Complex sentence: Next assume that all particles are hard spheres of the same finite radius "r" (the van der Waals radius) of the particles is to decrease the available void space in which the particles are free to move.
Split into simpler sentences: Next assume that all particles are hard spheres of the same finite radius "r" (the van der Waals radius). The effect of the finite volume of the particles is to decrease the available void space in which the particles are free to move.