Tin

Tin is a chemical element with the symbol Sn (Latin: Stannum) and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead, like the two possible oxidation states +2 and +4. Tin is the 49th most abundant element and has, with 10 isotopes, the largest number of stable isotopes in the periodic table. Tin is obtained chiefly from the mineral cassiterite, where it occurs as tin dioxide, SnO2.

This silvery, malleable poor metal is not easily oxidized in air, and is used to coat other metals to prevent corrosion. The first alloy used in large scale since 3000 BC was bronze, an alloy of tin and copper. After 600 BC pure metallic tin was produced. Pewter, which is an alloy of 85 % to 90 % tin with the remainder commonly consisting of copper, antimony and lead, was used for flatware from the Bronze Age until the 20th century. In modern times tin is used in many alloys, most notably tin/lead soft solders, typically containing 60% or more of tin. Another large application for tin is corrosion-resistant tin plating of steel. Due to its low toxicity, tin-plated metal is also used for food packaging, giving the name to tin cans, which are made mostly out of aluminium or tin-plated steel.

Characteristics

Physical and allotropes

Tin is a malleable, ductile, and highly crystalline silvery-white metal. It is malleable at ordinary temperatures but is brittle when it is cooled, due to the properties of its two major allotropes, α- and β-tin. When a bar of tin is bent, a crackling sound known as the tin cry can be heard due to the twinning of the crystals. The two allotropes that are encountered at normal pressure and temperature, α-tin and β-tin, are more commonly known as gray tin, and respectively white tin. Two more allotropes, γ and σ, exist at temperatures above 161 °C and pressures above several GPa. White tin, or the β-form, is metallic, and is the stable one at room conditions or at higher temperatures. Below 13.2 °C, tin exists in the gray α-form, which has a diamond cubic crystal structure, similar to diamond, silicon or germanium. Gray tin has no metallic properties at all, is a dull-gray powdery material, and has few uses, other than a few specialized semiconductor applications.

Although the α-β transformation temperature is nominally 13.2 °C, impurities (e.g. Al, Zn, etc.) lower the transition temperature well below 0 °C, and upon addition of Sb or Bi the transformation may not occur at all. This conversion is known as tin disease or tin pest. Tin pest was a particular problem in northern Europe in the 18th century as organ pipes made of tin alloy would sometimes be affected during long cold winters. Some 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, contributing to the defeat of the Grande Armée. The veracity of this story is debatable, because the transformation to gray tin often takes a reasonably long time.

Commercial grades of tin (99.8%) resist transformation because of the inhibiting effect of the small amounts of bismuth, antimony, lead, and silver present as impurities. Alloying elements such as copper, antimony, bismuth, cadmium, and silver increase its hardness. Tin tends rather easily to form hard, brittle intermetallic phases, which are often undesirable. It does not form wide solid solution ranges in other metals in general, and there are few elements that have appreciable solid solubility in tin. Simple eutectic systems, however, occur with bismuth, gallium, lead, thallium, and zinc.

Chemistry and compounds

Tin is classified as a semimetal, as its chemical properties fall between those of metals and non-metals, just as the semiconductors silicon and germanium do. It resists corrosion from distilled, sea and soft tap water, but can be attacked by strong acids, alkalis, and acid salts. Tin can be highly polished and is used as a protective coat for other metals in order to prevent corrosion or other chemical action. Tin acts as a catalyst when oxygen is in solution and helps accelerate chemical attack.

Tin forms the dioxide SnO2 (cassiterite) when it is heated in the presence of air. SnO2, in turn, is feebly acidic and forms stannate (SnO32−) salts with basic oxides. There are also stannates with the structure [Sn(OH)6]2−, like K2[Sn(OH)6], although the free stannic acid H2[Sn(OH)6] is unknown.

Tin combines directly with chlorine forming tin(IV) chloride, while reacting tin with hydrochloric acid in water gives tin(II) chloride and hydrogen. Several other compounds of tin exist in the +2 and +4 oxidation states, such as tin(II) sulfide and tin(IV) sulfide (Mosaic gold). There is only one stable hydride, however: stannane (SnH4), where tin is in the +4 oxidation state.

The most important salt is stannous chloride, which has found use as a reducing agent and as a mordant in the calico printing process. Electrically conductive coatings are produced when tin salts are sprayed onto glass. These coatings have been used in panel lighting and in the production of frost-free windshields.

Tin is added to some dental care products as stannous fluoride (SnF2). Stannous fluoride can be mixed with calcium abrasives while the more common sodium fluoride gradually becomes biologically inactive combined with calcium. It has also been shown to be more effective than sodium fluoride in controlling gingivitis.

Organotin compounds or stannanes are chemical compounds based on tin with hydrocarbon substituents. Organotin compounds usually have high toxicity and have been used as biocides, but their use is slowly being phased out. The first organotin compound was diethyltin diiodide (Sn(C2H5)2I2), discovered by Edward Frankland in 1849. Organotin compounds differ from their lighter analogues of germanium and silicon in that there is a greater occurrence of the +2 oxidation state due to the "inert pair effect"; it also has a greater range of coordination numbers, and the common presence of halide bridges between polynuclear compounds. Most organotin compounds are colorless liquids or solids that are usually stable to air and water. The tetraalkyl stannates (R4Sn) always have a tetrahedral geometry at the tin atom. The halide derivatives R3SnX often form chained structures with Sn-X-Sn bridges. Alkyltin compounds are usually prepared via Grignard reagent reactions such as in:

SnCl4 + 4 RMgBr → R4Sn + 4 MgBrCl.

Isotopes

Tin is the element with the greatest number of stable isotopes, ten; these include all those with atomic masses between 112 and 124, with the exception of 113, 121 and 123. Of these, the most abundant ones are 120Sn (at almost a third of all tin), 118Sn, and 116Sn, while the least abundant one is 115Sn. The isotopes possessing even atomic numbers have no nuclear spin while the odd ones have a spin of +1/2. Tin, with its three common isotopes 115Sn, 117Sn and 119Sn, is among the easiest elements to detect and analyze by NMR spectroscopy, and its chemical shifts are referenced against SnMe4.

This large number of stable isotopes is thought to be a direct result of tin possessing an atomic number of 50, which is a "magic number" in nuclear physics. There are 28 additional unstable isotopes that are known, encompassing all the remaining ones with atomic masses between 99 and 137. Aside from 126Sn, which has a half-life of 230,000 years, all the radioactive isotopes have a half-life of less than a year. The radioactive 100Sn is one of the few nuclides possessing a "doubly magic" nucleus and was discovered relatively recently, in 1994. Another 30 metastable isomers have been characterized for isotopes between 111 and 131, the most stable of which being 121mSn, with a half-life of 43.9 years.

Etymology

The Latin name Stannum is connected to "stagnum" and "stag" (Indo-European) for dripping because tin melts easily. The former "stagnum" was the word for a stale pool or puddle, with a cognate in the English word "stagnant." The English word "tin" has cognates in many Germanic and Celtic languages. The American Heritage Dictionary speculates that the word was borrowed from a pre-Indo-European language. The later name "stannum" and its Romance derivatives come from the lead-silver alloy of the same name for the finding of the silver in ores. The word definitely assumed its present meaning in the 4th century (H. Kopp).

According to Meyers Konversationslexikon Stannum is derived from Cornish stean (present orthography sten), and is proof that Cornwall in the first centuries AD was the main source of tin. Other sources, however, see the Cornish stean merely as a back-derivation from the Latin stannum. The Latin Stannum became the source for most European words. According to SMI the English word for the metal is named after an Etruscan god, Tinia. (variants include Old English: tin, Old Latin: plumbum candidum ("white lead"), Old German: tsin, Late Latin: stannum)

From http://en.wikipedia.org/