Lead

Lead (symbol Pb, from the Latin *plumbum*) is a chemical element with atomic number 82. It is a heavy metal, denser than most everyday materials, and is soft, malleable, and melts at a relatively low temperature. When freshly cut or melted, it has a shiny, silvery appearance with a bluish tint, but it quickly dulls to a gray color upon exposure to air. Lead holds the highest atomic number of any element that is stable, and three of its naturally occurring isotopes are the final products of major radioactive decay series involving heavier elements.

As a post-transition metal, lead is fairly unreactive. Its weak metallic nature shows up in its amphoteric behavior—lead and its oxides react with both acids and bases—and it tends to form covalent bonds. Lead compounds most often occur in the +2 oxidation state, rather than the +4 state common among lighter elements in the carbon group, except in organolead compounds. Like those lighter group members, lead can bond with itself, forming chains and polyhedral structures.

Because lead is easy to extract from its ores, it was known to prehistoric people in the Near East. Its principal ore, galena, often contains silver, and the desire for silver drove widespread mining and use of lead in ancient Rome. After Rome’s fall, lead production declined and did not recover until the Industrial Revolution. Lead was crucial to the printing press, since movable type could be cast easily from lead alloys. In 2022, global annual production was about twelve million tonnes, roughly two-thirds from recycling. Lead’s high density, low melting point, ductility, and resistance to oxidation make it useful. These traits, along with its relative abundance and low cost, led to extensive use in construction, plumbing, batteries, bullets, shot, weights, solders, pewter, fusible alloys, lead paints, leaded gasoline, and radiation shielding.

Lead is a neurotoxin that builds up in soft tissues and bones. It damages the nervous system, interferes with biological enzymes, and can cause neurological disorders ranging from behavioral issues to brain damage, as well as affecting the cardiovascular and renal systems. Its toxicity was noted by Ancient Greek and Roman writers but became widely recognized in Europe only in the late 19th century.

**Physical properties**

*Atomic* A lead atom has 82 electrons, arranged as [Xe]4f¹⁴5d¹⁰6s²6p². The combined energy needed to remove its two 6p electrons (first and second ionization energies) is similar to that of tin, the element directly above it in the carbon group. This is unusual because ionization energies generally decrease down a group as outer electrons get farther from the nucleus and are more shielded. However, the sum of the first four ionization energies of lead is higher than that of tin, contrary to periodic trends. This anomaly comes from relativistic effects, which become significant in heavy atoms. These effects contract the s and p orbitals, giving lead’s 6s electrons stronger binding energies than its 5s electrons. This leads to the inert-pair effect, where the 6s electrons are less likely to bond, stabilizing the +2 oxidation state and causing unusually long distances between nearest atoms in crystalline lead.

Lighter carbon-group elements form stable or metastable allotropes with a tetrahedrally coordinated, covalently bonded diamond cubic structure. In those elements, s- and p-orbital energy levels are close enough to mix into four hybrid sp³ orbitals. In lead, the inert-pair effect increases the separation between s- and p-orbitals so much that the energy gained from hybridization cannot overcome the gap. Instead of a diamond cubic arrangement, lead forms metallic bonds where only the p-electrons are delocalized and shared among Pb²⁺ ions. As a result, lead adopts a face-centered cubic structure, like the divalent metals calcium and strontium.

*Bulk* Pure lead has a bright, shiny gray appearance with a faint blue tint. It tarnishes in moist air, developing a dull surface whose color depends on conditions. Lead is characterized by high density, malleability, ductility, and corrosion resistance 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 common metals like iron (7.87 g/cm³), copper (8.93 g/cm³), and zinc (7.14 g/cm³). This high density is the origin of the phrase “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—is 22.59 g/cm³, nearly twice lead’s density.

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 aluminum, 137.8 GPa for copper, and 160–169 GPa for mild steel. Lead’s tensile strength is low, at 12–17 MPa (about six times lower than aluminum, 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 boils at 1749 °C (3180 °F), the lowest among carbon-group elements. Its electrical resistivity at 20 °C is 192 nanoohm-meters, almost ten times higher than good conductors like copper (15.43 nΩ·m), gold (20.51 nΩ·m), and aluminum (24.15 nΩ·m). Lead becomes a superconductor below 7.19 K, the highest critical temperature among type-I superconductors and the third highest among elemental superconductors.

**Isotopes**

Natural lead consists of four stable isotopes.