Element:	Iron
Symbol:	Fe
Atomic number:	26
Atomic weight:	55.845(2)
Electron configuration:	[Ar] 3d6 4s2
Ground level:	5D4
Ionization potential:	7.9024 ev
Physical form:	Silvery-white or gray metal
Melting point:	1538°C
Boiling point:	2861°C
Critical temperature:
Density:	7.87 g/cm3
Specific heat:	0.449 J/g•K
Valence	2, 3, 4, 6

Origin of name: Anglo-Saxon, iron; symbol Fe from L. ferrum

The use of iron is prehistoric. Genesis mentions that Tubal-Cain, seven generations from Adam, was “an instructor of every artificer in brass and iron.” A remarkable iron pillar, dating to about A.D. 400, remains standing today in Delhi, India. This solid shaft of wrought iron is about 7 m high by 40 cm in diameter. Corrosion to the pillar has been minimal although it has been exposed to the weather since its erection.

Iron is a relatively abundant element in the universe. It is found in the sun and many types of stars in considerable quantity. It has been suggested that the iron we have here on earth may have originated in a supernova. Iron is a very difficult element to produce in ordinary nuclear reactions, such as would take place in the sun. Iron is found native as a principal component of a class of iron-nickel meteorites known as siderites, and is a minor constituent of the other two classes of meteorites. The core of the earth, 2150 miles in radius, is thought to be largely composed of iron with about 10% occluded hydrogen. The metal is the fourth most abundant element, by weight, making up the crust of the earth. The most common ore is hematite (Fe2O3). Magnetite (Fe3O4) is frequently seen as black sands along beaches and banks of streams. Lodestone is another form of magnetite. Taconite is becoming increasingly important as a commercial ore. Iron is a vital constituent of plant and animal life, and appears in hemoglobin.

The pure metal is not often encountered in commerce, but is usually alloyed with carbon or other metals. The pure metal is very reactive chemically, and rapidly corrodes, especially in moist air or at elevated temperatures. It has four allotropic forms, or ferrites, known as α, β, γ, and δ, with transition points at 700, 928, and 1530°C. The α form is magnetic, but when transformed into the β form, the magnetism disappears although the lattice remains unchanged. The relations of these forms are peculiar. Pig iron is an alloy containing about 3% carbon with varying amounts of S, Si, Mn, and P. It is hard, brittle, fairly fusible, and is used to produce other alloys, including steel. Wrought iron contains only a few tenths of a percent of carbon, is tough, malleable, less fusible, and has usually a “fibrous” structure. Carbon steel is an alloy of iron with carbon, with small amounts of Mn, S, P, and Si. Alloy steels are carbon steels with other additives such as nickel, chromium, vanadium, etc. Iron is the cheapest and most abundant, useful, and important of all metals.

Natural iron contains four isotopes. Twenty-six other isotopes and isomers, all radioactive, are now recognized.