Iron
Iron is in period 4, group VIII B (the d-element family), Z = 26, configuration [Ar] 3d⁶ 4s²; in compounds it shows the oxidation states +2 and +3: it first loses the two 4s electrons, then one 3d electron. After aluminium it is the most abundant metal in the Earth's crust (roughly 4–5 %); its minerals are magnetite Fe₃O₄, hematite Fe₂O₃, limonite, siderite FeCO₃ and pyrite FeS₂. Iron is obtained from its oxides by reduction with carbon monoxide, hydrogen or aluminium: Fe₂O₃ + 3CO → 2Fe + 3CO₂. It is a silvery, soft, malleable metal attracted by a magnet (melting point 1539 °C, density about 7.87 g/cm³). Heated in air it forms iron scale: 3Fe + 2O₂ → Fe₃O₄; with sulfur Fe + S → FeS, with chlorine 2Fe + 3Cl₂ → 2FeCl₃ (with Cl₂ iron gives +3, with S +2). Since it stands before hydrogen in the activity series, it displaces hydrogen from dilute acids forming Fe²⁺ salts: Fe + 2HCl → FeCl₂ + H₂↑; it displaces a less active metal from a salt solution: Fe + CuSO₄ → FeSO₄ + Cu. Cold concentrated H₂SO₄ and HNO₃ passivate iron, so they are transported in iron containers. In moist air iron rusts: 4Fe + 6H₂O + 3O₂ → 4Fe(OH)₃.
Safe observation: use a magnet to separate iron filings from sand or flour; in class watch iron nails for several days in clean water, in salt water and oiled, and write where rust appears fastest. Burning iron with oxygen or testing it with acids is done only by the teacher; do not do such experiments at home.