Trends in period 3

Melting points generally increase going from sodium to silicon, then decrease going to argon (with a “bump” at sulphur).
Boiling points generally increase going from sodium to aluminium, then decrease to argon (again with a “bump” at sulphur).
Melting
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.
Boiling
When a substance boils, most of the remaining attractive forces are broken so the particles can move freely and far apart. The stronger the attractive forces are, the more energy is needed to overcome them and the higher the boiling temperature.

Silicon has a very high melting point and boiling point because:
all the silicon atoms are held together by strong covalent bonds ...
which need a very large amount of energy to be broken.