Why do metals boiling points increase across a periodWatch
But most importantly, the trend in charge (for metals)
Metals lose their outer shell electrons to become ions.
From left to right on the periodic table: Na+ Mg2+ Al3+
Melting and boiling points are dependent on the strength of metallic bonding.
an Aluminium ion has a higher positive charge but smaller atomic radius thus it has the strongest metallic bonding of the three. This is because the strength of metallic bonding is proportional to the magnitude of the charge to the radius of metal cation.
What I mean by this is : Al+3 has 13 protons and 10 electrons. Na+ has 11 protons and 10 electrons.
So Aluminium has more protons for the same amount of electrons (higher ratio), and thus has stronger metallic bonding (a tighter hold on its electrons) which produces a smaller ion.
Also, we generally know that atomic radii decreases across a period anyway, because nuclear charge increases by 1 each time (proton number) and this outweighs other factors.