The metallic structures of  Period 3 Elements

Melting and boiling points increase across the three metals because of the increasing strength of their metallic bonds. The number of electrons which each atom can contribute to the delocalized "sea of electrons" increases. The atoms also get smaller and have more protons as you go from sodium to magnesium to aluminum. The attractions and therefore the melting and boiling points increase because:

• The nuclei of the atoms are more positively charged.
• The "sea" is more negatively charged.
• The "sea" is progressively nearer to the nuclei and thus is more strongly attracted.