Answer:
Number of delocalized electrons
Explanation:
Magnesium has more delocalized electrons compared to sodium and this accounts for the higher melting point. 
- When magnesium atoms comes together to form a metallic bonds, they have more network of delocalized electrons. 
- There is more pull for the localized electrons due to the nuclear charge on the nucleus. 
- This strong intermolecular metallic bond increases the melting point of magnesium.