What is the trend in acidity of oxides across a period?

The acidity of oxides generally increases across a period from left to right on the periodic table.

In more detail, this trend is due to the increasing number of protons in the nucleus as you move across a period from left to right. This increase in nuclear charge attracts the bonding pair of electrons more strongly, making the oxide ion more polarised. As a result, the oxide ion becomes a stronger acid.

At the start of a period, elements form metallic oxides which are basic in nature. For example, sodium oxide (Na2O) and magnesium oxide (MgO) are basic oxides. They react with water to form alkalis (bases that are soluble in water).

As we move across the period, the nature of oxides changes from basic to amphoteric and then to acidic. Aluminium oxide (Al2O3) is an example of an amphoteric oxide, meaning it can behave as both an acid and a base. It reacts with both acids and alkalis to form salts and water.

Towards the end of the period, non-metallic elements form acidic oxides. For instance, sulphur forms sulphur dioxide (SO2) and sulphur trioxide (SO3), both of which are acidic oxides. They react with water to form acids.

In summary, the trend in acidity of oxides across a period on the periodic table increases from basic, through amphoteric, to acidic. This is due to the increasing nuclear charge which polarises the oxide ion, making it a stronger acid.