to put it simply. 'coz there's more "stuff" in them. but remember, protons and neutrons have a molecular mass of one, therefore as more are added, more mass is added and more also equals bigger

1) The statement "the Van der Waals radius of atoms always increases with molar mass for any group or family of elements" is false. For instance: H: 120 pm, Li: 182 pm, Na: 227 pm, K: 275 pm increases. Cu: 140 pm, Ag: 172 pm, Au: 166 pm does *not* increase! C: 170 pm, N: 155 pm, O: 152 pm, F: 147 pm, Ne: 154 pm does *not* increase! Source and further information: http://www.webelements.com/periodicity/van_der_waals_radius/ Further information: http://en.wikipedia.org/wiki/Van_der_Waals_radius 2) "The way the atomic radius varies with increasing atomic number can be explained by the arrangement of electrons in shells of fixed capacity. The shells are generally filled in order of increasing radius, since the negatively charged electrons are attracted by the positively charged protons in the nucleus. As the atomic number increases along each row of the periodic table, the additional electrons go into the same outermost shell; whose radius gradually contracts, due to the increasing nuclear charge. In a noble gas, the outermost shell is completely filled; therefore, the additional electron of next alkali metal will go into the next outer shell, accounting for the sudden increase in the atomic radius. The increasing nuclear charge is partly counterbalanced by the increasing number of electrons, a phenomenon that is known as shielding; which explains why the size of atoms usually increases down each column. However, there are two occasions where shielding is less effective: in these cases, the atoms are smaller than would otherwise be expected." Source and further information: http://en.wikipedia.org/wiki/Atomic_radius#Explanation_of_the_general_trends