[HmKwok]

Why Ca has a higher melting & boiling point than Mg?
if I consider Ca2+ has 3 filled electron shells and Mg2+ has 2, while they have the same no. of valence electrons for delocalization.
The metallic bond exists inside Ca should be weaker~ Why is it now Ca has a higher melting point?

[Donaldson Tan]

You think Mg²⁺ has a higher charge density than Ca²⁺, so Magnesium should have a stronger metallic bond. That is true, thus Calcium is more reactive than Magnesium, but it does not explain why Calcium has the higher melting point. Your argument is also valid when you compare the melting point of Sodium to Magnesium's, so what's different in your case? Ceteris paribus...