9.1.1—Period 3 physical property trends

Across Period 3, properties such as atomic radius, melting point, boiling point and electrical conductivity change as nuclear charge, bonding and structure change.

The trend is not one uniform line: metallic bonding dominates Na–Al, giant covalent silicon produces a very high melting point, and simple molecular P₄/S₈/Cl₂/Ar are governed by intermolecular forces.

Silicon has a much higher melting point than phosphorus because it forms a giant covalent lattice, whereas phosphorus exists as discrete P₄ molecules.

Do not explain the whole period using only increasing proton number. The type of structure and bonding changes at the same time.