A-Level chem- bonding and melting temperature

Magnesium has a higher melting point then sodium because it has a stronger metalic bond then sodium.
As the atomic number increase, the number of protons and electron also increase. Magnesium has one more proton than sodium, this increases the attractive force between the nucleus and the electrons. Since the extra electron is added to the same orbital as the original one (both are in 3s), the screening effect is not significant. Therefore the increase in attractive force outweights the increase in screening effect. As a result the metalic bond is strengthen and so magnesium has a higher melting point than sodium.

Mg has 2 valence electrons. Also it has a smaller size than Na. Hence the matallic bond strength of Mg > Na. so a larger amount of energy is needed to melt Mg. so mp of Mg > Na