Diagonal Relationship Of Beryllium and Aluminium

Beryllium shows resemblance with its diagonally opposite element aluminium.

Points of similarity are:

1) Both metals have a tendency to form covalent compounds e.g. the chlorides of both being covalent are soluble in organic solvents.

2) Both BeCl₂ and AlCl₃ act as a Lewis acids and are used as Friedel – Crafts catalyst.

3) These two elements have same electronegativity and the polarising power i.e. charge/ radius ratio of their ions are very similar.

4) BothBeCl₂ and AlCl₃ have chlorine bridged structures in the vapour phase.

5) Both Be and Al are resistant to the action of acids due to the formation of protective film of the oxides on their surface.

6) Both the metals dissolve in strong alkalies to form soluble complexes: beryllates and aluminates.

7) The oxides of both beryllium(BeO) and Aluminium(Al₂O₃) are hard, high melting insoluble solids.

8) The oxides and hydroxides of both Be and Al are amphoteric and dissolve in sodium hydroxide solution as well as in hydrochloric acid.

BeO + 2 HCl——–> BeCl₂ + H₂O

BeO + 2 NaOH —–> Na₂BeO₂ + H₂O

Al₂O₃ + 6 HCl ——–> 2 AlCl₃ + 3 H₂O

Al₂O₃ + 2 NaOH —–> 2 NaAlO₂ + H₂O

9) Carbides of both the metals react with water liberating methane gas.

10) Salts of both these elements form hydrated ions e.g.[ Be(OH)₄]²⁺ and [Al(H₂O)₆]³⁺ in aqueous solution.

11) Because of similar polarising power both beryllium and Aluminium form complexes. For eg: beryllium forms tetrahedral complexes such as [BeF₄]²– and Aluminium forms octahedral complexes like [AlF₆]^3-