Periodicity: Melting Points and Conductivity

The melting points and electrical conductivity of the Period 3 elements tell a fascinating story about how structure and bonding change across a period. By examining these trends, we can connect macroscopic physical properties to the microscopic world of atoms, bonds, and intermolecular forces.

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Melting Points Across Period 3

The melting points of Period 3 elements vary dramatically:

Element	Na	Mg	Al	Si	P₄	S₈	Cl₂	Ar
Melting Point (°C)	98	650	660	1414	44	115	−101	−189
Structure	Metallic	Metallic	Metallic	Giant covalent	Simple molecular	Simple molecular	Simple molecular	Simple molecular (monatomic)

The pattern shows a rise from sodium to silicon, then a dramatic drop at phosphorus, with continued low values through to argon. This reflects three fundamentally different types of structure.

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The Metals: Na, Mg, Al

Sodium, magnesium, and aluminium are metallic. In a metal, a lattice of positive ions is surrounded by a sea of delocalised electrons. The melting point depends on the strength of the metallic bonding.