Non-metals are electronegative in nature. They generally have 5, 6 or 7 electrons in their valence shells. They have tendency to form anion by gaining electrons.
Non-metals react with oxygen on heating or burning to form their oxides.
S (s) +O2 (g) → SO2 (g)
C (s) +O2 (g) → CO2 (g)
2H2 (g) +O2(g) → 2H2O (l)
Many non-metals form more than one oxide.
Carbon with limited supply of oxygen on burning forms CO which is a neutral oxide. However, in ample supply of air carbon forms CO2 which is an acidic oxide.
2C (g) + O2 (g) → 2CO
C (g) + O2 (g) → CO2
Nitrogen forms a series of oxides with oxygen:
Nature of oxides of non-metals
In general, oxides of non-metals are acidic in nature or after dissolving in water they form acids.
Carbon dioxide forms carbonic acid with water.
CO2 (g) + H2O (l) → H2CO3 (aq) (carbonic acid)
Sulphur trioxide forms sulphuric acid with water.
SO3 (g) + H2O (l) → H2SO4 (l) (Sulphuric acid)
Dinitrogen pentoxide forms nitric acid with water.
N2O5 (g) + H2O (l) → 2HNO3 (l) (nitric acid)