Iron – facts and properties
- Symbol
- Fe
- Atomic number
- 26
- Atomic mass
- 55.845 u
- Period
- 4
- Group
- 8
- Category
- transition metal
- Electron configuration
- [Ar] 3d⁶ 4s²
- Electronegativity
- 1.83
- Atomic radius
- 156 pm
- Valence electrons
- 8
- State at standard conditions
- solid
- Melting point
- 1538 °C
- Boiling point
- 2861 °C
- Density
- 7.874 g/cm³
- Discovered
- known since antiquity
Iron is a silvery-grey, magnetic metal with atomic number 26. It is by far the most widely used metal: bridges, cars, ships and tools are made mostly of iron and its alloys. The Iron Age is named after it, and it also does a vital job inside the body, where it carries oxygen in red blood cells. But iron has a weakness that everyone knows: it rusts.
Position in the periodic table
Occurrence and extraction
Iron is the fourth most abundant element in the Earth's crust, after oxygen, silicon and aluminium, and the most abundant metal in the planet as a whole, because the core consists mainly of iron and nickel. Native iron is found at the surface almost only in iron meteorites. The main ores are haematite, a red iron oxide, and magnetite, a black, magnetic iron oxide. Large deposits lie in Australia, Brazil and China.
Iron has been known since antiquity. The Iron Age, which began in Europe about three thousand years ago, replaced bronze tools and weapons with iron ones.
Today the work is done in the blast furnace. Iron ore, coke and limestone are fed in at the top, and hot air is blown in at the bottom. The coke burns to carbon monoxide, which strips the oxygen from the iron oxide: Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂. The molten pig iron still contains about four percent carbon and is therefore brittle. The limestone binds the waste rock into slag, which floats on top of the pig iron. In the steelworks, oxygen is blown through the pig iron to burn off most of the carbon. What remains is steel with less than two percent carbon, which can be forged and rolled. Scrap steel can be melted down and reused.
Properties and reactions
Iron is in period 4 and group 8, and its electron configuration is [Ar] 3d⁶ 4s². Its electronegativity is 1.83. The metal has a density of 7.874 g/cm³, melts at 1538 °C and boils at 2861 °C. Together with cobalt and nickel it is one of the three ferromagnetic elements at room temperature: a magnet attracts it, and it can be magnetised itself. Above about 770 °C it loses this property.
Iron is a base metal. It dissolves in dilute acids, releasing hydrogen and forming iron(II) salts. In compounds it usually appears as iron(II) or iron(III). Iron(II) solutions are pale green, iron(III) solutions yellow-brown.
Rust is a mixture of hydrated iron(III) oxides. It forms when iron is in contact with oxygen and water at the same time, and salt in the water speeds the reaction up. Unlike the oxide layer on aluminium, rust does not stick firmly. It is porous and flakes off, exposing fresh iron underneath, so the process continues. Paint, zinc coatings and alloying with chromium offer protection.
Uses in everyday life and technology
Pure iron is rarely used; nearly all of it is put to work as steel or cast iron, and the carbon content decides the properties. Mild structural steel is used for beams, sheets and the reinforcing bars in concrete, harder steel for tools, springs and rails. Cast iron, which has more carbon, can be poured into moulds, for example for engine blocks, frying pans and manhole covers. Alloying steel with chromium and nickel gives stainless steel; adding manganese gives a particularly tough steel.
For living things, iron is indispensable. An adult has about four grams of it in the body, most of it in the haemoglobin of the red blood cells. Each haem unit holds an iron ion that binds oxygen reversibly, which allows oxygen to travel from the lungs to the cells. The iron-containing pigment haem gives blood its red colour. A lack of iron causes anaemia, with tiredness and pale skin, and women of childbearing age are most often affected. Good sources are meat, pulses and oats, and vitamin C helps the body to absorb iron from plants. Too much iron from tablets is harmful and can be fatal for small children, so iron tablets belong out of children's reach.
Molecules containing this element
- Iron(II) oxide FeO
- Iron(III) oxide hydroxide FeO(OH)
- Iron disulfide (pyrite) FeS2
Quick question
Which position correctly describes iron (Fe)?
FAQ
Why does iron rust?
In the presence of water, iron reacts with oxygen from the air to form hydrated iron(III) oxides, which we call rust. The rust layer is porous and flakes off, so fresh iron is exposed again and again. Salt water and road salt speed things up. Dry air on its own does little harm.
What is the difference between iron and steel?
Iron is the chemical element. Steel is an alloy of iron with a little carbon, usually under two percent, often with other metals such as chromium, nickel or manganese. The carbon makes steel much harder and stronger than pure iron, yet it can still be shaped.
Why is blood red?
The red colour comes from haemoglobin, which contains iron ions that bind oxygen. Blood carrying oxygen is bright red, blood without it is darker. So the iron in blood is not a piece of metal but ions held inside a large protein.