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Lead

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Lead,  82Pb
Lead electrolytic and 1cm3 cube.jpg
General properties
Name, symbol lead, Pb
Appearance metallic gray
Pronunciation /ˈlɛd/
LED
Lead in the periodic table
Hydrogen (diatomic nonmetal)
Helium (noble gas)
Lithium (alkali metal)
Beryllium (alkaline earth metal)
Boron (metalloid)
Carbon (polyatomic nonmetal)
Nitrogen (diatomic nonmetal)
Oxygen (diatomic nonmetal)
Fluorine (diatomic nonmetal)
Neon (noble gas)
Sodium (alkali metal)
Magnesium (alkaline earth metal)
Aluminium (post-transition metal)
Silicon (metalloid)
Phosphorus (polyatomic nonmetal)
Sulfur (polyatomic nonmetal)
Chlorine (diatomic nonmetal)
Argon (noble gas)
Potassium (alkali metal)
Calcium (alkaline earth metal)
Scandium (transition metal)
Titanium (transition metal)
Vanadium (transition metal)
Chromium (transition metal)
Manganese (transition metal)
Iron (transition metal)
Cobalt (transition metal)
Nickel (transition metal)
Copper (transition metal)
Zinc (transition metal)
Gallium (post-transition metal)
Germanium (metalloid)
Arsenic (metalloid)
Selenium (polyatomic nonmetal)
Bromine (diatomic nonmetal)
Krypton (noble gas)
Rubidium (alkali metal)
Strontium (alkaline earth metal)
Yttrium (transition metal)
Zirconium (transition metal)
Niobium (transition metal)
Molybdenum (transition metal)
Technetium (transition metal)
Ruthenium (transition metal)
Rhodium (transition metal)
Palladium (transition metal)
Silver (transition metal)
Cadmium (transition metal)
Indium (post-transition metal)
Tin (post-transition metal)
Antimony (metalloid)
Tellurium (metalloid)
Iodine (diatomic nonmetal)
Xenon (noble gas)
Caesium (alkali metal)
Barium (alkaline earth metal)
Lanthanum (lanthanide)
Cerium (lanthanide)
Praseodymium (lanthanide)
Neodymium (lanthanide)
Promethium (lanthanide)
Samarium (lanthanide)
Europium (lanthanide)
Gadolinium (lanthanide)
Terbium (lanthanide)
Dysprosium (lanthanide)
Holmium (lanthanide)
Erbium (lanthanide)
Thulium (lanthanide)
Ytterbium (lanthanide)
Lutetium (lanthanide)
Hafnium (transition metal)
Tantalum (transition metal)
Tungsten (transition metal)
Rhenium (transition metal)
Osmium (transition metal)
Iridium (transition metal)
Platinum (transition metal)
Gold (transition metal)
Mercury (transition metal)
Thallium (post-transition metal)
Lead (post-transition metal)
Bismuth (post-transition metal)
Polonium (post-transition metal)
Astatine (metalloid)
Radon (noble gas)
Francium (alkali metal)
Radium (alkaline earth metal)
Actinium (actinide)
Thorium (actinide)
Protactinium (actinide)
Uranium (actinide)
Neptunium (actinide)
Plutonium (actinide)
Americium (actinide)
Curium (actinide)
Berkelium (actinide)
Californium (actinide)
Einsteinium (actinide)
Fermium (actinide)
Mendelevium (actinide)
Nobelium (actinide)
Lawrencium (actinide)
Rutherfordium (transition metal)
Dubnium (transition metal)
Seaborgium (transition metal)
Bohrium (transition metal)
Hassium (transition metal)
Meitnerium (unknown chemical properties)
Darmstadtium (unknown chemical properties)
Roentgenium (unknown chemical properties)
Copernicium (transition metal)
Ununtrium (unknown chemical properties)
Flerovium (post-transition metal)
Ununpentium (unknown chemical properties)
Livermorium (unknown chemical properties)
Ununseptium (unknown chemical properties)
Ununoctium (unknown chemical properties)
Sn

Pb

Fl
thalliumleadbismuth
Atomic number (Z) 82
Group, block group 14 (carbon group), p-block
Period period 6
Element category   post-transition metal
Standard atomic weight (±) (Ar) 207.2(1)[1]
Electron configuration [Xe] 4f14 5d10 6s2 6p2
per shell
2, 8, 18, 32, 18, 4
Physical properties
Phase solid
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)
vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 978 1088 1229 1412 1660 2027
Atomic properties
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
Miscellanea
Crystal structure face-centered cubic (fcc)
Face-centered cubic crystal structure for lead
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 ordering diamagnetic
Young's modulus 16 GPa
Shear modulus 5.6 GPa
Bulk modulus 46 GPa
Poisson ratio 0.44
Mohs hardness 1.5
Brinell hardness 38–50 MPa
CAS Number 7439-92-1
History
Discovery Middle Easterns (7000 BC)
Most stable isotopes of lead
iso NA half-life DM DE (MeV) DP
204Pb 1.4% >1.4×1017 y (α) 1.972 200Hg
205Pb syn 1.53×107 y ε 0.051 205Tl
206Pb 24.1% (α) 1.1366 202Hg
207Pb 22.1% (α) 0.3915 203Hg
208Pb 52.4% >2×1019 y (α) 0.5188 204Hg
210Pb trace 22.3 y α 3.792 206Hg
β 0.064 210Bi
Decay modes in parentheses are predicted, but have not yet been observed
· references

Lead (/lɛd/) is a chemical element in the carbon group with symbol Pb (from Latin: plumbum) and atomic number 82. Lead is a soft, malleable and heavy post-transition metal. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed to air. Lead has a shiny chrome-silver luster when it is melted into a liquid. It is also the heaviest (has the highest atomic number) non-radioactive element (two radioactive elements, namely technetium and promethium, are lighter).

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 has the highest atomic number of all of the stable elements, although the next higher element, bismuth, has one isotope with a half-life that is so long (over one billion times the estimated age of the universe) that it can be considered stable. Lead's four stable isotopes have 82 protons, a magic number in the nuclear shell model of atomic nuclei. The isotope lead-208 also has 126 neutrons, another magic number, and is hence double magic, a property that grants it enhanced stability: lead-208 is the heaviest known stable isotope.

If ingested, lead is poisonous to animals and humans, damaging the nervous system and causing brain disorders. Excessive lead also causes blood disorders in mammals. Lead is a neurotoxin that accumulates both in soft tissues and the bones. Lead poisoning has been documented since ancient Rome, ancient Greece, and ancient China.

Physical characteristics

Bulk properties

A sample of freshly solidified lead (from molten state)

Lead is a bright and silvery metal with a very slight shade of blue in a dry atmosphere.[2] On contact with air, it begins to tarnish by forming a complex mixture of compounds depending on the conditions. The color of the compounds can vary. The tarnish layer can contain significant amounts of carbonates and hydroxycarbonates.[3][4] Lead's characteristic properties include high density, softness, ductility, malleability, poor electrical conductivity compared to other metals, high resistance to corrosion, and ability to react with organic chemicals.[2]

Various traces of other metals change its properties significantly: the addition of small amounts of antimony or copper to lead increases the alloy's hardness and improves resistance to sulfuric acid corrosion.[2] Some other metals, such as cadmium, tin, and tellurium, also improve hardness and fight metal fatigue. Sodium and calcium also have this ability, but they reduce the alloy's chemical stability.[2] Finally, zinc and bismuth simply impair the corrosion resistance (0.1% bismuth content is the industrial usage threshold).[2] Conversely, lead impurities mostly worsen the quality of industrial materials, although there are exceptions: for example, small amounts of lead improve the ductility of steel.[2]

Lead has only one common allotrope, which is face-centered cubic, with the length of an edge of a unit cell being 349 pm.[5] At 327.5 °C (621.5 °F),[6] lead melts; the melting point exceeds that of tin (232 °C, 449.5 °F),[6] but is significantly below that of germanium (938 °C, 1721 °F).[7] The boiling point of lead is 1749 °C (3180 °F),[8] below those of both tin (2602 °C, 4716 °F)[6] and germanium (2833 °C, 5131 °F).[7] Densities increase down the group: the values of germanium and tin (5.23[9] and 7.29 g·cm−3,[10] respectively) are significantly below that of lead: 11.32 g·cm−3.[9]

Isotopes

Lead has an atomic number of 82; the number is even, and as such, lead has several observationally stable isotopes, four: lead-204, lead-206, lead-207, and lead-208.[11][lower-alpha 1] With its high atomic number, lead is the second heaviest element that occurs naturally in the form of isotopes that could be treated as stable for any practical applications: bismuth has a higher atomic number of 83, but its only stable isotope was found in 2003 to be actually very slightly radioactive.[lower-alpha 2] The four stable isotopes of lead could theoretically undergo alpha decay with release of energy as well, but this has not been observed for any of them.[12] As such, lead is often quoted as the heaviest stable element.

Three of these isotopes also found in three of the four major decay chains: lead-206, −207 and −208 are final decay products of uranium-238, uranium-235, and thorium-232, respectively; the decay chains are called uranium series, actinium series, and thorium series, respectively. Since the amounts of them in nature depend also on other elements' presence, the isotopic composition of natural lead varies by sample: in particular, the relative amount of lead-206 may vary between 20.84% and 27.78%.[11] (For this reason, the atomic weight of lead is given with such imprecision, one decimal place.[15]) As time goes, relative amounts of lead-206 and –207 to that of lead–204 increase, since the former two are regenerated by radioactivity of heavier elements and the latter is not; this allows for the lead–lead dating. Analogously, as uranium decays into (eventually) lead, their relative amounts change; this allows for uranium–lead dating.

Apart from the stable isotopes, which make up almost all of 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,[12] a period too short to allow any primordial lead-210 to still exist, some small quantities of it exist, because lead-210 is found in uranium series: even though it constantly decays away, its amount is also 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 well known 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); however, lead-214 is also present in that chain, and lead-212 is present in thorium series; therefore, traces of both isotopes exist naturally as well.[16]

In total, thirty-eight isotopes of lead have been synthesized, those with mass numbers of 178–215.[12] Lead-205 is the most stable radioisotope of lead, with a half-life of around 1.5×107 years.[lower-alpha 3] Additionally, 47 nuclear isomers (long-lived excited nuclear states), corresponding to 24 lead isotopes, have been characterized. The longest-lived isomer is lead-204m2 (half-life of about 1.1 hours).[12]

Chemical characteristics

A lead atom has 82 electrons, arranged in an electronic configuration of [Xe]4f145d106s26p2. The first and second ionization energies—energies required to remove an electron from a neutral atom and an electron from a resulting singly charged ion—of lead combined are close to those of tin, its upper group 14 neighbor; this proximity is caused by the 4f shell—no f shell is present in previous group 14 elements atoms—and the thereby following lanthanide contraction. However, the first four ionization energies of lead combined exceed those of tin,[18] opposite to what the periodic trends would predict. For that reason, unlike tin,[19] lead is reluctant[19] to form the +4 oxidation state in inorganic compounds.

Such unusual behavior is rationalized by relativistic effects, which are increasingly stronger closer to the bottom of the periodic table;[19] one of such effects is the spin–orbit (SO) interaction, particularly the inert pair effect, which stabilizes the 6s orbital.[lower-alpha 4] The inert pair effect in lead comes from the great difference in electronegativity between lead and the anions (oxide, halides, nitrides), which results in positive charge on lead and then leads to a stronger contraction of the 6s orbital than the 6p orbital, making the 6s orbital inert.[21] (However, this is not applicable to compounds in which lead forms covalent bonds; as such, lead, similar to carbon, is dominantly tetravalent in organolead compounds.) The SO interaction not only stabilizes the 6s electron levels, but also two of the six 6p levels; and lead has just two p electrons. This effect takes part in making lead slightly more stable chemically, but it is revealed to a greater extent for the period 7 elements, and as such, it is often not mentioned in a chemical description of lead.

The figures for electrode potential show that lead is only slightly easier to oxidize than hydrogen. Lead thus can dissolve in acids, but this is often impossible due to specific problems (such as the formation of insoluble salts).[22] 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)—1.87 and 2.33, accordingly. This marks the reversal of the trend of increasing stability of the +4 oxidation state in group 14 down the group into decreasing; tin, for comparison, has electronegativities of 1.80 and 1.96.[23]

Reactivity

Powdered lead burns with a bluish-white flame. As with many metals, finely divided powdered lead exhibits pyrophoricity.[24] Bulk lead released to the air forms a protective layer of insoluble lead oxide, which covers the metal from undergoing further reactions.[25] Other insoluble compounds, such as sulfate or chloride, may form the protective layer if lead is exposed to a different chemical environment.[26]

Fluorine reacts with lead at room temperature, forming lead(II) fluoride. The reaction with chlorine is similar, although it requires heating: the chloride layer diminishes the reactivity of the elements.[25][26] Molten lead reacts with chalcogens.[27]

Presence of carbonates or sulfates results in the formation of insoluble lead salts, which protect the metal from corrosion. So does carbon dioxide, as the insoluble lead carbonate is formed; however, an excess of the gas leads to the formation of the soluble bicarbonate, which makes the use of lead pipes dangerous.[22] Water in the presence of oxygen attacks lead to start an accelerating reaction.[28] Lead also dissolves in quite concentrated alkalis (≥10%) because of the amphoteric character and solubility of plumbites.[22]

The metal is normally not attacked by sulfuric acid; however, concentrated acid does dissolve lead thanks to complexation.[28] Lead does react with hydrochloric acid, albeit slowly, and nitric acid, quite actively, to form nitrogen oxides and lead(II) nitrate.[28] Organic acids, such as acetic acid, also dissolves lead, but this reaction requires oxygen as well.[26]

Inorganic compounds

In a vast majority of compounds lead forms, it occurs in oxidation states +2 and +4. One principal difference between lead(II) and lead(IV) compounds is that the former are normally ionic and the latter are often covalent. Even the strongest oxidizing elements (oxygen, fluorine) oxidize lead to only lead(II) initially.

Lead(II)

Most inorganic compounds lead forms are lead(II) compounds. This includes binary compounds; lead forms such compounds with many nonmetals, but not with every one: for example, there is no known lead carbide.

Even though most lead(II) compounds are ionic, 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.[29] Unlike tin(II) ions, these do not react as reducing agents in solution.

Lead monoxide exists in two allotropes, red α-PbO and yellow β-PbO, the latter being stable only from around 488 °C. It is the most commonly applicable compound of lead.[30] However, its hydroxide counterpart, lead(II) hydroxide, is not capable of existence outside solutions; in solution, it is known to form anions, plumbites. Lead commonly reacts with chalcogens other than oxygen: for that reason, it is classified as a chalcophile using the Goldschmidt classification. Lead sulfide is capable of dissolving only in strong acids;[31] 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.[32] The other two chalcogenides are photoconducting as well.[33]

Lead dihalides are known and well-characterized; this refers to not only the binary halides, to some extent even including diastatide,[34] but also mixed ones, such as PbFCl, etc. The difluoride is the first ionically conducting compound to have been discovered. The other dihalides decompose on exposure to light, especially notably for the diiodide. There are anion counterparts for the heavier three dihalides, such as PbCl4−
6
.

Lead(IV)

In general, few lead(IV) compounds are known: inorganic lead(IV) compounds are typically strong oxidants or exist only in highly acidic solutions.[19] Lead(II) oxide gives a mixed oxide on further oxidation, Pb
3
O
4
. It is described as lead(II,IV) oxide, or structurally 2PbOPbO
2
, and is the best-known mixed valence lead compound. A standing out lead dioxide is known as well; it is a strong oxidizing agent, capable of oxidizing hydrochloric acid. Like lead monoxide, lead dioxide is capable of forming anions, plumbates. Lead tetrafluoride, a yellow crystalline powder, is stable, but less stable than the difluoride; lead tetrachloride decomposes at room temperature; lead tetrabromide is less stable still; and existence of lead tetraiodide is questionable.[35][36] Lead disulfide, like the monosulfide, is a semiconductor.[37] Lead(IV) selenide is also known.[38]

Other oxidation states

A few compounds exist in oxidation states other than +2 and +4, but they don't have a great impact on lead chemistry from neither theoretical nor industrial perspective. Lead(III) may be obtained under specific conditions as an intermediate between lead(II) and lead(IV), in larger organolead complexes rather than by itself.[39][40] This oxidation state is not specifically stable, as lead(III) ion (as well as, consequently, larger complexes containing it) is a radical; same applies for lead(I), which can also be found in such species.[41] Negative oxidation states can occur as Zintl phases, as either free lead ions, for example, in Ba
2
Pb
, with lead formally being lead(−IV),[42] or cluster ions, for example, in a Pb5−
2
ion, where two lead atoms are lead(−I) and three are lead(0).[43]

Organolead

Since lead is a heavier carbon homolog, it displays a property common for carbon that allows building long chains of atoms, bonded via single or multiple bonds: catenation. Thanks to it, lead may behave somewhat similar to carbon with regards to covalent chemistry: Lead atoms can build metal–metal bonds of order up to three,[44] although lower orders are also possible. Alternatively, lead is also known to build bonds to carbon; the carbon–lead bonds are covalent, and compounds containing such bonds thus resemble typical organic compounds.[45] Compound containing the lead–carbon bond are called organolead compounds. In general, such compounds are not very stable chemically.

The simplest lead analog of an organic compound is plumbane, the lead analog of methane. It is unstable against heat, decaying in heated tubes,[46] and thermodynamically;[47] in general, little is known about chemistry of plumbane, as it is so unstable. A lead analog of the next alkane, ethane, is not known.[46] Two simple plumbane derivatives, tetramethyllead and tetraethyllead, are the best-known organolead compounds. These compounds are relatively unstable against heating—tetraethyllead starts to decompose at only 100 °C (210 °F)[45]—as well as sunlight or ultraviolet light.[48] (Despite such instability, tetraethyllead is produced in larger quantities than any other organometallic compound.[49]) Other organolead compounds, including homologs of the said compounds, are less stable chemically still.[45]

Lead readily forms an equimolar alloy with sodium metal that reacts with alkyl halides to form organometallic compounds of lead such as tetraethyllead.[50] Plumbane may be obtained in a reaction between metallic lead and atomic (not molecular) hydrogen.[46] Atoms of chlorine or bromine displace alkyls in tetramethyllead and tetraethyllead; hydrogen chloride, a by-product of the previous reaction, further reacts with the halogenated molecules to complete mineralization—chemical reaction or a series of reactions transforming an organic compound into an inorganic one—of the original compounds, yielding lead dichloride.[48]

Occurrence

In space

Solar System abundances[51]
Atomic
number
Element Relative
amount
42 Molybdenum 4.0
46 Palladium 1.3
50 Tin 3.6
52 Tellurium 6.42
56 Barium 4.8
80 Mercury 0.4
82 Lead 4
92 Uranium 0.0262

Since lead is located at the end of three major decay chains (see above), it has been constantly regenerated in nature from decay of heavier elements. The isotopes at the end of the chains make up around 98.02% lead in the universe, with non-radiogenic lead-204 making up slightly less than two percent.[51] Thanks to regeneration, lead is a very abundant element for its high atomic number, at … ppb. After element 40 (zirconium) no element is at least twofold as abundant as lead, and there is no element as abundant as lead starting after element 56 (barium). Lead is three times as abundant as platinum, ten times as mercury, and twenty times as gold.[51] Lead is found in the solar atmosphere, and much more abundantly in the atmospheres of some hot subdwarfs.[52]

On Earth

Lead is a quite common element in the Earth's crust for its high atomic number.

On Earth, lead becomes more abundant still, at 13 ppm.[15] As lead is a chalcophile (see above), lead is likely to form compounds that do not sink into the core but that stay above on Earth in its crust, even though without sinking deep into it. Uranium is greatly present on Earth as well (2.3 ppm); as such, the share of radiogenic stable lead isotopes is higher still, 98.6%.[15] Lead's pronounced chalcophilic character is close to those of zinc and copper; as such, it is usually found in ore and extracted together with these metals.[53]

Lead and zinc bearing carbonate and clastic deposits

Metallic lead does occur in nature, but it is rare. As a result of lead's chemistry, it occurs in minerals exclusively as lead(II), unlike tin, which always occurs as tin(IV).[15] The main lead mineral is galena (PbS). Other common varieties are cerussite (PbCO3) and anglesite (PbSO4).[54]

History

World lead production peaking in the Roman period and the rising Industrial Revolution[55]
Lead ingots from Roman Britain on display at the Wells and Mendip Museum
Lead mining in the upper Mississippi River region of the US in 1865

Lead has been commonly used for thousands of years because it is widespread, easy to extract and easy to work with. It is highly malleable as well as easy to smelt. Metallic lead beads dating back to 6400 BC have been found in Çatalhöyük in modern-day Turkey.[56] In the early Bronze Age, lead was used with antimony and arsenic.[57]

The largest preindustrial producer of lead was the Roman economy, with an estimated annual output of 80,000 tonnes, which was typically won as a by-product of extensive silver smelting.[55][58][59] Roman mining activities occurred in Central Europe, Roman Britain, the Balkans, Greece, Asia Minor and Hispania which alone accounted for 40% of world production.[55]

Roman lead pipes often bore the insignia of Roman emperors (see Roman lead pipe inscriptions). Lead plumbing in the Latin West may have been continued beyond the age of Theoderic the Great into the medieval period.[60] Many Roman "pigs" (ingots) of lead figure in Derbyshire lead mining history and in the history of the industry in other English centers. The Romans also used lead in molten form to secure iron pins that held together large limestone blocks in certain monumental buildings.[61] In alchemy, lead was thought to be the oldest metal and was associated with the planet Saturn. Alchemists accordingly used Saturn's symbol (the scythe, ) to refer to lead.[62]

Up to the 17th century, tin was often not distinguished from lead: lead was called plumbum nigrum (literally, "black lead"), while tin was called plumbum candidum (literally, "bright lead").[63] Their inherence through history can also be seen in other languages: the words "ołów" in Polish and "olovo" in Czech mean lead, but in Russian the cognate "олово" (olovo) means tin.[64] Lead's symbol Pb is an abbreviation of its Latin name plumbum for soft metals; the English words "plumbing", "plumber", "plumb", and "plumb-bob" also derive from this Latin root.[65]

Lead production in the US commenced as early as the late 1600s by the Indians in The Southeast Missouri Lead District, commonly called the Lead Belt, is a lead mining district in the southeastern part of Missouri. Significant among Missouri's lead mining concerns in the district was the Desloge Family and Desloge Consolidated Lead Company in Desloge, Missouri and Bonne Terre – having been active in lead trading, mining and lead smelting from 1823 in Potosi to 1929.

Production

Ore processing

Historical evolution of the extracted lead ore grade extracted in different countries.

Most ores contain less than 10% lead, and ores containing as little as 3% lead can be economically exploited. Ores are crushed and concentrated by froth flotation typically to 70% or more. Sulfide ores are roasted, producing primarily lead oxide and a mixture of sulfates and silicates of lead and other metals contained in the ore.[66] Lead oxide from the roasting process is reduced in a coke-fired blast furnace to the metal.[67] Additional layers separate in the process and float to the top of the metallic lead. These are slag (silicates containing 1.5% lead), matte (sulfides containing 15% lead), and speiss (arsenides of iron and copper). These wastes contain concentrations of copper, zinc, cadmium, and bismuth that can be recovered economically, as can their content of unreduced lead.[66]

Galena, lead ore

Metallic lead that results from the roasting and blast furnace processes still contains significant 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 silver, gold, and bismuth. The oxidized contaminants are removed by drossing, where they float to the top and are skimmed off.[66][68] Since lead ores contain significant concentrations of silver, the smelted metal also is commonly contaminated with silver. Metallic silver as well as gold is removed and recovered economically by means of the Parkes process.[32][66][68] Desilvered 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.[66][68] 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.[66][68]

Production and recycling

Historical evolution of the production of lead, as extracted in different countries.

Production and consumption of lead is increasing worldwide. Total annual production is about 8 million tonnes; about half is produced from recycled scrap. The top lead producing countries, as of 2008, are Australia, China, USA, Peru, Canada, Mexico, Sweden, Morocco, South Africa and North Korea.[69] Australia, China and the United States account for more than half of primary production.[70] In 2010, 9.6 million tonnes of lead were produced, of which 4.1 million tonnes came from mining.[71]

At current use rates, the supply of lead is estimated to run out in 42 years.[72] Environmental analyst Lester Brown has suggested lead could run out within 18 years based on an extrapolation of 2% growth per year.[73] This may need to be reviewed to take account of renewed interest in recycling, and rapid progress in fuel cell technology. According to the International Resource Panel's Metal Stocks in Society report, 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).[74]

Applications

Lead when mined contains an unstable isotope, lead-210, which has a half life of 22 years. This makes lead slightly radioactive. As such ancient lead which has almost no radioactivity is sometimes desired for scientific experimentation.[75][76]

Elemental form

Lead bricks are commonly used as radiation shielding.

Contrary to popular belief, pencil leads in wooden pencils have never been made from lead. The term comes from the Roman stylus, called the penicillus, a small brush used for painting.[77] 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).[78][79]

Lead is used in applications where its low melting point, ductility and high density are advantageous. The low melting point makes casting of lead easy, and therefore small arms ammunition and shotgun pellets can be cast with minimal technical equipment. It is also inexpensive and denser than other common metals.[80]

Because of its high density and resistance to corrosion, lead is used for the ballast keel of sailboats.[81] 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 offering the least underwater resistance. For the same reason it is used in scuba diving weight belts to counteract the diver's natural buoyancy and that of his equipment.[82] It does not have the weight-to-volume ratio of many heavy metals, but its low cost increases its use in these and other applications.

Roman lead water pipes with taps
Lead pipe in Roman baths
Multicolor lead-glazing in a Tang dynasty Chinese sancai ceramic cup dating from the 8th century AD
Punched lead cast in a Venice bridge wall fixing the hard-metal connecting bar

More than half of the US lead production (at least 1.15 million tonnes in 2000) is used for automobiles, mostly as electrodes in the lead–acid battery, used extensively as a car battery.[83]

Cathode (reduction)

PbO2 + 4 H+ + SO2−
4
+ 2e → PbSO4 + 2 H2O

Anode (oxidation)

Pb + SO2−
4
→ PbSO4 + 2e[84][85]

Lead is used as electrodes in 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).[86] Molten lead is used as a coolant (e.g., for lead cooled fast reactors).[87]

Lead is added to brass to reduce machine tool wear. In the form of strips or tape, lead is used for the customization of tennis rackets. Tennis rackets in the past sometimes had lead added to them by the manufacturer to increase weight.[88] It is also used to form glazing bars for stained glass or other multi-lit windows. The practice has become less common, not for danger 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 where levels of airborne and mechanically produced sound are targeted for reduction or virtual elimination.[89][90] It is the traditional base metal of organ pipes, mixed with varying amounts of tin to control the tone of the pipe.[91][92]

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 used to fix lead sheet. Lead is still widely used in statues and sculptures. Lead is often used to balance the wheels of a car; this use is being phased out in favor of other materials for environmental reasons. Owing to its half-life of 22.20 years, the radioactive isotope 210Pb is used for dating material from marine sediment cores by radiometric methods.[93][94][95]

Compounds

Lead compounds are used as a coloring element in ceramic glazes, notably in the colors red and yellow.[96] Lead is frequently used in polyvinyl chloride (PVC) plastic, which coats electrical cords.[97][98]

Lead is used in some candles to treat the wick to ensure a longer, more even burn. Because of the dangers, European and North American manufacturers use alternatives such as zinc.[99][100] Lead glass is composed of 12–28% lead oxide. It changes the optical characteristics of the glass and reduces the transmission of radiation.[101]

Some artists using oil-based paints continue to use lead carbonate white, citing its properties in comparison with the alternatives. Tetra-ethyl lead is used as an anti-knock additive for aviation fuel in piston-driven aircraft. Lead-based semiconductors, such as lead telluride, lead selenide and lead antimonide are finding applications in photovoltaic (solar energy) cells and infrared detectors.[102]

Lead, in either pure form or alloyed with tin, or antimony is the traditional material for bullets and shot in firearms use.

Historical applications

Lead pigments were used in lead paint for white as well as yellow, orange, and red. Most uses have been discontinued due to the dangers of lead poisoning. Beginning April 22, 2010, US federal law requires that contractors performing renovation, repair, and painting projects that disturb more than six square feet of paint in homes, child care facilities, and schools built before 1978 must be certified and trained to follow specific work practices to prevent lead contamination. Lead chromate is still in industrial use. Lead carbonate (white) is the traditional pigment for the priming medium for oil painting, but it has been largely displaced by the zinc and titanium oxide pigments. It was also quickly replaced in water-based painting mediums. Lead carbonate white was used by the Japanese geisha and in the West for face-whitening make-up, which was detrimental to health.[103][104][105]

Lead was the principal component of the alloy used in hot metal typesetting. It was used for plumbing (hence the name) as well as a preservative for food and drink in Ancient Rome. Until the early 1970s, lead was used for joining cast iron water pipes and used as a material for small diameter water pipes.[106]

Tetraethyllead was used in leaded fuels to reduce engine knocking, but this practice has been phased out across many countries of the world in efforts to reduce toxic pollution that affected humans and the environment.[107][108][109][110]

Lead was used to make bullets for slings. Lead is used for shotgun pellets (shot). Waterfowl hunting in the US with lead shot is illegal and it has been replaced with steel and other non-toxic shot for that purpose. 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.[111]

Lead was a component of the paint used on children's toys – now restricted in the United States and across Europe (ROHS Directive). Lead solder was used as a car body filler, which was used in many custom cars in the 1940s–60s; hence the term Leadsled. Lead is a superconductor with a transition temperature of 7.2 K, and therefore IBM tried to make a Josephson effect computer out of a lead alloy.[112]

Lead was also used in pesticides before the 1950s, when fruit orchards were treated especially against the codling moth.[113] A lead cylinder attached to a long line was used by sailors for the vital navigational task of determining water depth by heaving the lead at regular intervals. A soft tallow insert at its base allowed the nature of the sea bed to be determined, to assess its suitability for anchoring.[114]

Bioremediation

Fish bones are being researched for their ability to bioremediate lead in contaminated soil.[115][116] The fungus Aspergillus versicolor is both greatly effective and fast at removing lead ions.[117] Several bacteria have been researched for their ability to reduce lead; including the sulfate reducing bacteria Desulfovibrio and Desulfotomaculum; which are highly effective in aqueous solutions.[118]

Health effects

Lead is 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. However, even at this low level, a prolonged intake can be hazardous to human beings.[119][120]

Much of its toxicity comes from how Pb2+ ions are confused for Ca2+ ions, and lead as a result gets into bones.

The main target for lead toxicity is the nervous system, both in adults and children. Long-term exposure of adults can result in decreased performance in some tests that measure functions of the nervous system.[121] Long-term exposure to lead or its salts (especially soluble salts or the strong oxidant PbO2) can cause nephropathy, and colic-like abdominal pains. It may also cause weakness in 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.[122] 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.[123] 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.[124] The treatment for lead poisoning consists of dimercaprol and succimer.[125]

NFPA 704
"fire diamond"
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oil Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Fire diamond for lead granules

The concern about lead's role in cognitive deficits in children has brought about widespread reduction in its use (lead exposure has been linked to learning disabilities).[126] Most cases of adult elevated blood lead levels are workplace-related.[127] High blood levels are associated with delayed puberty in girls.[128] Lead has been shown many times to permanently reduce the cognitive capacity of children at extremely low levels of exposure.[129]

During the 20th century, the use of lead in paint pigments was sharply reduced because of the danger of lead poisoning, especially to children.[130][131] By the mid-1980s, a significant shift in lead end-use patterns had taken place. Much of this shift was a result of the U.S. lead consumers' compliance 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.[132] Lead may still be found in harmful quantities in stoneware,[133] vinyl[134] (such as that used for tubing and the insulation of electrical cords), and Chinese brass. Old houses may still contain substantial amounts of lead paint.[134] 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.[135]

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 (OSHA) has set the legal limit (permissible exposure limit) for lead exposure in the workplace as 0.050 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 (NIOSH) has set a recommended exposure limit (REL) of 0.050 mg/m3 over an 8-hour workday, and recommends that workers' blood concentrations of lead stay below 0.060 mg per 100 g blood. At levels of 100 mg/m3, lead is immediately dangerous to life and health.[136]

Lead salts used in pottery glazes have on occasion caused poisoning, when acidic drinks, such as fruit juices, have leached lead ions out of the glaze.[137] It has been suggested that what was known as "Devon colic" arose from the use of lead-lined presses to extract apple juice in the manufacture of cider. Lead is considered to be particularly harmful for women's ability to reproduce. Lead(II) acetate (also known as sugar of lead) was used in the Roman Empire as a sweetener for wine, and some consider this a plausible explanation for the dementia of many Roman emperors, and that chronic lead poisoning contributed to the empire's gradual decline. (see Decline of the Roman Empire#Lead poisoning)[138]

Biochemistry of poisoning

In the human body, lead inhibits porphobilinogen synthase and ferrochelatase, preventing both porphobilinogen formation and the incorporation of iron into protoporphyrin IX, the final step in heme synthesis. This causes ineffective heme synthesis and subsequent microcytic anemia.[139] At lower levels, it acts as a calcium analog, interfering with ion channels during nerve conduction. This is one of the mechanisms by which it interferes with cognition. Acute lead poisoning is treated using disodium calcium edetate: the calcium chelate of the disodium salt of ethylene-diamine-tetracetic acid (EDTA). This chelating agent has a greater affinity for lead than for calcium and so the lead chelate is formed by exchange. This is then excreted in the urine leaving behind harmless calcium.[140] According to the Agency for Toxic Substance and Disease Registry, a small amount of ingested lead (1%) will store itself in bones, and the rest will be excreted by an adult through urine and feces within a few weeks of exposure. However, only about 32% of lead will be excreted by a child.[141]

Exposure to lead and lead chemicals can occur through inhalation, ingestion and dermal contact. Most exposure occurs through ingestion or inhalation; in the U.S. the skin exposure is unlikely as leaded gasoline additives are no longer used. Lead exposure is a global issue as lead mining and lead smelting are common in many countries. Most countries had stopped using lead-containing gasoline by 2007.[142] Lead exposure mostly occurs through ingestion. Lead 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.[142] 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 that was used before the Environment Protection Agency issued the regulation around 1980.[143] 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.[144] 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.[142] Dermal exposure may be significant for a narrow category of people working with organic lead compounds, but is of little concern for general population. The rate of skin absorption is also low for inorganic lead.[142]

See also

Notes

  1. An even number of either protons or neutrons generally increases nuclear stability of isotopes, compared to isotopes with odd such numbers. For example, elements with odd atomic numbers have no more than two stable isotopes, while even-numbered elements have multiple stable isotopes, which tin (element 50) having the highest number of isotopes of all elements, ten.[12] See Even and odd atomic nuclei for more details.
  2. The half-life found in the experiment was 1.9×1019 years.[13] A kilogram of natural bismuth, would thus be radioactive with an activity value of approximately 0.003 becquerels—decays per second. For comparison, the natural radiation within human body would make an adult human have radioactivity of 65 becquerels per kilogram of body weight (around 4500 becquerels on average).[14]
  3. It can be enhanced by ionizing atoms of the isotope, which become absolutely stable in the fully ionized state—with all 82 electrons removed.[17]
  4. About 10% of the lanthanide contraction has also been attributed to relativistic effects.[20]

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Bibliography

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Further reading

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  • Casas, Jose S.; Sordo, Jose, eds. (2006). Lead Chemistry, Analytical Aspects. Environmental Impacts and Health Effects. Elsevier. ISBN 0-444-52945-4.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>

External links