In the period Na-Ar, sodium, magnesium and aluminium have metallic structures. Their melting points increase from sodium to aluminium. This happens because the metallic bonds stronger due to:
• Decreasing atomic radius,
• Increasing nuclear charge and
• Increasing number of delocalised electrons.
Silicon has a giant molecular structure. It has a high melting point as strong covalent bonds must be broken to melt it.
From phosphorus to argon, the elements have a molecular structure. They all have low melting and boiling points as little energy is required to break the very weak Van der Waals forces between the molecules. The melting and boiling points are in the same order as the mass and size of the molecules, as show in Table 2.10. this is because the strength of Van der Waals forcers depends on the size of the moleculers.