Van der Waals constant a is a indirect measure of attractive force. Gases with higher values of a can be easily liquified.
Van der Waals constant b is also called co-volume or excluded volume. It is the effective volume of a gas. When gas molecules have constant value of b over wider range of temperature and pressure, the gas molecules are incompressible.
b has units L mol−1 or ml3 mol−1.
The units of a are atm L2 mol−1.