Lead

Lead is a chemical element with the symbol Pb 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, it appears shiny gray 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 unreactive 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, 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.
Image:Angeln zubehoer grundblei 01.jpg|thumb|left|Lead fishing weights
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, copper, and zinc. 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. Its strength can be increased by alloying with small amounts of copper or antimony.
Lead melts at 327.5 °C, a relatively low melting point compared to most metals, and has a boiling point of 1749 °C, 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. 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

Natural lead consists of four stable isotopes with mass numbers 204, 206, 207, and 208, along with traces of six short-lived radioisotopes with mass numbers 209–214. The relatively high number of isotopes is consistent with lead's even atomic number. Lead has a magic number of protons, making its nucleus especially stable according to the nuclear shell model. Lead-208 also has 126 neutrons, another magic number, which may account for its exceptional stability.
With its high atomic number, lead is the heaviest element whose natural isotopes are considered stable; lead-208 is the heaviest stable nucleus known. This distinction previously belonged to bismuth until its sole primordial isotope, bismuth-209, was found in 2003 to decay extremely slowly. Although the four stable isotopes of lead could theoretically undergo alpha decay to mercury isotopes with an energy release, no such decay has been observed; their predicted half-lives range from 10³⁵ to 10¹⁸⁹ years, at least 10²⁵ times the current age of the universe.
File:Holsinger Meteorite.jpg|thumb|left|The Holsinger meteorite, the largest piece of the Canyon Diablo meteorite. Uranium–lead dating and lead–lead dating on this meteorite allowed refinement of the age of the Earth to 4.55 billion ± 70 million years.|alt=A piece of a gray meteorite on a pedestal
Three of lead's stable isotopes—lead-206, lead-207, and lead-208—are the end products of the three major natural decay chains: the uranium chain, the actinium chain, and the thorium chain, respectively. The isotopic composition of a rock sample depends on the presence of these parent isotopes; for example, lead-208 abundance can vary from about 52% in ordinary samples to as much as 90% in thorium ores. For this reason, the standard atomic weight of lead is reported to only one decimal place. Over time, the ratios of these isotopes to lead-204 increase as they are produced by radioactive decay. These variations allow for lead–lead and uranium–lead dating. Lead-207 exhibits nuclear magnetic resonance, a property used to study its compounds in both solution and solid states, including in biological systems such as the human body.

Chemistry

When exposed to moist air, bulk lead develops a protective surface layer of variable composition. Lead carbonate is a common constituent, and in urban or maritime environments, lead sulfate or lead chloride may also be present. This layer renders bulk lead effectively inert under atmospheric conditions. In contrast, finely powdered lead, like many metals, is pyrophoric and burns with a bluish-white flame.
Lead reacts with fluorine at room temperature to form lead fluoride. Its reaction with chlorine is similar but requires heating, as the resulting chloride layer reduces further reactivity. Molten lead combines with the chalcogens to produce lead chalcogenides.
The metal resists attack by sulfuric and phosphoric acids but not by hydrochloric or nitric acids; the difference arises from the insolubility and subsequent passivation of certain lead salts. Organic acids, such as acetic acid, dissolve lead in the presence of oxygen. Concentrated alkalis can also dissolve lead, producing plumbites.

Inorganic compounds

Lead exhibits two principal oxidation states: +4 and +2. While the tetravalent state is characteristic of the carbon group, the divalent state is rare for carbon and silicon, less common for germanium, significant but not dominant for tin, and the most prevalent for lead. This predominance is linked to relativistic effects—specifically the inert pair effect—which occurs when there is a large electronegativity difference between lead and anions such as oxide, halide, or nitride. In such cases, lead develops a pronounced partial positive charge, causing a stronger contraction of the 6s orbital compared to the 6p orbital and rendering it relatively unreactive in ionic compounds. The inert pair effect is less pronounced in compounds where lead forms covalent bonds with elements of similar electronegativity, such as carbon in organolead compounds. In these, the 6s and 6p orbitals remain comparable in size, and sp³ hybridization remains energetically favorable, making lead predominantly tetravalent in such cases.
The electronegativity values further reflect this behavior: lead has a value of 1.87, and lead has 2.33. This represents a reversal in the general trend of increasing stability of the +4 oxidation state down the carbon group; by comparison, tin has electronegativities of 1.80 and 1.96.