Simple Oxides

Simple Oxides

When oxygen combines with any other element, the resulting binary compound is called an oxide. Because oxygen forms compounds with almost every element in the periodic table, oxides are extremely numerous, and a single element often produces more than one oxide, each differing widely in properties.

Oxides can be simple, built around one type of metal or non-metal centre (e.g., $MgO$, $Al_2O_3$), or mixed, which behave as though they were a combination of two simple oxides of the same element in different oxidation states (e.g., $Pb_3O_4$ and $Fe_3O_4$).

Simple oxides are classified according to their acid-base behaviour into four categories:

• Acidic oxides — these dissolve in water to give an acid, e.g. $SO_2$, $Cl_2O_7$, $CO_2$, $N_2O_5$. Sulphur dioxide, for instance, reacts with water to form sulphurous acid:

As a general rule only non-metal oxides are acidic, but a few metals in unusually high oxidation states also give acidic oxides, such as $Mn_2O_7$, $CrO_3$ and $V_2O_5$.

• Basic oxides — these react with water to give a base, e.g. $Na_2O$, $CaO$, $BaO$. Calcium oxide, for example, forms calcium hydroxide:

Metallic oxides, in general, are basic in nature.

• Amphoteric oxides — a small set of metallic oxides show dual behaviour, reacting with both acids and alkalies. Aluminium oxide is the classic example: …