Re: Why does the atomic volume of Oxygen less than Nitrogen?

The reason that O2, despite a larger molecular weight (MW 32), has a 
smaller diameter than N2 (MW 28), lies in the electronic structure of the 
molecules. The Quantum mechanics theory of atomic and molecular structure 
says that electrons of a molecule exist as a delocalized cloud of 
negative charge surrounding the nuclei of the atoms in the molecule. The 
electron cloud around the oxygen nuclei in the O2 molecule is smaller due 
to attractive electrostatic interactions between the electrons in the 
cloud and the greater positive charge of the nuclei of the O atoms in the 
O2 molecule. Each oxygen atom has 8 protons in its nucleus, while each 
nitrogen atom has only 7 protons in its nucleus. Thus, the overall size 
of the electron cloud of the O2 molecule is smaller than for N2, in part 
because its electron cloud is drawn in or compressed closer to the O 
nuclei by the greater positive charge on the O nuclei.