9.1 Periodicity of physical properties of the elements in Period 3

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.

Melting points rise when stronger attractions must be overcome. Electrical conductivity requires mobile charged particles or delocalised electrons.

Na, Mg and Al generally strengthen metallic bonding as charge density increases; silicon forms a giant covalent network; molecular elements have lower melting points and limited conductivity.

Al conducts as a metal, silicon conducts less effectively as a semiconductor-like giant covalent solid, and sulfur does not conduct because its S₈ molecules have no mobile charge carriers.

A high melting point does not guarantee conductivity. Silicon and many ionic solids are strong lattices but lack freely moving charge carriers in the solid state.