Explain why the boiling point increases from sodium to aluminium.

The elements from Na to Al have metallic bonding - the electrostatic attraction between positively charged metal ions and the delocalised electrons. Sodium forms Na+ ions, therefore 1 electron is lost from each Na atom, whereas aluminium forms Al3+ ions, so 3 electrons are lost per each Al atom. The number of delocalised electrons therefore increases from sodium to aluminium, and charge density of the ions increases as the ionic charge increases and size decreases. This means there will be increased electrostatic attraction between electrons and ions as you go from Na to Al, therefore more energy is required to break the stronger metallic bonds, so boiling point increases.