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tht means the max no. of electrons in the first shell is 2. the max no. of electrons in the second shell is 8. and the max number of electrons in the third shell is 8....it goes on as the no. of electron in the atom increases. so the electron configuration for magnesium is 2.8.2. this shows tht the first and second shells are filled to the max. the third shell isn't. it has a 2+ charge because it has lost 2 electrons( which carries a negative charge), hence the atom is more positive than normal.
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. (Note this is different to atomic radius.) 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.