|
|
Atomic Number: |
50 |
Melting Point: | 231.93 șC | |
| Atomic Symbol: | Sn | Boiling Point: | 2602 șC | ||
| Atomic Weight: | 118.69 amu | Density: | 7310
kg/m 3 |
||
| Atomic Radius: |
140.5 pm |
Oxidation States: | 4, 2 | ||
| Covalent Radius: | 141 pm | Electron Configuration: | [Kr]5s24d105p2 | ||
| van der Waals Radius: |
217 pm |
State of Matter: | solid |
(anglo-Saxon, tin; L. stannum) Tin is one of the earliest metals known and was used as a component of bronze from antiquity. Because of its hardening effect on copper, tin was used in bronze implements as early as3,500 BC. However the pure metal was not used until about 600 BC.
Ordinary tin is a silver-white metal, is malleable, somewhat ductile, and has a highly crystalline structure. Due to the breaking of these crystals, a "tin cry" is heard when a bar is bent. This metal resists corrosion from distilled sea and soft tap water, but can be attacked by strong acids, alkalis, and by acid salts. Tin acts as a catalyst when oxygen is in solution and helps accelerate chemical attack. Tin can be highly polished and is used as a protective coat for other metals in order to prevent corrosion or other chemical action. It becomes a superconductor below 3.72 K. In fact, tin was one of the first superconductors to be studied; the Meissner effect, one of the characteristic features of superconductors, was first discovered in superconducting tin crystals.
About 35 countries mine tin throughout the world with most of the world's supply coming from Malaya, Bolivia, Indonesia, Zaire, Thailand, and Nigeria. Nearly every continent has an important tin-mining country. Tin is obtained by reducing the ore with coal in a reverberatory furnace. Most of the world's tin is produced from placer deposits; at least one-half comes from Southeast Asia. The only mineral of commercial importance as a source of tin is cassiterite (SnO2), although small quantities of tin are recovered from complex sulfides such as stanite, cylindrite, frankeite, canfieldite, and teallite. Secondary, or scrap, tin is also an important source of the tin.
Tin bonds readily to iron, and has been used for coating lead or zinc and steel to prevent corrosion. Tin-plated steel containers are widely used for food preservation, and this forms a large part of the market for metallic tin.
The element has two allotropic forms at normal pressure. On warming, gray, or alpha tin, with a cubic structure, changes at 13.2oC into white, or beta tin, the ordinary form of the metal. White tin has a tetragonal structure. When tin is cooled below 13.2oC, it changes slowly from white to gray. This change is affected by impurities such as aluminum and zinc, and can be prevented by small additions of antimony or bismuth. This change from the alpha to beta form is called the tin pest. There are few if any uses for gray tin. Tin takes a high polish and is used to coat other metals to prevent corrosion or other chemical action. Such tin plate over steel is used in the so-called tin can for preserving food.
Alloys of tin are very important. Soft solder, type metal, fusible metal, pewter, bronze, bell metal, Babbitt metal, White metal, die casting alloy, and phosphor bronze are some of the important alloys using tin.
When heated in air, tin forms Sn2, which is feebly acid, forming stannate salts with basic oxides. The most important salt is the chloride, which is used as a reducing agent and as a mordant in calico printing. Tin salts sprayed onto glass are used to produce electrically conductive coatings. These have been used for panel lighting and for frost-free windshields. Most window glass is now made by floating molten glass on molten tin (float glass) to produce a flat surface (Pilkington process).
Also interesting is a crystalline tin-niobium alloy that is superconductive at very low temperatures. This promises to be important in the construction of superconductive magnets that generate enormous field strengths but use practically no power. Such magnets, made of tin-niobium wire, weigh only a few pounds and produce magnetic fields that, when started with a small battery, are comparable to that of a 100 ton electromagnet operated continuously with a large power supply.
Tin is the element with the greatest number of stable isotopes with a total of ten. Eighteen additional unstable isotopes are known.
The small amount of tin found in canned foods is quite harmless. The agreed limit of tin content in U.S. foods is 300 mg/kg. The trialkyl and triaryl tin compounds are used as biocides and must be handled carefully.
