Tin is a chemical element; it has symbol Sn (from Latin stannum) and atomic number 50. A silvery-coloured metal, tin is soft enough to be cut with little force,^[9] and a bar of tin can be bent by hand with little effort. When bent, the so-called "tin cry" can be heard as a result of twinning in tin crystals;^[10] this trait is shared by indium, cadmium, zinc, and mercury in its solid state.

Pure tin after solidifying presents a mirror-like appearance similar to most metals. In most tin alloys (e.g. pewter) the metal solidifies with a dull grey colour.

Tin is a post-transition metal in group 14 of the periodic table of elements. It is obtained chiefly from the mineral cassiterite, which contains stannic oxide, SnO
₂. Tin shows a chemical similarity to both of its neighbors in group 14, germanium and lead, and has two main oxidation states, +2 and the slightly more stable +4. Tin is the 49th-most abundant element on Earth and has, with 10 stable isotopes, the largest number of stable isotopes in the periodic table, due to its magic number of protons.

It has two main allotropes: at room temperature, the stable allotrope is β-tin, a silvery-white, malleable metal; at low temperatures it is less dense grey α-tin, which has the diamond cubic structure. Metallic tin does not easily oxidize in air and water.

The first tin alloy used on a large scale was bronze, made of 1⁄8 tin and 7⁄8 copper (12.5% and 87.5% respectively), from as early as 3000 BC. After 600 BC, pure metallic tin was produced. Pewter, which is an alloy of 85–90% tin with the remainder commonly consisting of copper, antimony, bismuth, and sometimes lead and silver, has been used for flatware since the Bronze Age. In modern times, tin is used in many alloys, most notably tin-lead soft solders, which are typically 60% or more tin, and in the manufacture of transparent, electrically conducting films of indium tin oxide in optoelectronic applications. Another large application is corrosion-resistant tin plating of steel. Because of the low toxicity of inorganic tin, tin-plated steel is widely used for food packaging as "tin cans". Some organotin compounds can be extremely toxic.

Characteristics

Tin is a soft, malleable, ductile and highly crystalline silvery-white metal. When a bar of tin is bent a crackling sound known as the "tin cry" can be heard from the twinning of the crystals.^[10] Tin melts at about 232 °C (450 °F), the lowest in group 14. The melting point is further lowered to 177.3 °C (351.1 °F) for 11 nm particles.^[11][12]

β-tin, also called white tin, is the allotrope (structural form) of elemental tin that is stable at and above room temperature. It is metallic and malleable, and has body-centered tetragonal crystal structure. α-tin, or gray tin, is the nonmetallic form. It is stable below 13.2 °C (55.8 °F) and is brittle. α-tin has a diamond cubic crystal structure, as do diamond and silicon. α-tin does not have metallic properties because its atoms form a covalent structure in which electrons cannot move freely. α-tin is a dull-gray powdery material with no common uses other than specialized semiconductor applications.^[10] γ-tin and σ-tin exist at temperatures above 161 °C (322 °F)  and pressures above several GPa.^[13]

In cold conditions β-tin tends to transform spontaneously into α-tin, a phenomenon known as "tin pest" or "tin disease".^[14] Some unverifiable sources also say that, during Napoleon's Russian campaign of 1812, the temperatures became so cold that the tin buttons on the soldiers' uniforms disintegrated over time, contributing to the defeat of the Grande Armée,^[15] a persistent legend.^[16][17][18]

The α-β transformation temperature is 13.2 °C (55.8 °F), but impurities (e.g. Al, Zn, etc.) lower it well below 0 °C (32 °F). With the addition of antimony or bismuth the transformation might not occur at all, increasing durability.^[19]

Commercial grades of tin (99.8% tin content) resist transformation because of the inhibiting effect of small amounts of bismuth, antimony, lead, and silver present as impurities. Alloying elements such as copper, antimony, bismuth, cadmium, and silver increase the hardness of tin.^[20] Tin easily forms hard, brittle intermetallic phases that are typically undesirable. It does not mix into a solution with most metals and elements so tin does not have much solid solubility. Tin mixes well with bismuth, gallium, lead, thallium and zinc, forming simple eutectic systems.^[19]

Tin becomes a superconductor below 3.72 K^[21] and was one of the first superconductors to be studied.^[22] The Meissner effect, one of the characteristic features of superconductors, was first discovered in superconducting tin crystals.^[22]

Chemical[edit]

Tin resists corrosion from water, but can be corroded by acids and alkalis. Tin can be highly polished and is used as a protective coat for other metals,^[10] a protective oxide (passivation) layer prevents further oxidation.^[23] Tin acts as a catalyst triggering a chemical reaction of a solution containing oxygen and helps to increase the speed of the chemical reaction that results.^[24]

Isotopes[edit]

Tin has ten stable isotopes, the greatest number of any element. Their mass numbers are: 112, 114, 115, 116, 117, 118, 119, 120, 122, and 124, although Sn-112, Sn-122, and Sn-124 are theoretically unstable and can undergo double beta decay. Tin-120 makes up almost a third of all tin. Tin-118 and tin-116 are also common. Tin-115 is the least common stable isotope. The isotopes with even mass numbers have no nuclear spin, while those with odd mass numbers have a nuclear spin of 1/2. It is thought that tin has such a great multitude of stable isotopes because of tin's atomic number being 50, which is a "magic number" in nuclear physics.

Tin is one of the easiest elements to detect and analyze by NMR spectroscopy, which relies on molecular weight and its chemical shifts are referenced against tetramethyltin (SnMe
₄).^{[notes 1][25]}

Of the stable isotopes, tin-115 has a high capture cross section for fast neutron energies, at 30 barns. Tin-117 ranks one below, with a cross section of 2.3 barns, while tin-119 has a slightly smaller cross section of 2.2 barns. ^[26] Before these cross sections were well known, it was proposed to use tin-lead solder as a reactor coolant for fast reactors because of its low melting point. Current studies are for lead or lead-bismuth reactor coolants because both heavy metals are nearly transparent to fast neutrons, with very low capture cross sections. ^[27] In order to use a tin or tin-lead coolant the tin would first have to go through isotopes separation to remove the 115, 117 and 119 tin isotopes. Combined, these three isotopes make up about 17% of natural tin but represent nearly all of the capture cross section. Of the remaining seven isotopes tin-112 has a capture cross section of 1 barn. The other six isotopes forming 82.7% of natural tin have capture cross sections of 0.3 barns or less making them effectively transparent to neutrons, like lead and bismuth are.

Tin has 31 unstable isotopes, ranging in mass number from 99 to 139. The unstable tin isotopes have half-lives of less than a year except for tin-126, which has a half-life of about 230,000 years. Tin-100 and tin-132 are two of the very few nuclides with a "doubly magic" nucleus which despite being unstable, as they have very uneven neutron–proton ratios, are the endpoints beyond which tin isotopes lighter than tin-100 and heavier than tin-132 are much less stable.^[28] Another 30 metastable isomers have been identified for tin isotopes between 111 and 131, the most stable being tin-121m, with a half-life of 43.9 years.^[29]

The relative differences in the number of tin's stable isotopes can be explained by how they are formed during stellar nucleosynthesis. Tin-116 through tin-120 are formed in the s-process (slow neutron capture) in most stars which leads to them being the most common tin isotopes, while tin-122 and tin-124 are only formed in the r-process (rapid neutron capture) in supernovae and are less common. Tin isotopes 117 through 120 are also produced in the r-process.^{[citation needed]}. Tin isotopes 112, 114, and 115, cannot be made in significant amounts in the s- or r-processes and are among the p-nuclei whose origins are not well understood. Some theories about their formation include proton capture and photodisintegration. Tin-115 might be partially produced in the s-process both directly and as the daughter of long-lived indium-115.

RECTANGLE AUDIO DESIGN TIN BOX

Etymology[edit]

The word tin is shared among Germanic languages and can be traced back to reconstructed Proto-Germanic *tin-om; cognates include German Zinn, Swedish tenn and Dutch tin. It is not found in other branches of Indo-European, except by borrowing from Germanic (e.g., Irish tinne from English).^[31][32]

The Latin name for tin, stannum, originally meant an alloy of silver and lead, and came to mean 'tin' in the fourth century^[33]—the earlier Latin word for it was plumbum candidum, or "white lead". Stannum apparently came from an earlier stāgnum (meaning the same substance),^[31] the origin of the Romance and Celtic terms for tin, such as French étain, Spanish estaño, Italian stagno, and Irish stán.^[31][34] The origin of stannum/stāgnum is unknown; it may be pre-Indo-European.^[35]

The Meyers Konversations-Lexikon suggests instead that stannum came from Cornish stean, and is evidence that Cornwall in the first centuries AD was the main source of tin.^{[citation needed]}

History

Tin extraction and use can be dated to the beginnings of the Bronze Age around 3000 BC, when it was observed that copper objects formed of polymetallic ores with different metal contents had different physical properties.^[36] The earliest bronze objects had a tin or arsenic content of less than 2% and are believed to be the result of unintentional alloying due to trace metal content in the copper ore.^[37] The addition of a second metal to copper increases its hardness, lowers the melting temperature, and improves the casting process by producing a more fluid melt that cools to a denser, less spongy metal.^[37] This was an important innovation that allowed for the much more complex shapes cast in closed molds of the Bronze Age. Arsenical bronze objects appear first in the Near East where arsenic is commonly found with copper ore, but the health risks were quickly realized and the quest for sources of the much less hazardous tin ores began early in the Bronze Age.^[38] This created the demand for rare tin metal and formed a trade network that linked the distant sources of tin to the markets of Bronze Age cultures.^{[citation needed]}

Cassiterite (SnO
₂), the oxide form of tin, was most likely the original source of tin. Other tin ores are less common sulfides such as stannite that require a more involved smelting process. Cassiterite often accumulates in alluvial channels as placer deposits because it is harder, heavier, and more chemically resistant than the accompanying granite.^[37] Cassiterite is usually black or dark in color, and these deposits can be easily seen in river banks. Alluvial (placer) deposits may incidentally have been collected and separated by methods similar to gold panning.^[39]

Compounds and chemistry[edit]

In the great majority of its compounds, tin has the oxidation state II or IV. Compounds containing bivalent tin are called stannous while those containing tetravalent tin are termed stannic.

Inorganic compounds[edit]

Halide compounds are known for both oxidation states. For Sn(IV), all four halides are well known: SnF₄, SnCl₄, SnBr₄, and SnI₄. The three heavier members are volatile molecular compounds, whereas the tetrafluoride is polymeric. All four halides are known for Sn(II) also: SnF₂, SnCl
₂, SnBr₂, and SnI₂. All are polymeric solids. Of these eight compounds, only the iodides are colored.^[40]

Tin(II) chloride (also known as stannous chloride) is the most important commercial tin halide. Illustrating the routes to such compounds, chlorine reacts with tin metal to give SnCl₄ whereas the reaction of hydrochloric acid and tin produces SnCl
₂ and hydrogen gas. Alternatively SnCl₄ and Sn combine to stannous chloride by a process called comproportionation:^[41]

SnCl₄ + Sn → 2 SnCl
₂

Tin can form many oxides, sulfides, and other chalcogenide derivatives. The dioxide SnO
₂ (cassiterite) forms when tin is heated in the presence of air.^[40] SnO
₂ is amphoteric, which means that it dissolves in both acidic and basic solutions.^[42] Stannates with the structure [Sn(OH)
₆]²⁻, like K
₂[Sn(OH)
₆], are also known, though the free stannic acid H
₂[Sn(OH)
₆] is unknown.

Sulfides of tin exist in both the +2 and +4 oxidation states: tin(II) sulfide and tin(IV) sulfide (mosaic gold).

Hydrides[edit]

Stannane (SnH
₄), with tin in the +4 oxidation state, is unstable. Organotin hydrides are however well known, e.g. tributyltin hydride (Sn(C₄H₉)₃H).^[10] These compound release transient tributyl tin radicals, which are rare examples of compounds of tin(III).^[44]

Organotin compounds[edit]

Organotin compounds, sometimes called stannanes, are chemical compounds with tin–carbon bonds.^[45] Of the tin compounds, the organic derivatives are commercially the most useful.^[46] Some organotin compounds are highly toxic and have been used as biocides. The first organotin compound to be reported was diethyltin diiodide ((C₂H₅)₂SnI₂), reported by Edward Frankland in 1849.^[47]

Most organotin compounds are colorless liquids or solids that are stable to air and water. They adopt tetrahedral geometry. Tetraalkyl- and tetraaryltin compounds can be prepared using Grignard reagents:^[46]

SnCl
₄ + 4 RMgBr → R
₄Sn + 4 MgBrCl

The mixed halide-alkyls, which are more common and more important commercially than the tetraorgano derivatives, are prepared by redistribution reactions:

SnCl
₄ + R
₄Sn → 2 SnCl
₂R₂

Divalent organotin compounds are uncommon, although more common than related divalent organogermanium and organosilicon compounds. The greater stabilization enjoyed by Sn(II) is attributed to the "inert pair effect". Organotin(II) compounds include both stannylenes (formula: R₂Sn, as seen for singlet carbenes) and distannylenes (R₄Sn₂), which are roughly equivalent to alkenes. Both classes exhibit unusual reactions.^[48]

Occurrence[edit]

Tin is generated via the long s-process in low-to-medium mass stars (with masses of 0.6 to 10 times that of the Sun), and finally by beta decay of the heavy isotopes of indium.^[49]

Tin is the 49th most abundant element in Earth's crust, representing 2 ppm compared with 75 ppm for zinc, 50 ppm for copper, and 14 ppm for lead.^[50]

Tin does not occur as the native element but must be extracted from various ores. Cassiterite (SnO
₂) is the only commercially important source of tin, although small quantities of tin are recovered from complex sulfides such as stannite, cylindrite, franckeite, canfieldite, and teallite. Minerals with tin are almost always associated with granite rock, usually at a level of 1% tin oxide content.^[51]

Because of the higher specific gravity of tin dioxide, about 80% of mined tin is from secondary deposits found downstream from the primary lodes. Tin is often recovered from granules washed downstream in the past and deposited in valleys or the sea. The most economical ways of mining tin are by dredging, hydraulicking, or open pits. Most of the world's tin is produced from placer deposits, which can contain as little as 0.015% tin.^[52]

About 253,000 tonnes of tin were mined in 2011, mostly in China (110,000 t), Indonesia (51,000 t), Peru (34,600 t), Bolivia (20,700 t) and Brazil (12,000 t).^[53] Estimates of tin production have historically varied with the market and mining technology. It is estimated that, at current consumption rates and technologies, the Earth will run out of mine-able tin in 40 years.^[54] In 2006 Lester Brown suggested tin could run out within 20 years based on conservative estimates of 2% annual growth.^[55]

Scrap tin is an important source of the metal. Recovery of tin through recycling is increasing rapidly.^{[when?][citation needed]} Whereas the United States has neither mined (since 1993) nor smelted (since 1989) tin, it was the largest secondary producer, recycling nearly 14,000 tonnes in 2006.^[53]

New deposits are reported in Mongolia,^[56] and in 2009, new deposits of tin were discovered in Colombia.