|Name, symbol||lead, Pb|
|Lead in the periodic table|
|Atomic number (Z)||82|
|Group, block||group 14 (carbon group), p-block|
|Element category||post-transition metal|
|Standard atomic weight (±) (Ar)||207.2(1)|
|Electron configuration||[Xe] 4f14 5d10 6s2 6p2|
|2, 8, 18, 32, 18, 4|
|Melting point||600.61 K (327.46 °C, 621.43 °F)|
|Boiling point||2022 K (1749 °C, 3180 °F)|
|Density near r.t.||11.34 g/cm3|
|when liquid, at m.p.||10.66 g/cm3|
|Heat of fusion||4.77 kJ/mol|
|Heat of vaporization||179.5 kJ/mol|
|Molar heat capacity||26.650 J/(mol·K)|
|Oxidation states||4, 3, 2, 1, −1, −2, −4 (an amphoteric oxide)|
|Electronegativity||Pauling scale: 1.87|
|Ionization energies||1st: 715.6 kJ/mol
2nd: 1450.5 kJ/mol
3rd: 3081.5 kJ/mol
|Atomic radius||empirical: 175 pm|
|Covalent radius||146±5 pm|
|Van der Waals radius||202 pm|
|Crystal structure||face-centered cubic (fcc)|
|Speed of sound thin rod||1190 m/s (at r.t.) (annealed)|
|Thermal expansion||28.9 µm/(m·K) (at 25 °C)|
|Thermal conductivity||35.3 W/(m·K)|
|Electrical resistivity||208 nΩ·m (at 20 °C)|
|Magnetic susceptibility (χmol)||−23.0·10−6 cm3/mol (298 K)|
|Young's modulus||16 GPa|
|Shear modulus||5.6 GPa|
|Bulk modulus||46 GPa|
|Brinell hardness||38–50 MPa|
|Discovery||Middle Easterns (7000 BCE)|
|Most stable isotopes of lead|
Lead (//) is a chemical element with atomic number 82 and symbol Pb (from Latin: plumbum). It is a soft, malleable, and heavy metal. Freshly cut lead has a bluish-white color that soon tarnishes to a dull grayish color when exposed to air; as a liquid, it has a shiny chrome-silver luster. Lead's density of 11.34 g/cm3 exceeds that of most common materials. It has the second highest atomic number of all practically stable elements. As such, lead is located at the end of some decay chains of heavier elements, which in part accounts for its relative abundance: it exceeds those of other similarly-numbered elements.
Lead is a post-transition metal and is relatively inert unless powdered. Its weak metallic character is illustrated by its general amphoteric nature: lead and lead oxides react with both acids and bases. Lead also displays a marked tendency toward covalent bonding. Its compounds are most commonly found in the +2 oxidation state, rather than +4, unlike the lighter group 14 elements. Exceptions are mostly limited to organolead compounds, where the positive charge on lead is dispersed and stabilized. Like the lighter group 14 elements, lead shows a tendency to bond to itself, forming complicated chain, ring, or polyhedral structures.
Lead is easily extracted from ore, and it was known to prehistoric people in Western Asia. A principal ore of lead, galena, often bears silver, and this helped initiate lead production in ancient Rome, when lead became widely available. After the fall of Rome, lead production declined and did not reach levels seen during ancient Rome until the Industrial Revolution. Today, lead is produced in quantities of around ten thousand tonnes annually; secondary production from recycling is gaining ground, accounting for around 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 its relative abundance and low cost, these factors have led to its widespread employment. Lead is used in building construction, lead–acid batteries, bullets and shot, weights, as part of solders, pewters, fusible alloys, and as a radiation shield. Lead was established as poisonous in the late nineteenth century, and this is why it is being phased out for some applications. If ingested or inhaled, lead and its compounds are poisonous to animals and humans. Lead is a neurotoxin that accumulates in soft tissues and bones, damaging the nervous system and causing brain disorders. Lead can also cause blood disorders in mammals.
- 1 Etymology
- 2 Properties
- 3 Chemistry
- 4 Origin and occurrence
- 5 History
- 6 Production
- 7 Applications
- 8 Biological and environmental effects
- 9 See also
- 10 Notes
- 11 References
- 12 Bibliography
- 13 Further reading
- 14 External links
The modern English word "lead" is of Germanic origin; it comes from the Middle English leed and Old English lēad (with the macron above the "e" signifying that the vowel sound of that letter is long). The Old English word is derived from the hypothetical reconstructed Proto-Germanic *lauda- ("lead"). According to accepted linguistic theory, this word bore descendants in most Germanic languages of exactly the same meaning: the major exception is German, which used a different root instead.
The origin of the Proto-Germanic *lauda- is not agreed within the linguistic community. One hypothesis suggests it is derived from Proto-Indo-European *lAudh- ("lead"; capitalization of the vowel is equivalent to the macron). Another hypothesis suggests it is borrowed from Proto-Celtic *ɸloud-io- ("lead"). This word is related to the Latin plumbum, which gave the element its chemical symbol Pb. The word *ɸloud-io- may also be the origin of Proto-Germanic *bliwa- (which also means "lead"), from which stemmed the German Blei.
The name of the chemical element is not related to the verb of the same spelling, which is instead derived from (eventually) Proto-Germanic *laidijan- ("to lead").
A neutral lead atom has 82 electrons, arranged in an electronic configuration of [Xe]4f145d106s26p2. The combined first and second ionization energy of lead—the total energy required to remove the two 6p electrons from a lead atom—is close to that of tin, its upper neighbor in group 14. This is unusual since ionization energies generally fall going down a group as an element's electrons become more distant from its nucleus. The similarity is attributable to the lanthanide contraction— the decrease in element radii from atomic number 57, lanthanum, to 71, lutetium, and relatively small ionic radii for the subsequent elements starting with 72, hafnium. This contraction results from poor shielding of the nucleus by the lanthanide 4f electrons; the outer electrons are drawn towards the nucleus, thus resulting in a smaller atomic radius. The combined first four ionization energies of lead exceed those of tin, contrary to what the periodic trends would predict. For this reason lead, unlike tin, rarely has a +4 oxidation state in inorganic compounds. Such behavior is attributable to relativistic effects, which become particularly prominent at the bottom of the periodic table; the result is that the 6s electrons of lead become reluctant to participate in bonding,[a] a phenomenon referred to as the inert pair effect.
Aside from lead, the lighter elements in group 14 have a stable or metastable allotrope in which they crystallize in the diamond cubic structure, involving covalent bonds. In this structure, each atom is tetrahedrally coordinated, indicating that all four bonds are equivalent, having each attained the lowest possible energy. Orbital hybridization is invoked to explain this phenomenon. Despite the fact that two of the electrons are in s-orbitals and the other two in higher-energy p-orbitals, one electron is "promoted" from an s-orbital to a p-orbital, and then all form four intermediate hybrid orbitals in a process called sp3 hybridization. In lead, on the other hand, the inert pair effect means that the promotion energy of a 6s-electron becomes larger than the amount of energy that would be released from the additional bonds formed. Thus, rather than having the diamond-cubic covalent structure, lead forms metallic bonds, in which only the p-electrons are delocalized and shared between the Pb2+ ions, resulting in a face-centered cubic structure like those of the similarly-sized divalent calcium and strontium.
Freshly prepared or fractured lead has a bright silvery appearance with a very slight hint of blue. Lead tarnishes on contact with moist air, forming a complex surface mixture of compounds whose color and composition will vary depending on the prevailing conditions. The characteristic properties of lead include high density, softness, malleability, ductility, poor electrical conductivity compared to other metals, high resistance to corrosion (conferred by its surface patina), and a propensity to react with organic reagents.
Lead's face-centered cubic structure and high atomic weight give it a high density of 11.34 g/cm3. This figure exceeds that of common metals such as iron (7.87 g/cm3), copper (8.93 g/cm3), and zinc (7.14 g/cm3). Some rarer metals are denser: tungsten and gold are both 19.3 g/cm3, while the densest metal known—osmium—has a density of 22.59 g/cm3, almost twice that of lead. The high density of lead is behind the idiom go down like a lead balloon.
Lead is a very soft metal with a Mohs hardness of 1.5; it can be scratched with a fingernail. It is malleable and ductile[b], with its malleability exceeding its ductility. The compressive strength of lead is high and it can therefore be rolled into extremely thin sheets. The bulk modulus—a measure of the ease of compressibility of a material—of lead is 45.8 GPa. (For comparison, that of aluminium is 75.2 GPa; copper 137.8 GPa; and mild steel 160–169 GPa.) Lead's tensile strength is comparatively low: 12–17 MPa (that of aluminium is 6 times higher; copper 10 times higher; mild steel 15 times higher); this value is easily improved by adding small concentrations of other metals or metalloids, such as copper or antimony.
The melting point of lead—at 327.5 °C (621.5 °F),—is considered low from an industrial perspective.[c] Its boiling point is 1749 °C (3180 °F). The electrical resistivity of lead at 20 °C is 208 nano-ohm-meters; this almost an order of magnitude higher than those of other industrially useful metals (that of copper is 17.12 nΩ·m; gold 22.55 nΩ·m; aluminium 27.09 nΩ·m). Lead is a superconductor at temperatures lower than 7.19 K; this is the highest critical temperature of all type-I superconductors and the third highest of all elemental superconductors.
Bulk lead exposed to moist air forms a protective layer of varying composition. A common reaction is the formation of the oxide which in turn reacts with carbon dioxide to give lead carbonate. Other insoluble compounds, such the sulfate or chloride, may form the protective layer in differing chemical environments.
Fluorine reacts with lead at room temperature, forming lead(II) fluoride. The reaction with chlorine is similar, but requires heating: the chloride layer diminishes the reactivity of the elements. Molten lead reacts with the chalcogens to give lead(II) chalcogenides.
The presence of carbonates or sulfates results in the formation of insoluble lead salts, which protect the metal from corrosion. So does carbon dioxide, due to the formation of insoluble lead carbonate; however an excess of the gas will result in the formation of soluble lead bicarbonate, which makes the use of lead pipes dangerous. Water in the presence of oxygen attacks lead and starts an self-fueling reaction. Lead also dissolves in concentrated alkalis thanks to its ability to form anions—plumbites—in solution and the general solubility of the said anions.
Lead is not attacked by dilute sulfuric acid; the concentrated acid dissolves the metal thanks to complexation. Lead reacts slowly with hydrochloric acid; nitric acid reacts vigorously to form nitrogen oxides and lead(II) nitrate. Organic acids, such as acetic acid, dissolve lead, but this reaction requires the presence oxygen.
Lead has four stable isotopes, lead-204, lead-206, lead-207, and lead-208. The high number of stable isotopes relies on the fact that lead's atomic number of 82 is even, and is a magic number.[d] With its high atomic number, lead is the second-heaviest element that occurs naturally in the form of isotopes regarded as stable: bismuth has a higher atomic number of 83, but its only primordial isotope was found in 2003 to be very slightly radioactive.[e] The four stable isotopes of lead could theoretically undergo alpha decay to isotopes of mercury with a release of energy, but this has not been observed for any of them: accordingly, their predicted half-lives are extremely long, ranging up to over 10100 years.[f] As such, lead is often quoted as the heaviest stable element.
Three of these isotopes are also found in three of the four major decay chains: lead-206, lead-207, and lead-208, are the final decay products of uranium-238, uranium-235, and thorium-232, respectively; the decay chains are called the uranium series, actinium series, and thorium series. Since the amounts of them in nature depend on the presence of other elements, the isotopic composition of natural lead varies between samples: in particular, the relative amount of lead-206 may vary between 20.84% and 27.78%, and the abundance of lead-208 may vary between 52.4% in normal samples to 90% in thorium ores. (For this reason, the atomic weight of lead is given to only one decimal place.) As time passes, the ratio of lead-206 and lead-207 to lead-204 increase, since the former two are supplemented by radioactive decay of heavier elements and the latter is not; this allows for lead–lead dating. Analogously, as uranium decays (eventually) into lead, their relative amounts change; this allows for uranium–lead dating.
Apart from the stable isotopes, which make up almost all lead that exists naturally, there are trace quantities of a few radioactive isotopes. One of them is lead-210; although it has a half-life of 22.3 years, a period too short to allow any primordial lead-210 to exist, some small non-primordial quantities of it occur in nature, because lead-210 is found in the uranium series: thus, even though it constantly decays away, it is constantly regenerated by decay of its parent, polonium-214, which, while also constantly decaying, is also supplied by decay of its parent, and so on, all the way up to original uranium-238, which has been present for billions of years on Earth. Lead-210 is particularly useful for helping to identify ages of samples containing it, which is performed by measuring lead-210 to lead-206 ratios (both isotopes are present in a single decay chain). Lead-214 is also present in the decay chain of natural uranium-238, lead-212 is present in that of natural thorium-232, and lead-211 is present in that of natural uranium-235; therefore, traces of all three of these isotopes exist naturally as well. Lastly, very minute traces of lead-209 are also present from the cluster decay of radium-223, one of the daughter products of natural uranium-235. Hence, natural lead consists of not only the four stable isotopes, but also minute traces of another five short-lived radioisotopes.
In total, thirty-eight isotopes of lead have been synthesized, with mass numbers of 178–215. Lead-205 is the most stable radioisotope of lead, with a half-life of around 1.5×107 years.[g] The second-most stable radioisotope is the synthetic lead-202, which has a half-life of about 53,000 years, longer than any of the natural trace radioisotopes. Additionally, 47 nuclear isomers (long-lived excited nuclear states), of 24 lead isotopes, have been characterized. The longest-lived isomer is lead-204m2 with a half-life of about 1.1 hours).
Lead shows two main oxidation states: +4 and +2. The tetravalent state is common for group 14. The divalent state is rare for carbon and silicon, minor for germanium, important (but not prevailing) for tin, and is the more important for lead: even the strongest oxidizing agents, oxygen and fluorine, initially oxidize lead only to lead(II). This is caused by relativistic effects, specifically the inert pair effect, which manifests itself when there is a large difference in electronegativity between lead and, for example, oxide, halide, or nitride anions, leading to a significant partial positive charge on lead. The result is a stronger contraction of the lead 6s orbital than is the case for the 6p orbital, making it rather inert in ionic compounds. This is not quite as applicable to compounds in which lead forms covalent bonds to elements of similar electronegativity such as carbon in organolead compounds. Here the 6s and 6p orbitals remain similarly sized and sp3 hybridization in compounds is still energetically favorable; as such, lead, like carbon, is predominantly tetravalent in organolead compounds. The 5s electron pair tends to be stereochemically active in tin(II) compounds, but is much less so in lead(II) compounds. Consequently, there are often structural similarities between lead(II) compounds and analogous compounds of the divalent cations of calcium, strontium, barium, europium, and ytterbium.
The electrode potential of lead shows that it is only slightly easier to oxidize than hydrogen. Lead can therefore dissolve in acids, but this is often impossible due to factors such as the formation of insoluble salts. Electronegativity, although often thought to be constant for each element, is a variable property; lead shows a high electronegativity difference between values for lead(II) and lead(IV) of —1.87 and 2.33, respectively. This difference marks a reversal in the trend of increasing stability of the +4 oxidation state down group 14; tin, by comparison, has electronegativities of 1.80 and 1.96 in the +2 and +4 oxidation states.
Lead(II) compounds are characteristic of the inorganic chemistry of lead. Even strong oxidizing agents like fluorine and chlorine react with lead at room temperature to give only PbF2 and PbCl2. Lead forms binary compounds with many nonmetals, but not all of them; for example there is no known lead carbide.
Most lead(II) compounds are ionic, but they are not as ionic as those of many other metals. In particular, many lead(II) compounds are water-insoluble. In solution, lead(II) ions are colorless, but under specific conditions, lead is capable of changing its color. Unlike tin(II) ions, they do not react as reducing agents in solution. Lead(II) ions partially hydrolyze in aqueous solution to form Pb(OH)+ and finally Pb4(OH)4 (in which the hydroxyls ions act as bridging ligands).
Lead monoxide exists in two allotropes, red α-PbO and yellow β-PbO, the latter being stable only above around 488 °C. It is the most commonly used compound of lead. Its hydroxide counterpart, lead(II) hydroxide, is not capable of existence outside of solution; in solution, it is known to form plumbite anions. Lead commonly reacts with the heavier chalcogens. Lead sulfide can only be dissolved in strong acids. It is a semiconductor, a photoconductor, and an extremely sensitive infrared radiation detector. A mixture of the monoxide and the monosulfide, when heated, forms the metal. The other two chalcogenides are likewise photo-conducting. They are quite unusual in that their color becomes lighter down the group.
Lead dihalides are well-characterized; this includes the diastatide, and mixed examples, such as PbFCl. The relative insolubility of the latter forms a useful basis for the gravimetric determination of fluorine. The difluoride was the first ionically conducting compound to be discovered (in 1838, by Michael Faraday). The other dihalides decompose on exposure to ultraviolet or visible light, especially the diiodide. Many pseudohalides are also known. Lead(II) forms a tremendous variety of coordination complexes, such as [PbCl4]2−, [PbCl6]4−, and the chain anion [Pb2Cl9]n5n−, although most of them are not yet adequately characterized structurally.
Lead(II) sulfate is well known for its insolubility in water, like the sulfates of the other heavy divalent cations; lead(II) nitrate and lead(II) acetate, in contrast, are very soluble, and this property is exploited in the synthesis of other lead compounds.
Few inorganic lead(IV) compounds are known, and they are typically strong oxidants or exist only in highly acidic solutions. Lead(II) oxide gives a mixed oxide on further oxidation, Pb
4. It is described as lead(II,IV) oxide, or structurally 2PbO•PbO
2, and is the best-known mixed valence lead compound. Lead dioxide is a strong oxidizing agent, capable of oxidizing hydrochloric acid to chlorine gas. This is because the expected PbCl4 that would be produced is unstable and spontaneously decomposes to PbCl2 and Cl2. Analogously to lead monoxide, lead dioxide is capable of forming plumbate anions. Lead tetrafluoride, a yellow crystalline powder, is stable, but less stable than the difluoride. Lead tetrachloride (a yellow oil) decomposes at room temperature, lead tetrabromide is less stable still and the existence of lead tetraiodide is questionable. Lead disulfide, like the monosulfide, is a semiconductor. Lead(IV) selenide is also known.
Other oxidation states
Some lead compounds exist in formal oxidation states other than +4 or +2. Lead(III) may be obtained as an intermediate between lead(II) and lead(IV), in larger organolead complexes (rather than on its own). This oxidation state is not specifically stable, as the lead(III) ion (and, consequently, the larger complexes containing it) is a radical; the same applies for lead(I), which can also be found in such species.
Negative oxidation states can occur as Zintl phases, as either free lead anions, for example, in Ba
2Pb, with lead formally being lead(−IV), or in oxygen-sensitive cluster ions, for example, in a trigonal bipyramidal Pb2−
5 ion, where two lead atoms are lead(−I) and three are lead(0). In such anions, each atom is at a polyhedral vertex and contributes two electrons to each covalent bond along an edge from their sp3 hybrid orbitals, the other two being an external lone pair. They may be made in liquid ammonia via the reduction of lead by sodium 
Many mixed lead(II,IV) oxides are known. When PbO2 is heated in air, it becomes Pb12O19 at 293 °C, Pb12O17 at 351 °C, Pb3O4 at 374 °C, and finally PbO at 605 °C. A further sesquioxide Pb2O3 can be obtained at high pressure, along with several non-stoichiometric phrases. Many of them show defect fluorite structures in which some oxygen atoms are replaced by vacancies: for instance, PbO can be considered as such a structure with every alternate layer of oxygen atoms absent.
Lead can form long singly- or multiply-bonded chains—catenas—and so shares some covalent chemistry with its lighter homolog carbon. This tendency is much lower for lead because the Pb–Pb bond energy (98 kJ/mol) is much lower than that of the C–C bond (356 kJ/mol). Lead atoms can build metal–metal bonds of an order up to three. Lead also forms covalent bonds with carbon to produce organolead compounds similar to, but generally less stable than, typical organic compounds, as the Pb–C bond is rather weak. Nevertheless, the organometallic chemistry of lead is far less wide-ranging than that of tin. Almost all are organolead(IV) compounds. Very few organolead(II) compounds are known: even starting with inorganic lead(II) reactants always results in organolead(IV) products. The most well-characterized exceptions are the purple bis(disyl)plumbylene, Pb[CH(SiMe)3)2]2 and lead cyclopentadienide, Pb(η5-C5H5)2.
The simplest organic compound of lead is plumbane, the analog of methane. Plumbane may be obtained in a reaction between metallic lead and atomic (not molecular) hydrogen. Plumbane is unstable but two simple derivatives, tetramethyllead and tetraethyllead, are the best-known organolead compounds. They may be made by the addition of trimethyllead or triethyllead to alkenes or alkynes; these precursors may themselves be made from the corresponding lead halides and lithium aluminium hydride at −78 °C. These compounds are relatively stable—tetraethyllead only starts to decompose at 100 °C (210 °F)—or if exposed to sunlight or ultraviolet light. (Tetraphenyllead is even more thermally stable, decomposing only at 270 °C.) With sodium metal, lead readily forms an equimolar alloy that reacts with alkyl halides to form organometallic compounds such as tetraethyllead. The oxidizing nature of many organolead compounds is usefully exploited: lead tetraacetate is an important laboratory reagent for oxidation in organic chemistry; tetraethyllead was once produced in larger quantities than any other organometallic compound. Other organolead compounds, including homologs of said compounds, are less chemically stable; a lead analog of the next alkane—ethane—is not even known. Polyplumbanes are not well-characterized and are generally highly thermally unstable and reactive.
Origin and occurrence
Lead is not a common element in general—its per-particle abundance in the universe is 0.06 ppb (parts per billion). Even so, it is three times as abundant as platinum, ten times that of mercury, and twenty times more common than gold. The amount of lead in the universe is increasing, although noticeably only from the perspective of millions of years. The cause is nuclides with mass numbers 232, 235, and 238 and above that soon decay to the extremely long-lived isotopes of thorium and uranium which, in turn, very slowly decay to lead.
Primordial lead—which comprises the isotopes lead-204, lead-206, lead-207, and lead-208—was mostly created as a result of repetitive neutron capture processes occurring in stars. The two main modes of capture are the s-process and the r-process.
In the s-process (s is for "slow"), captures are separated by years or decades, allowing less stable nuclei to beta decay. For example, a stable thallium-203 nucleus captures a neutron and becomes thallium-204; this is unstable, and undergoes beta decay to give stable lead-204; on capturing another neutron, it becomes lead-205, which is stable enough to generally last longer than a capture takes (its half-life is around 15 million years). Further captures result in lead-206, lead-207, and lead-208. On capturing another neutron, lead-208 becomes lead-209, which quickly decays into bismuth-209 which, on capturing another neutron, becomes bismuth-210 and which either undergoes alpha decay into thallium-206 (which would beta decay into lead-206) or beta decay to yield polonium-210 (which would inevitably alpha decay into lead-206). The cycle ends at lead-206, lead-207, lead-208, and bismuth-209.
In the r-process (r is for "rapid"), captures happen faster than nuclei can decay. This occurs in environments with a high neutron density, possibly in a supernova or during the merger of two neutron stars. The neutron flux involved may be on the order of 1022 neutrons/(cm2·second). The r-process does not form as much lead as the s-process. This is because the r-process tends to stop once very neutron-rich nuclei reach 126 neutrons. At this point the neutrons are arranged in complete shells within the atomic nucleus and it becomes harder to energetically accommodate more of them. When the neutron flux subsides, these nuclei beta decay into stable isotopes of osmium, iridium and platinum.
Lead commonly reacts with sulfur (see Lead(II)) and, as such, is classified as a chalcophile under the Goldschmidt classification. Many lead minerals are relatively light and have remained in the Earth's crust. Lead is easily extracted from ore, and, indeed, the mineral form of lead sulfide, galena, has been known for millennia, as was the metal itself (see below). Lead's chalcophilic character is close to those of zinc and copper; as such, it is usually extracted together with these metals. Metallic lead occurs in nature but is rare. As a result of lead's chemistry, it occurs in primary minerals exclusively as lead(II), unlike tin, which always occurs as tin(IV).[h] Lead ore can be found in hydrothermal vein, impregnation, and replacement deposits; and in volcanogenic, and hydrothermal or marine sedimentary deposits. World lead resources exceed 2 billion tons. Significant deposits are located in Australia, China, Ireland, Mexico, Peru, Portugal, Russia, and the United States. World reserves—resources that are economically feasible to extract—totaled 89 million tons in 2015, of which Australia had 35 million, China 15.8 million, and Russia 9.2 million.
The main lead-bearing mineral is galena (PbS), which is mostly found with zinc ores. Most other lead minerals are related to galena in some way; for example, boulangerite, Pb
11, is a mixed sulfide derived from galena; anglesite, PbSO
4, is a product of galena oxidation; and cerussite or white lead ore, PbCO
3, is a decomposition product of galena. Zinc, copper, arsenic, tin, anitmony, silver, gold, and bismuth are common impurities in lead minerals.
Lead has been used for thousands of years because it is widespread, easy to extract and work with. Metallic lead beads dating back to at least 7000–6500 BCE have been found in Asia Minor and may represent the first example of metal smelting. At this time lead had few (if any) applications due its softness and dull appearance. The major reason for the spread of lead production, rather than its utility, was its association with silver, which may be obtained by burning galena, a widespread lead mineral. The Ancient Egyptians and they were the first to use lead in cosmetics, an application that would spread to Ancient Greece and beyond; the Egyptians might have used lead for sinkers in fishing nets, in glazes, glasses and enamels, and for ornaments. Various civilizations of the Fertile Crescent used lead as a writing material, as currency, and for construction. Lead was used in the Ancient Chinese royal court as a stimulant, as currency, and as a contraceptive; lead was also used for making amulets by the Indus Valley civilization the Mesoamericans, and by eastern and southern Africa peoples in wire drawing.
Because silver was extensively used as a decorative material and an exchange medium, lead deposits came to be worked in Asia Minor from 3000 BCE and, subsequently, from 2000 BCE in the Iberian peninsula by the Phoenicians; and in Athens, Carthage, and Sicily. Rome's territorial expansion in Europe and across the Mediterranean, and its concurrent development of mining, led to it becoming the greatest producer of lead during the classical era, with an estimated annual output peaking at 80,000 tonnes. The Romans obtained lead mostly as a by-product of extensive silver smelting. Lead mining occurred in Central Europe, Britain, the Balkans, Greece, Anatolia, and Hispania, which alone accounted for 40% of world production. Lead was used for making water pipes in the Roman Empire, as it was formable and resistant to corrosion. Consequently, the Latin word for the metal, plumbum, was the origin of the periodic table symbol Pb and the English word "plumbing" and its derivatives—even though some Romans, such as Vitruvius, were able to recognize the health dangers of lead. Some researchers have suggested that lead poisoning was one of the reasons behind the fall of Rome.[i] Lead poisoning—a condition in which one becomes dark and cynical—was called "saturnine", after the ghoulish god of Saturn; the metal was also considered the father of all metals. It was easily available in the Roman society, and as such, its social status was low.
During the ancient and classical eras, and even until the 17th century, tin was often not clearly distinguished from lead: Romans called lead plumbum nigrum (literally, "black lead"), while tin was called plumbum candidum (literally, "bright lead"). The association of lead and tin can be seen in other languages: the word olovo in Czech translates to "lead", but in Russian the cognate олово (olovo) means "tin". Lead also bore a close relation to antimony: both elements commonly occur as sulfides (galena and stibnite), often together. Pliny declared that stibnite would give lead on heating, whereas the mineral produced on heating was actually antimony. The originally South Asian surma—"galena" in English—spread across Asia with that meaning, and gave its name to antimony in a number of Central Asian languages, and in Russian.
After the fall of the Western Roman Empire and into the medieval era, lead continued to be used in plumbing in Western Europe, but lead mining in Europe declined, with the only region having a significant production being Arabian Iberia. The largest production of lead occurred in South and East Asia, especially China and India, where lead output underwent a strong growth. In Europe, lead production only began to revive in the 11th and 12th centuries, and it was again used for roofing and piping; from the 13th century, it was used to create stained glass. During the period, lead was used increasingly for adulterating wine. This practice was declared forbidden in 1498 by a papal bull, but it continued long past that time and resulted in numerous mass poisonings up to late 18th century. Lead was a key material in parts of the printing press, which was invented around 1440, and lead dust was commonly inhaled by press operators, causing lead poisoning. Firearms were invented at around the same time, and lead, despite being more expensive than iron, became the chief material for making bullets because it was less damaging to iron gun barrels, had a higher density (which allowed for better retention of velocity); lead's lower melting point made the production of bullets easier because they could be made using a wood fire. Lead was extensively used in cosmetics by Western European aristocracy, as whitened faces were seen as a sign of modesty. The practice eventually expanded to white wigs and eyeliners, and only faded out with the French Revolution in the late 18th century; one effect of such prolonged contact with lead was rotting teeth; replacements were often made of lead (which temporarily gave a sweet breath), inducing further damage to the person. A similar fashion appeared in Japan in the 18th century with the emergence of the geishas, a practice that continued long into the 20th century. The white face become a "symbol of a Japanese woman"; lead was commonly used as the whitener.)
In the New World, lead was produced soon after the arrival of European settlers. The earliest recorded lead production dates to 1621, in the Colony of Virginia that had been founded fourteen years earlier. In Australia, colonists opened the first mine on the continent—a lead mine—in 1841. Centuries before the Europeans were able to start colonizing Africa in the late 19th century, lead mining was known in the Benue Trough and the lower Congo basin, where lead was used for trade with the Europeans and as a currency.[j]
In the second half of the 18th century, Britain and later continental Europe and then the United States experienced the Industrial Revolution. During the period, lead mining proved important; the Industrial Revolution was the first time during which lead production rates exceeded those of Rome. Britain was the leading producer, losing this status by the mid-19th century with the depletion of its mines and the development of lead mining in Germany, Spain, and the United States. Lead production in the United States dominated by 1900; other non-European nations—in particular, Canada, Mexico, and Australia—started massive lead production activities, and by 1900, Europe's output of lead fell below that elsewhere. A great share of the demand for lead came from plumbing and painting—lead paints had been invented and were regularly used; with invention of gasoline in the late 19th century, lead was extensively used as an additive. At this time, more people—the working class—contacted the metal; lead poisoning cases escalated. This led to research into the effects of lead intake: lead was proven to be more dangerous in its fume form than as a solid metal; lead poisoning and gout were linked (Alfred Baring Garrod noted a third of his gout patients were plumbers and painters); the effects of chronic ingestion of lead, including mental disorders, were all studied in the 19th century. The first laws to decrease the degree of lead poisoning in factories followed during the 1870s and 1880s in the United Kingdom.
Further evidence of the threat that lead posed to humans was discovered in the late 19th and early 20th centuries—mechanisms of harm were better understood, and lead blindness was documented—and countries in Europe and the United States started efforts to reduce the amount of lead that people came into contact with. The last major innovation to impose contact with lead on humans was adding tetraethyllead to gasoline, a practice originating in the United States in 1921; it was phased out in the U.S. and the European Union by 2000. Most European countries banned usage of lead paint for interiors by 1930. The result of many regulations and bans put on lead products was significant: in the last quarter of the 20th century, the percentage of people with excessive lead blood levels dropped from over three quarters of the population to slightly over two percent in the U.S. By the end of the 20th century, the main product made of lead was the lead–acid battery, which possesses no direct threat to humans. This facilitated consistent lead production in industrialized countries. From 1960 to 1990, lead output in the Western Bloc grew by 31%. The share of the world's lead production by the Eastern Bloc increased from 10% to 30% from 1950 to 1990, with the Soviet Union being world's largest producer during the mid-1970s and the 1980s, and China starting massive lead production in the late 20th century. Unlike the European communist countries, China was largely unindustrialized by the mid-20th century; in 2004, China surpassed Australia as the largest producer of lead. Like the experience of European industrialization, lead has had a negative effect on health in China.
Production and consumption of lead is increasing worldwide. There are two major categories of production: primary, from minded ores; and secondary from scrap. In 2013, 4.74 million metric tons came from primary production and 5.74 million tons from secondary production. The top mining countries for lead in that year were China, Australia, Russia, India, Bolivia, Sweden, North Korea, South Africa, Poland, and Ireland. The top lead producing countries were China, United States, India, South Korea, Germany, Mexico, United Kingdom, Canada, Japan, and Australia. According to the International Resource Panel's Metal Stocks in Society report of 2010, the global per capita stock of lead in use in society is 8 kg. Much of this is in more developed countries (20–150 kg per capita) rather than less developed countries (1–4 kg per capita).
Production processes for primary and secondary lead are similar. Some primary production plants now also use scrap, and this trend is likely increase in the future. Given adequate techniques, secondary lead is indistinguishable from primary lead. Scrap lead from the building trade is usually fairly clean and is re-melted without the need for smelting, though some refining may be necessary; as such, secondary lead is cheaper to produce than primary in terms of energy spent on production, often by 50% or more.
Most lead ores contain only a very low percentage of lead, which must be concentrated during processing. During initial processing, ores typically undergo crushing, dense-medium separation, grinding, froth flotation, and drying. The resulting concentrate, which has a lead content fraction of 30–80%,  is then turned into (impure) lead metal. The main route for doing so involves a two-stage process. First, the sulfide concentrate is roasted in the air, in order to oxidize the lead sulfide:
- 2PbS + 3O2 → 2PbO + 2SO2↑
As the original concentrate was not pure lead sulfide, roasting yields lead oxide and a mixture of sulfates and silicates of lead and other metals contained in the ore. This impure lead oxide is reduced in a coke-fired blast furnace to the (again, impure) metal: 
- 2PbO + C → Pb + CO2↑
Research on a cleaner less energy intensive process continues, with some success; a major drawback is that the alternative results in either an exceedingly high sulfur content in the resulting lead metal, or too much lead is lost as waste. A promising alternative involves direct smelting without an intermediate compound involved; hydrometallurgical extraction, in which anodes of impure lead and a cathodes of pure lead are dissolved in an electrolyte) is another technique that is being explored.
Impurities in the resulting metal are still significant; these are mostly contaminants of arsenic, antimony, bismuth, zinc, copper, silver, and gold. The melt is treated in a reverberatory furnace with air, steam, and sulfur, which oxidizes the contaminants except for silver, gold, and bismuth. The oxidized contaminants are removed by drossing, where they float to the top and are skimmed off. Since lead ores contain significant concentrations of silver, the smelted metal is commonly contaminated with silver. Metallic silver as well as gold is removed and recovered economically by means of the Parkes process, in which zinc is added to lead and adsorbs silver, which dissolves in zinc many times more actively than in lead. De-silvered lead is freed of bismuth according to the Betterton–Kroll process by treating it with metallic calcium and magnesium, which forms a bismuth dross that can be skimmed off. Very pure lead can be obtained by processing smelted lead electrolytically by means of the Betts process. The process uses anodes of impure lead and cathodes of pure lead in an electrolyte of silica fluoride. Once electrical potential is applied, impure lead at the anode dissolves and plates out on the cathode, while the impurities remain in solution.
Smelting, an essential part of the primary production, is often skipped during secondary production. The reason for this is that scrap lead itself is commonly reduced to its metallic form. As such, smelting is only performed when metallic lead had undergone significant chemical transformation, such as oxidation or rusting. When smelting is performed, the process is similar to that of the primary production in either a blast furnace or a rotary furnace (though both, with the essential difference being the greater variability of what could be extracted as the final product after the latter). The Isasmelt process is a more recent method that may act as an extension to primary production; the essence of this process is that the input battery paste is deprived of its sulfur content (by, for example, treating it with alkalies) and then treated in a coal-fueled furnace in the presence of oxygen, which eventually yields impure lead, with antimony being the most common impurity.
Refining of secondary lead is similar to that of primary lead; some refining processes may be skipped depending on the material recycled and its potential contamination, with bismuth and silver most commonly being accepted as impurities.
Of the sources of lead for recycling, lead–acid batteries is the most important one; lead pipe, sheet, and cable sheathing are other significant sources.
Contrary to popular belief, pencil leads in wooden pencils have never been made from lead. When the pencil originated as a wrapped graphite writing tool, the particular type of graphite being used was named plumbago (lit. act for lead, or lead mockup).
Lead metal has a number of useful mechanical properties: high density, low melting point, ductility, and relative inertness. While many metals are superior to lead in some of these aspects, lead is more common than most of these metals; moreover, lead minerals are easier to mine and prcoess than many other metals. One disadvantage of using lead is its toxicity, which explains why is has been or is being phased out for some uses.
Lead has been used for bullets since their invention (see above); with the development of firearms, round bullets became pointed and later, lead was jacketed with, for example, copper. Lead is sometimes alloyed with tin or antimony: this increases the cost and time of making the bullet, but increases its hardness (thereby making the bullet more effective against hard targets), reduces tension on the gun barrel and does not contaminate it with lead, as simple lead bullets do. Concerns have been raised over whether lead bullets used for hunting can damage the environment.[k]
Because of its high density and resistance to corrosion, lead is used as ballast in sailboat keels. Its high density allows it to counterbalance the heeling effect of wind on the sails while at the same time occupying a small volume and thus minimizing underwater resistance. On a related note, lead is used in scuba diving weight belts to counteract the diver's buoyancy.
Lead is alloyed with copper and its alloys (namely, brass and bronze) to increase their machinability and to reduce machine tool wear. Rather than forming a solid solution with copper, lead forms granules within copper. It acts like a lubricant and, in low concentrations, also as a chip breaker.
Lead is used to form glazing bars for stained glass or other multi-lit windows. The practice has become less common, not due to concerns about lead toxicity but for stylistic reasons. Lead, or sheet-lead, is used as a sound deadening layer in some areas in wall, floor and ceiling design in sound studios. It is the traditional base metal of organ pipes, mixed with varying amounts of tin to control the tone of the pipe.
Lead has many uses in the construction industry (e.g., lead sheets are used as architectural metals in roofing material, cladding, flashing, gutters and gutter joints, and on roof parapets). Detailed lead moldings are used as decorative motifs to fix lead sheet. Lead is still widely used in statues and sculptures. It is often used to balance the wheels of a car; for environmental reasons this use is being phased out in favor of other materials.
Apart from its mechanical properties, lead is also useful in lead–acid batteries. The reactions in the battery between lead, lead dioxide, and sulfuric acid provides a reliable source of voltage.[l] This has been the largest use of lead in early 21st century, since the lead in batteries undergoes no direct contact with humans (and thus there are no immediate toxicity concerns).
Lead is also used in electrodes for the process of electrolysis. It is used in solder for electronics, although this usage is being phased out by some countries to reduce the amount of environmentally hazardous waste, and in high voltage power cables as sheathing material to prevent water diffusion into insulation. Lead is one of three metals used in the Oddy test for museum materials, helping detect organic acids, aldehydes, and acidic gases. It is also used as shielding from radiation (e.g., in X-ray rooms). Molten lead is used as a coolant (e.g., for lead cooled fast reactors).
Lead compounds are used as, or in, coloring agents, oxidants, plastic, candles, glass, and semiconductors. Lead-based coloring agents are used in ceramic glazes, notably for red and yellow shades. Lead tetraacetate (LTA) and lead dioxide have been used as oxidizing agents in organic chemistry. Lead is frequently used in polyvinyl chloride (PVC) plastic, which coats electrical cords. Lead is used to treat some candle wicks to ensure a longer, more even burn. Because of its toxicity, European and North American manufacturers use alternatives such as zinc. Lead glass is composed of 12–28% lead oxide. It changes the optical characteristics of the glass and reduces the transmission of ionizing radiation. Lead-based semiconductors, such as lead telluride, lead selenide and lead antimonide are finding applications in photovoltaic (solar energy) cells and infrared detectors.
Biological and environmental effects
Along with such elements as cadmium and mercury, lead has no biological role. It is considered a highly poisonous metal (whether inhaled or swallowed), affecting almost every organ and system in the body. The component limit of lead (1.0 μg/g) is a test benchmark for pharmaceuticals, representing the maximum daily intake an individual should have. Even at this level, a prolonged intake can be hazardous. Exposure to lead and lead chemicals occurs primarily through ingestion, to a lesser extent through inhalation and occasionally by direct contact.
The main target for lead toxicity in humans is the central nervous system. By mimicking calcium, lead is able to cross the blood-brain barrier. It subsequently degrades the myelin sheaths of neurons, reduces their numbers, interferes with neurotransmission routes, and decreases neuronal growth. In a child's developing brain, lead interferes with synapse formation in the cerebral cortex, neurochemical development (including that of neurotransmitters), and the organization of ion channels.
The primary cause of lead's toxicity is its predilection for binding to the sulfhydryl groups found on many enzymes, thereby interfering with their proper functioning. Part of lead's toxicity results from its ability to mimic and displace other metals which act as cofactors in many enzymatic reactions. Lead salts are thus very quickly and efficiently absorbed by the body, accumulating in it and leading to both chronic and acute poisoning.
Among the essential metals with which lead interacts are calcium, iron, and zinc. Thus high levels of calcium and iron tend to protect one somewhat from lead poisoning, while low levels of these metals render one more susceptible.
According to the United States Agency for Toxic Substance and Disease Registry, a small amount of ingested lead (1%) will be stored in bones, and the rest will be excreted by an adult through urine and feces within a few weeks of exposure. Only about a third of lead will be excreted by a child.
Chronic exposure to lead or its salts (especially soluble salts or the strong oxidant PbO2) in adults can result in decreased performance in some tests that measure functions of the nervous system. Symptoms include nephropathy, and colic-like abdominal pains and possibly weakness in the fingers, wrists, or ankles. Lead exposure also causes small increases in blood pressure, particularly in middle-aged and older people and can cause anemia. Exposure to high lead levels can cause severe damage to the brain and kidneys in adults or children and ultimately cause death. In pregnant women, high levels of exposure to lead may cause miscarriage. Chronic, high-level exposure has been shown to reduce fertility in males.
Lead also damages nervous connections (especially in young children) and causes blood and brain disorders. Lead poisoning typically results from ingestion of food or water contaminated with lead, but may also occur after accidental ingestion of contaminated soil, dust, or lead-based paint. It is rapidly absorbed into the bloodstream and is believed to have adverse effects on the central nervous system, the cardiovascular system, kidneys, and the immune system.
|Fire diamond for lead granules|
Treatment for lead poisoning normally involves the administration of dimercaprol and succimer. Acute cases may require the use of disodium calcium edetate, this being the calcium chelate of the disodium salt of ethylenediaminetetraacetic acid (EDTA). This chelating agent has a greater affinity for lead than calcium with the result that lead chelate is formed by exchange. This is excreted in the urine leaving behind harmless calcium.
The role of lead in causing cognitive deficits in children has brought about a widespread reduction in its use. Most cases of adult elevated blood lead levels are workplace-related. High blood levels are associated with delayed puberty in girls. Lead has been shown many times to permanently reduce the cognitive capacity of children at extremely low levels of exposure.
Despite the toxicity of lead in significant amounts, there is some evidence that trace amounts are beneficial in pigs and rats, and that its absence causes deficiency signs including depressed growth, anemia, and disturbed iron metabolism. If true in humans as well, this would make lead an essential element; nevertheless, these findings are still uncertain, and even if lead does turn out to be beneficial in small quantities, the threshold of toxicity is so low that lead toxicity would remain a much higher priority to address than lead deficiency.
Sources of exposure
Ingestion of lead-based paint is the major source of lead exposure for children. As lead paint deteriorates, it peels, is pulverized into dust and then enters the body through hand-to-mouth contact or through contaminated food, water, or alcohol. Ingesting certain home remedy medicines may also expose people to lead or lead compounds. Lead can be ingested through fruits and vegetables contaminated by high levels of lead in the soils they were grown in. Soil is contaminated through particulate accumulation from lead in pipes, lead paint and residual emissions from leaded gasoline (before use of the latter was generally phased out). The use of lead for water pipes is problematic in areas with soft or (and) acidic water. Hard water forms insoluble layers in the pipes while soft and acidic water dissolves the lead pipes.
Inhalation is the second major pathway of exposure, especially for workers in lead-related occupations. Almost all inhaled lead is absorbed into the body, the rate is 20–70% for ingested lead; children absorb more than adults.
Dermal exposure may be significant for a narrow category of people working with organic lead compounds. The rate of skin absorption is also low for inorganic lead.
The extraction, production, use, and disposal of lead and its products have caused significant contamination of the Earth's soils and waters, posing a hazard to living organisms because of its toxicity. Atmospheric emissions of lead were at their peak during the Industrial Revolution and the period of leaded petrol in the second half of the twentieth century; although these periods are over, elevated concentrations of lead persist in soils and sediments in post-industrial and urban areas. Meanwhile, industrial emissions continue in many parts of the world.
Lead accumulates in soil, especially in soil with high organic content, where it remains for a long time (hundreds and thousands of years.) It can take the place of other metals within plants and can accumulat on their surfaces, thereby retarding photosynthesis, and preventing the growth of the plant or killing it. Contamination of soils and plants, in turn, affects microorganisms and animals. Affected animals have a reduced ability to synthesize red blood cells. Sources of lead contamination are therefore being curtailed.[m]
Research has been conducted on how to remove lead from biosystems via biological organisms. Fish bones are being researched for their ability to bioremediate lead in contaminated soil. The fungus Aspergillus versicolor is particularly effective at removing lead ions. Several bacteria have been researched for their ability to reduce lead; including the sulfate reducing bacteria Desulfovibrio and Desulfotomaculum, both of which are highly effective in aqueous solutions.
Restriction of lead usage
During the 20th century, the use of lead in paint pigments was sharply curtailed because of the danger of lead poisoning, especially to children. By the mid-1980s, a significant shift in lead end-use patterns had taken place. Much of this shift was a result of compliance, in the U.S., with environmental regulations that significantly reduced or eliminated the use of lead in non-battery products, including gasoline, paints, solders, and water systems. Lead use is being further curtailed by the European Union's RoHS directive. Lead may be found in harmful quantities in stoneware, vinyl (such as that used for tubing and the insulation of electrical cords), and Chinese brass. Old houses may contain substantial amounts of lead paint. White lead paint has been withdrawn from sale in industrialized countries, but the yellow lead chromate is still in use. Old paint should not be stripped by sanding, as this produces inhalable dust.
People can be exposed to lead in the workplace by breathing it in, swallowing it, skin contact, and eye contact. In the United States, the Occupational Safety and Health Administration has set the permissible exposure limit for lead exposure in the workplace as 0.05 mg/m3 over an 8-hour workday, which applies to metallic lead, inorganic lead compounds, and lead soaps. The National Institute for Occupational Safety and Health has set a recommended exposure limit of 0.05 mg/m3 over an 8-hour workday, and recommends that workers' blood concentrations of lead stay below 0.06 mg per 100 g blood. At levels of 100 mg/m3, lead is immediately dangerous to life and health.
- 2009 Chinese lead poisoning scandal
- Adult Blood Lead Epidemiology and Surveillance
- Consumer Product Safety Improvement Act
- Devon colic
- Flint water crisis
- Medical geology
- Restriction of Hazardous Substances Directive
- Roman lead pipe inscription
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- The difference between the two terms is that malleability refers to deformability under compression (i.e., pressing a tablet of a material into a sheet) while ductility refers to its ability to stretch (i.e., elongating a rod of a material into a wire).
- In addition to that, this, for example, allows dipping a finger into molten lead without burning it.
- An even number of either protons or neutrons generally increases the nuclear stability of isotopes, compared to isotopes with odd numbers. For example, elements with odd atomic numbers have no more than two stable isotopes, while even-numbered elements have multiple stable isotopes, with tin (element 50) having the highest number of isotopes of all elements, ten. See Even and odd atomic nuclei for more details.
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- It is suggested that the sweeteners the Romans made were often prepared in lead vessels; this led to the formation of poisonous lead(II) acetate, which accumulated in the sweeteners and, accordingly, the products they were used for; in particular, wine. Lead containers did further sweeten their contents as well as help to preserve them. In comparison, copper vessels spoiled the taste of wine in them. The fact that Julius Caesar managed to father only one child, as well as the alleged sterility of his successor, Caesar Augustus, have also been attributed to lead poisoning. The Romans were aware of the potential health problems lead could cause, as well as the fact that copper was used far more commonly for Roman vessels than lead.
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- For instance, the U.S. state of California banned lead bullets for hunting on that basis in April 2015.
- See  for details on how a lead–acid battery works.
- For example, in the Netherlands, the use of lead shot for hunting and sport shooting was banned in 1993, which caused a large drop in lead emission, from 230 tonnes in 1990 to 47.5 tonnes in 1995, two years after the ban.
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