Chromium: position, atomic structure and some properties
Chromium is in period 4, side subgroup of group VI (VI B) (Z = 24), electron configuration [Ar] 3d⁵ 4s¹; in compounds it shows the oxidation states +2, +3 and +6 (the most stable is +3). In nature it occurs mainly as chromite FeO·Cr₂O₃. Pure chromium is obtained by reducing Cr₂O₃ with aluminium: Cr₂O₃ + 2Al → Al₂O₃ + 2Cr; reducing chromite with coke gives a mixture of chromium and iron (ferrochrome): FeO·Cr₂O₃ + 4C → 2Cr + Fe + 4CO. Chromium is a silvery-white, hard, shiny metal; a thin oxide film makes it stable in air. On heating it reacts with dilute HCl: Cr + 2HCl → CrCl₂ + H₂↑ (in air Cr²⁺ quickly changes into Cr³⁺); in concentrated HNO₃ it is passivated. At high temperature: 4Cr + 3O₂ → 2Cr₂O₃, 2Cr + 3Cl₂ → 2CrCl₃, 2Cr + 3S → Cr₂S₃. Because it resists corrosion, chromium is used for coating objects (chrome plating) and added to steel to make stainless steel (about 12 % chromium or more).
Observe chrome-plated objects (taps, car parts, tools) and write why they shine and do not rust. Experiments with chromium and its compounds are done only by the teacher; chromium(VI) compounds are poisonous.