a chemical element belonging to the carbon family, Group IVa of the periodic table. It is a soft, silvery-white metal with a bluish tinge, known to the ancients in bronze, an alloy with copper. Tin is widely used for plating steel cans used as food containers, in metals used for bearings, and in solder.

Occurrence, properties, and uses

The element is present in the igneous rocks of the Earth's crust to the extent of about 0.001 percent, occurring in grains of the native metal but chiefly as the oxide in the mineral cassiterite, from which the metal is obtained by reduction (removal of the oxygen) with coal or coke in smelting furnaces. The oldest tin mines were those in England and Spain, but those in Indonesia, Thailand, Congo (Kinshasa), Nigeria, and China have surpassed them in modern production. Several processes have been devised for reclaiming the metal from scrap tin or tin-plated articles. For a full treatment of tin mining, refining, and recovery, see tin processing.

Tin is nontoxic, ductile, malleable, and adapted to all kinds of cold-working, such as rolling, spinning, and extrusion. The colour of pure tin is retained during exposure because a thin, invisible, protective film of tin(IV) oxide is formed spontaneously by reaction with the oxygen of the air. The low melting point of tin and its firm adhesion to clean surfaces of iron, steel, copper, and copper alloys facilitate its use as an oxidation-resistant coating material. Tin exists in two different forms, or allotropes: the familiar form, white (or beta) tin, and gray (or alpha) tin, which is powdery and of little use. The gray form changes to the white above 13.2° C (55.8° F), rapidly at temperatures above 100° C (212° F); the reverse transformation, called tin pest, occurs at low temperatures but is prevented by small amounts of antimony, bismuth, copper, lead, silver, or gold normally present in commercial grades of tin.

White tin has a body-centred tetragonal crystal structure, and gray tin has a face-centred cubic structure. When bent, tin makes an eerie, crackling “cry” as its crystals crush each other. Tin is attacked by strong acids and alkalies, but nearly neutral solutions do not affect it appreciably. Chlorine, bromine, and iodine react with tin, but fluorine reacts with it only slowly at room temperature.

Because pure tin is relatively weak, it is not put to structural uses unless alloyed with other metals. Numerous alloys of tin are used, including soft solder, type metal, pewter, bronze, bell metal, babbitt metal, and low-temperature casting alloys. A crystalline alloy with niobium is a superconductor at temperatures as high as 18 K (−427° F) and retains this property in very strong magnetic fields.

Tin has 10 stable isotopes, occurring in the following percentages in natural tin: tin-112, 0.97; tin-114, 0.65; tin-115, 0.36; tin-116, 14.53; tin-117, 7.68; tin-118, 24.22; tin-119, 8.58; tin-120, 32.59; tin-122, 4.63; and tin-124, 5.79.

Compounds

Tin forms two series of compounds: stannous, or tin(II), compounds and stannic, or tin(IV), compounds. Some of the more commercially important compounds of tin(II) are stannous chloride, SnCl₂, used in tin galvanizing and as a reducing agent in the manufacture of polymers and dyes; stannous oxide, SnO, employed in making tin salts for chemical reagents and for plating; and stannous fluoride, SnF₂, an active ingredient in toothpastes. Tin(IV) compounds of significance include stannic chloride, SnCl₄, widely used as a stabilizer for perfumes and as a starting material for other tin salts; and stannic oxide, SnO₂, a useful catalyst in certain industrial processes and a polishing powder for steel.

Tin can form a bond with carbon, as in the more than 500 known organotin compounds. Organotin stabilizers are used to prevent changes in polyvinyl chloride upon exposure to light and heat. A number of organotin compounds are major ingredients in biocides and fungicides.