# Explain why the melting points of Period 3 elements first increase and then decrease across the period, from sodium to argon.

> AQA A-Level Chemistry (7405) — 3.2.1 Periodicity · Explain · 5 marks

> The melting points of Period 3 elements vary considerably. Sodium has a melting point of 98 °C, silicon has a melting point of 1414 °C, and argon has a melting point of −189 °C.

## Mark scheme (5 marks)

1. Sodium, magnesium and aluminium are metals with metallic bonding / giant metallic structures, so melting points increase from Na to Al
2. More delocalised electrons / stronger metallic bonding as the number of delocalised electrons per atom increases from Na to Al, so more energy is needed to overcome these forces
3. Silicon has the highest melting point because it has a giant covalent / giant atomic structure
4. Phosphorus, sulfur, chlorine and argon are simple molecular / simple covalent structures, so they have low melting points
5. Argon (noble gas) exists as individual atoms with very weak intermolecular forces, so it has the lowest melting point

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