Periodicity of chemical properties of the elements in Period 3 — common mistakes

When asked to compare the pH of solutions of sodium chloride and aluminium chloride, a complete answer must mention the difference in cation charge density (Al³⁺ > Na⁺), the resulting polarisation of water molecules by Al³⁺, and the subsequent release of H⁺ ions (hydrolysis). Simply stating 'AlCl₃ is acidic' is not enough for full marks.

The key difference lies in the cation. The Al³⁺ ion is smaller and has a greater charge than the Mg²⁺ ion, giving it a much higher charge density. This polarises the Al-O bond, giving it significant covalent character. This allows it to react with strong alkalis. The Mg-O bond is more purely ionic, and the O²⁻ ion is strongly basic, but the compound itself does not show acidic properties.

Argon is a noble gas in Group 18. It has a full outer shell of electrons, making it exceptionally stable and chemically inert under normal conditions. It does not react with oxygen or chlorine, so it does not form an oxide or a chloride.

Not perfectly. The solution is very slightly acidic, with a pH of around 6.5. The Mg²⁺ ion has a higher charge density than Na⁺ and does cause a very small degree of hydrolysis. However, for the purposes of comparison in A-Level Chemistry, it is considered essentially neutral relative to the strongly acidic solution formed by AlCl₃.