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Element Fact
Selenium is an essential trace element in the diets that acts as antioxidants.

Iron, Fe

iron sample

Atomic number: 26
Atomic mass: 55.845(2)
Natural abundance: 62000 ppm
Isotopes:
54Fe (53.939615) - 5.8%
56Fe (55.934942) - 91.72%
57Fe (56.935399) - 2.1%
58Fe (57.933280) - 0.28%

History and Production

The word iron is derived from Anglo-Saxon iron. In Latin, it is called ferrum, of which the symbol Fe was derived. Iron was known since the prehistoric times. There is some historic evidence to indicate that iron was first smelted by the Hittites as early as the third millennium BC. However, the smelting process was not known in other places until around 1200 BC when the Hittite empire fell. Iron is perhaps the most important metal known to man. It is produced from the reduction of coke with various ores such as hematite (Fe2O3) and magnetite (Fe3O4) in a blast furnace. In the past, the ore was reduced by using ordinary charcoal (derived from timber). In around 1773, A. Darby has developed a process of obtaining the carbon source (coke) from coal which dramatically improved the production process. The impact was so dramatic that it ushered mankind into the Industrial Revolution. Pure iron is rarely used in the industry. Instead, the mechanical properties are dramatically improved when additives were added. For example, Pig iron, which is hard and brittle, contains 3% carbon with varying amounts of phosphorus, sulfur, silicon and manganese. Carbon steel is an alloy of iron with carbon while the alloy steel is the carbon steel containing other metallic additives such as chromium, nickel etc.

Physical Data

iron pyrite mineral Pure iron is soft, lustrous and silvery in color. It readily corrodes (rust) in moist air or at elevated temepratures. It is the second most abundant metal, after aluminium, in the earth's crustal rocks (6.2 %). Indeed, the earth's core, with a radius of about 3500 km, is thought to constitute mainly of iron. On the Earth's surface, it is widely distributed in a variety of minerals such as haematite, magnetite, siderite (FeCO3) and pyrite (FeS), which is shown on left. The element also plays crucial roles in biological processes such as storage and transport of oxygen in the blood.

Interatomic distance: 248.2 pm
Melting point: 1538C
Boiling point: 2861C
Thermal conductivity/Wm-1K-1: 80.2 (27C)
Density/kgm-3: 7874 (20C), 7035 (m.p.)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 416.3 kJ/mol
Gibbs free energy of formation: 370.7 kJ/mol
Entropy: 180.5 J/mol K
Heat capacity: 25.7 J/mol K

Electronic data

Electronic configuration: [Ar] 3d6 4s2
Term symbol: 5D4
Electron affinity: 14.5693 kJ/mol   Electronegativity (Pauline): 1.83
Ionization energy (first, second, third): 762.466, 1561.89, 2957.47 kJ/mol

Chemical properties

Iron combines directly with most non-metals. Fine iron wire will burn in oxygen to give Fe3O4. The oxide also forms when steam is passed over red hot iron.

Test for iron(II) and iron(III)
(1) Ammonia or sodium hydroxide solutions give green precipitate with iron(II) solution, which turning brown on exposure to air. With iron(III) solution gives red-brown precipitate.
(2) Potassium hexacyanoferrate(II) gives white precipitate rapidly turning blue with iron(II) solution but form Prussian blue precipitate with iron(III) solution.
(3) Potassium hexacyanoferrate(III) gives dark blue (Turnbull's blue) precipitate with iron(II) solution but gives reddish-brown coloration with no precipitate in iron(III) solution.
(3) Potassium thiocyanate, KCNS, gives blood red coloration with iron(III) solution. With iron(II) no coloration. However, the test is very sensitive to Fe(III) and can turn the iron(II) solution blood red due to small amount of Fe(III) present.


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