A binary compound of oxygen with another element is called oxide. On the basis of acid-base characteristics, the oxides may be classified into the following four types,

(i) Basic oxides :Alkali, alkaline earth and transition metals form basic oxides - Na₂O, MgO, Fe₂O₃ etc. their relative basic character decreases in the order :alkali metal oxides>alkaline earth metal oxides>transition metal oxides.

(ii) Acidic oxides :Non-metal oxides are generally acidic - CO₂, SO₂, SO₃, NO₂, N₂O₅, P₄O₁₀, Cl₂O₇etc.

(iii) Amphoteric oxides: Al₂O₃, SnO₂ etc.

(iv) Neutral oxides : H₂O, CO, N₂O, NOetc.

Trends of oxides in the periodic Table : On moving from left to the right in periodic table, the nature of the oxides change from basic to amphoteric and then to acidic. For example, the oxides of third period has the following behaviour,

However, on moving down a group, acidic character of the oxides decreases. For example in the third group, the acidic character of oxides decreases as:

On the basis of oxygen content the oxides may be classified into the folloiwng types,

Normal oxides: These contain oxygen atoms according to the normal oxidation number i.e., - 2. For example, MgO, H₂O, CaO, Li₂O, Al₂O₃ etc.

Polyoxides : These contain oxygens atoms more than permitted by the normal valency. Therefore, these contain oxygen atoms in oxidation state different than –2.

Peroxides : These contains O^2–₂ ion having oxidation number of oxygen as –1. For example,

H₂O₂, Na₂O₂, BaO₂, PbO₂ etc.

Superoxides : These contains O^–₂ ion having oxidation number of oxygen as –1/2. For example, KO₂, PbO₂ etc.

Suboxides : These oxides contain less oxygen than expected from the normal valency. For example, N₂O.

Mixed oxides : These oxides are made up of two simple oxides. For example, red lead Pb₃O₄(2PbO₂ + PbO₂) magnetic oxide of iron, Fe₃O₄ (FeO + Fe₂O₃) and mixed oxide of manganese, Mn₃O₄ (MnO₂ + 2MnO)