Trends in melting points of elements in Period 3

Melting points generally increase going from sodium to silicon, then decrease going to argon (with a “bump” at sulphur).

Explanation of the trends:

When a substance melts, some of the attractive forces holding the particles together are broken or loosened so that the particles can move freely around each other but are still close together. The stronger these forces are, the more energy is needed to overcome them and the higher the melting temperature.

Sodium, magnesium and aluminium

Sodium, magnesium and aluminium are all metals. They have metallic bonding, in which positive metal ions are attracted to delocalised electrons. Going from sodium to aluminium:
the charge on the metal ions increases from +1 to +3 (with magnesium at +2)
the number of delocalised electrons increases
so the strength of the metallic bonding increases and
the melting points increase.

Silicon

Silicon is a metalloid (an element with some of the properties of metals and some of the properties of non-metals). Silicon has giant covalent bonding. It has a giant lattice structure similar to that of diamond, in which each silicon atom is covalently-bonded to four other silicon atoms in a tetrahedral arrangement. This extends in three dimensions to form a giant molecule or macromolecule.
Silicon has a very high melting point because:
all the silicon atoms are held together by strong covalent bonds.
which need a very large amount of energy to be broken.

Phosphorus, sulphur, chlorine and argon

These are all non-metals, and they exist as small, separate molecules. Phosphorus, sulphur and chlorine exist as simple molecules, with strong covalent bonds between their atoms. Argon exists as separate atoms (it is monatomic).
Their melting points are very low because:
when these four substances melt or boil, it is the van der Waals’ forces between the molecules which are broken,
which are very weak bonds,
so little energy is needed to overcome them.
Sulphur has a higher melting point and boiling point than the other three because:
phosphorus exists as P4 molecules
sulphur exists as S8 molecules
chlorine exists as Cl2 molecules
argon exists individual Ar atoms
the strength of the van der Waals’ forces decreases as the size of the molecule decreases
so the melting points and boiling points decrease in the order S8 > P4 > Cl2 > Ar