What is the difference between nuclear charge and ionic charge? There was a question asking why magnesium metal has a higher melting point than sodium. 

In the answer, they said:


  
  Magnesium has more delocalised electrons. 
  Magnesium ions have a greater positive charge. 
  Magnesium has a greater electrostatic attraction between ions and delocalised electrons.
  


However, in the mark scheme in the additional guidance, it also said the mark couldn't be given if you said:


  Magnesium ions have a great nuclear charge rather than greater positive charge. 


I was confused as to what the difference is.
Nuclear charge is the charge present inside a nucleus, due to the protons. For example, a carbon atom has $+6e$ nuclear charge.

Ionic charge is the charge due to the gain or loss of electrons in the valence shell. So, the sodium ion has an ionic charge of $+1e$ (due to loss of one valence electron). That ion's nuclear charge is still $+11e$ (there are still 11 protons only).

The reason because nuclear charge as an answer to this question is unaccetable is because nuclear charge - being inside the nucleus - has largely no affect on the intermolecular forces between the magnesium atoms. Those magnesium atoms have intermolecular forces governed only by the valence electrons only.