Describe the variation in melting points and electrical conductivities of the Period 3 elements from sodium to chlorine in terms of their structure and bonding.

Sodium, magnesium and aluminium have giant metallic structures with strong metallic bonds. They have relatively high melting point. Silicon has a giant covalent structure with strong covalent bonds. This accounts for its exceptionally high melting point. The rest are simple molecules with weak intermolecular van der Waals forces. The strength of the van der Waals force and the melting point increases in the order: Ar ˂ Cl₂ ˂ P₄ ˂ S₈. Sodium, magnesium and aluminium have mobile electrons. They are conductors. Silicon is a metalloid. The energy gap between the conduction band and the valence bond is small. It is a semi-conductor. Phosphorous, sulphur, chlorine and argon do not have mobile electrons. They are non-conductors.