The Van Der Waals equation is derived to make an approach to the ideal gas law PV = nRT because the value of these constants stands at zero. The constant A is meant for providing a correction for the intermolecular forces exerted. The constant B is meant for correcting the finite molecular size. The value of constant B is equal to the volume of one mole of the atoms or the molecules. Since we can use constant B for calculating molecular volume, thus it can be used to calculate the radius of an atom or a molecule.