It is written in the third point ( shown in the picture) that presence of d orbitals results in stronger metallic bond.
1) We check the strength of metallic bonding to compare the melting and boiling point in case of metals. According to the general trend, melting and boiling point decreases down in the group( in s block) because the strength of metallic bonding decreases.
2) Presence of d orbitals decrease the size of atom thus increasing the electrostatic attraction between the delocalized electrons in the metal lattice and the cation and as a result, increasing metallic bonding. But magnesium IS IN A DIFFERENT PERIOD.
Can the ineffective shielding by d electrons be so much effective in decreasing the size of cation?
3) We didn't use any of this d orbital argument in comparing melting and boiling point of alkali metals. It decreased down the group following the expected trend.
How should we explain this fact ?