The volume occupied by the gas is much larger than the space occupied by the molecules themselves.
Intermolecular forces are quite weak at high temperature and low pressure.
Under these conditions the molecules are far apart and act independently of one another. They are moving fast and the molecular size and A.F. can be ignored. Liquids and solids occupy a much smaller volume - there is much less empty space compared to gases. The solid is rigid. In crystalline solids the atoms, molecules or ions are ordered in relatively fixed positions.