Tin is a chemical element or group-14 metal of the periodic table with the symbol Sn and atomic number 50. It is an important metal used in the early days of human civilization. Tin is used mainly for making non-toxic corrosion-resistant coating in steel, particularly for the packaging of food. Tin has two crystalline solid allotropic forms. The common form is white metallic β-tin having a destroyed tetragonal close-packed crystal lattice with a density of 7.265 g/cm3. Another form is gray metallic α-tin which has a much lower density (5.769 g/cm3). The +2 oxidation number or state is common for tin and lead due to the inert pair effect.
Where is tin found
Tin is slightly more abundant than germanium but it contains specific ores like cassiterite or tinstone (SnO2). Small quantities of Sn are found in sulfides like stannite, cylindrite, franckeite, canfieldite, and teallite. Tin scrap is also an important secondary source of metal. Recovery or recycling of scrap tin is increasing rapidly due to the huge global demand. Malaysia, Russia, Bolivia, Indonesia, Peru Thailand, Brazil, and China are the major producer of the element Sn. In India, tinstone is found mostly in Bihar and Orissa.
Tin has ten stable isotopes with mass number 112, 114, 115, 116, 117, 118, 119, 120, 112 and 124. Of these 120Sn, 118Sn and 116Sn are most stable. The isotope 119Sn (nuclear spin 1/2) is used for NMR studies and Mossbauer spectroscopy.
Production of tin
The mineral cassiterite or tinstone contains only 1 to 5 percent of SnO2, together with silica and wolframite. The major steps for extraction of metal are,
- The ore is crushed and washed. Light siliceous matter is largely removed.
- The washed ore roasted in a current of hot air. Sulfur and arsenic compounds are driven out. Iron forms Fe3O4.
- The nonmagnetic tinstone is separated from magnetic wolframite and Fe2O3 by the magnetic separation technique. SnO2 is concentrated by further washed.
- The concentrate or black Sn is reduced with coke at 1200 to 1300 °C to produced metal.
Properties of tin
The chemistry and chemical properties of group 14 elements follow from their electronic configuration. All the elements contain an s2p2 valence shell electronic configuration. Although the elements Ge, Sn, and Pb have d and f electrons. Therefore, the properties of these elements differ widely from carbon and silicon. The +2 state is the common and stable oxidation state of tin and lead but for other elements +4 state is common. It liberates hydrogen from hot concentrated HCl but concentrated sulfuric acid gives SnSO4 and SO2. Some common properties of the metal are given below the table,
|Properties of tin|
|Electronic configuration||[Kr] 4d10 5s2 5p2|
|Melting point||231.93 °C|
|Boiling point||2602 °C|
|Density (g/cm3)||white, β-Sn||gray, α-Sn|
|Molar heat capacity||27.112 J mol-1K-1 (white, β-Sn)|
|Electrical resistivity||115 nΩ·m|
|Crystal structure||white, β-Sn||gray, α-Sn|
|body-centered tetragonal||face-centered diamond-cubic|
|Common oxidation state||+2, +4|
|Electronegativity||Pauling scale: 1.96|
All the elements of group-14 form gaseous hydrides like MH4. The stability of the group-14 hydrides decreases sharply from Ge to Pb. SnH4 decomposes slowly at room temperature and is passive towards dilute aqueous acids and alkalis. But it is decomposed by concentrated acids or alkalis. SnH4 is a good reducing agent in organic chemistry. It is used for transformation benzaldehyde to benzyl alcohol or nitrobenzene to aniline.
All the four tetrahalides of tin are known. Except for SnF4, all these halides are formed by covalent bonding with volatile in nature. SnF4 is a hygroscopic white solid formed by treating SnCl4 with anhydrous HF. The other SnX4 compounds are best made by the direct union of metal and halide. SnCl4 and SnBr4 are colorless liquids while SnI4 is a bright orange solid.
As mentioned earlier, the divalent state of Sn is more stable than the tetravalent state. Therefore, Sn forms all the stable dihalides. The dihalide, SnF2 has been used in toothpaste but it is replaced by NaF due to the toxic nature of Sn.
Tin occurs in nature largely as cassiterite or tinstone (SnO2). It is a white color solid and amphoteric in nature. The amphoteric nature of Sn is shown by its reaction with fused NaOH and concentrated H2SO4.
SnO2 + 2NaOH + H2O → Na2Sn(OH)6
SnO2 + 2H2SO4 → Sn(SO4)2 + 2H2O
Sn also formed stable divalent oxide with molecular formula SnO. It is more basic than SnO2. SnO is obtained by thermal decomposition of stannous oxalate in absence of air.
SnC2O4 → SnO + CO + CO2
Uses of Tin
- Tin is one of the important metals used in early civilizations. It is mainly used as a non-toxic corrosion resistant coating on food packaging.
- Tin alloying with lead produce solder which is used mainly for joining pipes and electric circuits.
- Tin is used for coating lead, zinc, and steel to prevent corrosion. Tin-plated steel containers are widely used for food preservation. Sheet steel coated with a Pb-Sn alloy is used for roofing and gasoline tanks.
- It combines with other elements to form different types of alloys. For example, pewter (Sn 90 – 95%), Sb (1 -8%), Cu (0.5 – 3%). It is used for making trays, plates, and trophies. An alloy with 90 to 95% Sn and Pb is used in organ pipes.
- Tin is sometimes used for making American and Canadian pennies. Copper is the major content in such coins.
- Niobium tin compounds are used for making coils. These types of coils are used for making superconducting magnets due to their high critical temperature.