An oxide // is a chemical compoond that contains at least ane oxygen atom an ane ither element in its chemical formula. Metal oxides typically contain an anion o oxygen in the oxidation state o −2. Maist o the Yird's crust consists o solid oxides, the result o elements bein oxidized bi the oxygen in air or in watter. Hydrocarbon combustion affords the twa principal carbon oxides: carbon monoxide an carbon dioxide. Even materials considered pure elements eften develop an oxide coatin. For example, aluminium foil develops a thin skin of Al2O3 (called a passivation layer) that protects the foil from further corrosion. Different oxides o the same element are distinguished bi Roman numerals denotin thair oxidation nummer, e.g. iron(II) oxide versus iron(III) oxide.