Question

Why few molecules have compressibility factor less than 1 and few greater than 1?

Answer 1

The inter-molecular forces of various molecules varies.

For molecules like that of Hydrogen ($H_{2}$, the inter-molecular forces are predominantly repulsive, and hence the compressibility factor is greater than one, but for molecules like that of Sulphur Dioxide ($SO_{2}$, these forces are attractive, and the compressibility factor is less than one.

Further elaboration:

The compressibility factor is defined as the ratio of the actual volume occupied by a gas and the volume of the gas, if it were ideal.

If the forces between the gas molecules are attractive, they are more tightly bound, and under the same conditions of pressure and temperature, occupy a lesser volume than if they were ideal.

On the other hand, if the gas molecules' interactive forces are repulsive, the gas occupies a higher volume than that that should be occupied ideally.