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Thank you for your reply.
Maybe I was not clear enough.

Silberberg (Chemistry: The Molecular Nature of Matter and Change. 4th ed. 2006) defines Charge Density as:

"The ratio of the charge of an ion to its volume."

The factors playing into the strength of an ion's charge density are therefore the charge of the ion (e.g. 2+ for Mg, 1+ for Na) and the effective volume which that charge acts over - which is where the ionic radius comes in.

Charge density = charge/volume

As the magnesium ion and sodium ion both have the same electronic configuration i.e. that of neon (because Mg has lost 2 electrons and Na has lost 1 and they now both have 10 electrons in total) the charge of the electrons around the ions is the same. But - magnesium has one more proton in its nucleus. This means magnesium has a smaller radius, and thus higher charge density.

For Na+ the ionic radius is 1.16 Å, and the charge is +1
For Mg2+ the ionic radius is 0.86 Å and the charge is +2

The corresponding charge densities are (assuming spherical geometry):
Na+: 1/[(1.16)^3 * 4π/3)] = 0.15 e/Å^3
Mg2+: 2/[(0.86)^3 * 4π/3)] = 0.75 e/Å^3

For ions where the charge is localized only on one portion of the molecule, I need to determine the volume only of that portion, and not the whole ion.

What I want to achieve is something as described here:

https://bcs.whfreeman.com/WebPub/Chemis … 20Ions.pdf

I hope now my question is clearer