest in silver helped initiate widespread extraction and use of lead in ancient Rome. Lead production declined after the fall of Rome and did not reach comparable levels until the Industrial Revolution. Lead played a crucial role in the development of the printing press, as movable type could be relatively easily cast from lead alloys. In 2022, the annual global production of lead was about twelve million tonnes, about two thirds of which was from recycling. Lead's high density, low melting point, ductility and relative inertness to oxidation make it useful. These properties, combined with its relative abundance and low cost, resulted in its extensive use in construction, plumbing, batteries, bullets, shots (pellets), weights, solders, pewter, fusible alloys, lead paints, leaded gasoline, and radiation shielding. Lead is a neurotoxin that accumulates in soft tissues and bones. It damages the nervous system, interferes with biological enzymes, and can cause neurological disorders ranging from behavioral problems to brain damage. It also affects cardiovascular and renal systems. Lead's toxicity was noted by Ancient Greek and Roman writers, but became widely recognized in Europe in the late 19th century.
Physical properties
Atomic A lead atom has 82 electrons, with the electron configuration [Xe]4f145d106s26p2. The combined first and second ionization energies—the total energy required to remove the two 6p electrons—are similar to those of tin, lead's immediate neighbor above in the carbon group. This is unusual, as ionization energies typically decrease down a group due to the outer electrons being farther from the nucleus and more shielded by inner orbitals. However, the sum of the first four ionization energies of lead is higher than that of tin, contrary to periodic trends. This anomaly is explained by relativistic effects, which become significant in heavier atoms. These effects contract the s and p orbitals, giving lead's 6s electrons greater binding energies than its 5s electrons. This leads to the inert-pair effect, where the 6s electrons are less likely to participate in bonding. The result is stabilization of the +2 oxidation state and unusually long distances between nearest atoms in crystalline lead. Lighter carbon-group congeners of lead form stable or metastable allotropes with the tetrahedrally coordinated, covalently bonded diamond cubic structure. In these elements, the s- and p-orbital energy levels are close enough to allow mixing into four hybrid sp3 orbitals. In lead, however, the inert pair effect increases the separation between s- and p-orbitals so much that the energy gain from hybridization is insufficient to overcome this gap. Instead of a diamond cubic arrangement, lead forms metallic bonds in which only the p-electrons are delocalized and shared among Pb2+ ions. Consequently, lead adopts a face-centered cubic structure, similar to the divalent metals calcium and strontium.
Bulk Pure lead has a bright, shiny gray appearance with a faint blue tint. It tarnishes when exposed to moist air, developing a dull surface whose color depends on environmental conditions. Lead is characterized by high density, malleability, ductility, and resistance to corrosion due to passivation.
Its close-packed face-centered cubic structure and high atomic mass give lead a density of 11.34 g/cm3, greater than that of common metals such as iron (7.87 g/cm3), copper (8.93 g/cm3), and zinc (7.14 g/cm3). This high density is the origin of the idiom to go over like a lead balloon. Some rarer metals are denser: tungsten and gold are both 19.3 g/cm3, while osmium—the densest known metal—has a density of 22.59 g/cm3, nearly twice that of lead. Lead is soft, with a Mohs hardness of 1.5, and can be scratched with a fingernail. It is very malleable and moderately ductile. Its bulk modulus—a measure of resistance to compression—is 45.8 GPa, compared with 75.2 GPa for aluminium, 137.8 GPa for copper, and 160–169 GPa for mild steel. Lead's tensile strength is low, at 12–17 MPa (around six times lower than aluminium, ten times lower than copper, and fifteen times lower than mild steel). Its strength can be increased by alloying with small amounts of copper or antimony.
Lead melts at 327.5 °C (621.5 °F), a relatively low melting point compared to most metals, and has a boiling point of 1749 °C (3180 °F), the lowest among the carbon-group elements. Its electrical resistivity at 20 °C is 192 nanoohm-meters, almost an order of magnitude higher than that of good conductors (copper: 15.43 nΩ·m; gold: 20.51 nΩ·m; aluminium: 24.15 nΩ·m). Lead becomes a superconductor below 7.19 K, which is the highest critical temperature among type-I superconductors and the third highest among the elemental superconductors.
Isotopes
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