The sharp decrease in melting point marks a change from giant covalent to simple molecular structures.

The trend in melting points across periods 2 and 3 is:

1. Increase from group 1 to group 4.
2. Sharp decreases from group 4 to group 5.
3. Remains comparitively low from group 5 to group 0.

The graphs below show the trend in melting point as we move across periods 2 (left) and 3 (right).

The melting point of period 3 elements increases from sodium to aluminium due to the increasing strength of metallic bonding. This is because:

1. Ions have larger positive charge (Na: 1+, Mg: 2+, Al: 3+)
2. Ions have smaller ionic radius
3. More delocalised electrons per ion (Na: 1, Mg: 2, Al: 3)

These factors lead to a stronger electrostatic attraction between the metal cations and the delocalised electrons, requiring more energy to overcome and resulting in higher melting points.