The elements which form metals usually form cations through electron loss. Most will react with oxygen in the air to form oxides over various timescales (potassium burns in seconds while iron rusts over years) which depend upon whether the native oxide forms a passivation layer that acts as a diffusion barrier. Some others, like palladium, platinum, and gold, do not react with the atmosphere at all; gold can form compounds where it gains an electron (aurides, e.g. caesium auride). The oxides of elemental metals are often basic. However, oxides with very high oxidation states such as CrO 3, Mn 2 O 7, and OsO 4 often have strictly acidic reactions; and oxides of the less electropositive metals such as BeO, Al 2 O 3, and PbO, can display both basic and acidic properties. The latter are termed amphoteric oxides.