# The ionic radius of aluminium(III) vs. the covalent radius of neon?

I was wondering if it is a valid comparison to look at the ionic radius (noble-gas-like configuration) of a metal and the covalent radius of a noble gas. Specifically, $\ce{Al^{3+}}$ and $\ce{Ne}$.

This website gives me the impression that for comparable radii definitions, neon is slightly smaller than $\ce{Mg^{2+}}$ but larger than $\ce{Al^{3+}}$.

Someone I know is wondering why we should/shouldn't just use the atomic radius of neon, rather than the covalent radius, to compare to the ionic radius of metals like aluminum and magnesium (yes, even though they are on the next period).

• Covalent radius of neon? I wonder how it was obtained, not experimentally I'm afraid. – Mithoron Jun 10 '16 at 21:43