lithium

lithium (Li), chemical element of Group 1 (Ia) in the periodic table, the alkali metal group, lightest of the solid elements. The metal itself—which is soft, white, and lustrous—and several of its alloys and compounds are produced on an industrial scale.

Occurrence and production

Discovered in 1817 by Swedish chemist Johan August Arfwedson in the mineral petalite, lithium is also found in brine deposits and as salts in mineral springs; its concentration in seawater is 0.1 part per million (ppm). Lithium is also found in pegmatite ores, such as spodumene (LiAlSi₂O₆) and lepidolite (of varying structure), or in amblygonite (LiAlFPO₄) ores, with Li₂O contents ranging between 4 and 8.5 percent. It constitutes about 0.002 percent of Earth’s crust.

Until the 1990s the lithium chemical and metal market was dominated by American production from mineral deposits, but by the turn of the 21st century most production was derived from non-U.S. sources; Australia, Chile, and Portugal were the world’s largest suppliers. (Bolivia has half the world’s lithium deposits but is not a major producer of lithium.) The major commercial form is lithium carbonate, Li₂CO₃, produced from ores or brines by a number of different processes. Addition of hydrochloric acid (HCl) produces lithium chloride, which is the compound used to produce lithium metal by electrolysis. Lithium metal is produced by electrolysis of a fused mixture of lithium and potassium chlorides. The lower melting point of the mixture (400–420 °C, or 750–790 °F) compared with that of pure lithium chloride (610 °C, or 1,130 °F) permits lower-temperature operation of the electrolysis. Since the voltage at which decomposition of lithium chloride takes place is lower than that of potassium chloride, lithium is deposited at a purity level greater than 97 percent. Graphite anodes are used in the electrolytic production of lithium, while the cathodes are made of steel. The pure lithium formed at the cathode coalesces at the surface of the electrolyte to form a molten pool, which is protected from reaction with air by a thin film of the electrolyte. The lithium is ladled from the cell and cast by pouring it into a mold at a temperature only slightly above the melting point, leaving the solidified electrolyte behind. The solidified lithium is then remelted, and materials insoluble in the melt either float to the surface or sink to the bottom of the melt pot. The remelting step reduces the potassium content to less than 100 parts per million. Lithium metal, which can be drawn into wire and rolled into sheets, is softer than lead but harder than the other alkali metals and has the body-centred cubic crystal structure.

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Many lithium alloys are produced directly by the electrolysis of molten salts, containing lithium chloride in the presence of a second chloride, or by the use of cathode materials that interact with the deposited lithium, introducing other elements into the melt.

The table lists the major producers of lithium.

Significant uses

The principal industrial applications for lithium metal are in metallurgy, where the active element is used as a scavenger (remover of impurities) in the refining of such metals as iron, nickel, copper, and zinc and their alloys. A large variety of nonmetallic elements are scavenged by lithium, including oxygen, hydrogen, nitrogen, carbon, sulfur, and the halogens. Lithium is utilized to a considerable extent in organic synthesis, both in laboratory reactions and industrially. A key reagent that is produced commercially on a large scale is n-butyllithium, C₄H₉Li. Its principal commercial use is as an initiator of polymerization, for example, in the production of synthetic rubber. It is also extensively used in the production of other organic chemicals, especially pharmaceuticals. Because of its light weight and large negative electrochemical potential, lithium metal, either pure or in the presence of other elements, serves as the anode (negative electrode) in many nonrechargeable lithium primary batteries. Since the early 1990s much work has been done on high-power rechargeable lithium storage batteries for electric vehicles and for power storage. The most successful of these provides for separation of the anode and a cathode such as LiCoO₂ by a solvent-free conducting polymer that permits migration of the lithium cation, Li⁺. Smaller rechargeable lithium batteries are extensively used for cell phones, cameras, and other electronic devices.

Lightweight lithium-magnesium alloys and tough lithium-aluminum alloys, harder than aluminum alone, have structural applications in the aerospace and other industries. Metallic lithium is used in the preparation of compounds such as lithium hydride.

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Chemical properties

In many of its properties, lithium exhibits the same characteristics as do the more common alkali metals sodium and potassium. Thus, lithium, which floats on water, is highly reactive with it and forms strong hydroxide solutions, yielding lithium hydroxide (LiOH) and hydrogen gas. Lithium is the only alkali metal that does not form the anion, Li⁻, in solution or in the solid state.

Lithium is chemically active, readily losing one of its three electrons to form compounds containing the Li⁺ cation. Many of these differ markedly in solubility from the corresponding compounds of the other alkali metals. Lithium carbonate (Li₂CO₃) exhibits the remarkable property of retrograde solubility; it is less soluble in hot water than in cold.

Lithium and its compounds impart a crimson colour to a flame, which is the basis of a test for its presence. It is commonly kept in mineral oil because it reacts with the moisture in the air.

Organolithium compounds, in which the lithium atom is not present as the Li⁺ ion but is attached directly to a carbon atom, are useful in making other organic compounds. Butyllithium (C₄H₉Li), which is used in the manufacture of synthetic rubber, is prepared by the reaction of butyl bromide (C₄H₉Br) with metallic lithium.

In many respects lithium also shows similarities to the elements of the alkaline-earth group, especially magnesium, which has similar atomic and ionic radii. This similarity is seen in oxidation properties, the monoxide being normally formed in each case. Reactions of organolithium compounds are also similar to the Grignard reactions of organomagnesium compounds, a standard synthetic procedure in organic chemistry.

A number of the lithium compounds have practical applications. Lithium hydride (LiH), a gray crystalline solid produced by the direct combination of its constituent elements at elevated temperatures, is a ready source of hydrogen, instantly liberating that gas upon treatment with water. It also is used to produce lithium aluminum hydride (LiAlH₄), which quickly reduces aldehydes, ketones, and carboxylic esters to alcohols.

Lithium hydroxide (LiOH), commonly obtained by the reaction of lithium carbonate with lime, is used in making lithium salts (soaps) of stearic and other fatty acids; these soaps are widely used as thickeners in lubricating greases. Lithium hydroxide is also used as an additive in the electrolyte of alkaline storage batteries and as an absorbent for carbon dioxide. Other industrially important compounds include lithium chloride (LiCl) and lithium bromide (LiBr). They form concentrated brines capable of absorbing aerial moisture over a wide range of temperatures; these brines are commonly employed in large refrigerating and air-conditioning systems. Lithium fluoride (LiF) is used chiefly as a fluxing agent in enamels and glasses.

Nuclear properties

Lithium, which exhibits no natural radioactivity, has two isotopes of mass number 6 (92.5 percent) and 7 (7.5 percent). The lithium-7/lithium-6 ratio is between 12 and 13.

Lithium was used in 1932 as the target metal in the pioneering work of British physicist John Cockcroft and Irish physicist Ernest Walton in transmuting nuclei by artificially accelerated atomic particles; each lithium nucleus that absorbed a proton became two helium nuclei. The bombardment of lithium-6 with slow neutrons produces helium and tritium (³H); this reaction is a major source of tritium production. Tritium so produced is employed in the manufacture of hydrogen bombs, among other uses such as providing a radioactive hydrogen isotope for biological research.

Lithium has potential value as a heat-transfer fluid for high power-density nuclear reactors. The lithium-7 isotope, the more common stable isotope, has a low nuclear cross section (that is, it absorbs neutrons very poorly) and thus has potential as a primary coolant for nuclear reactors in which coolant temperatures above about 800 °C (1,500 °F) are required. The isotopes lithium-8 (half-life 0.855 second) and lithium-9 (half-life 0.17 second) have been produced by nuclear bombardment.

Biological properties

The widespread occurrence of lithium in plants results in a wide, although low-level, distribution of lithium in animals. Lithium salts have complex effects when absorbed into the body. They are not highly toxic, although high levels can be fatal. The use of lithium salts and mineral water containing them to treat gout (unsuccessfully) and to ward off depression (successfully) dates to the last half of the 19th century but fell into medical disrepute in the early 20th century. The use of lithium carbonate to treat manic-depression (also known as bipolar disorder) was demonstrated clinically in 1954. Fears about lithium toxicity delayed its approval for many years, but it is now the major drug for the treatment of manic episodes and for maintenance therapy in bipolar patients.