Each atom from Na to Ar is larger than its second-row counterpart
because of the additional shell of electrons in its core. Within
each row, atoms become smaller with increasing atomic number because
of the increased charge on the nucleus.
This is illustrated well by the ions in the series above,which
extends through the end of the second row and the beginning of the
third. The ions and Ne are isoelectronic because they all have the
identical 2,8 electron-shell structure.
The only difference between them is the charge on
the nucleus: +7 for nitrogen through +13 for aluminum. This increasing
charge causes the ions to shrink steadily as shown above.
The "ionic radius" of neon has been set
at its van der Waals packing radius, because neon can be considered
as a 2,8-electron ion with zero charge.
By the same logic, the ionic radii listed for He
and Ar also are their van der Waals radii. There are no covalent
radii for these atoms because they make no covalent bonds.