Beryllium and aluminium

  Representative chemical properties of Be that make it diagonally related to Al rather than vertically related to the later group 2 metals are given below.

• The Be^2‏+ ion is hydrated in aqueous solution, forming [Be(H₂O)₄]^2‏+in which the Be^2‏+ centre significantly polarizes the already polar O^_H bonds, leading to loss of H similarly, the highly polarizing Al³⁺ makes [Al(H₂O)₆]³⁺ acidic (pKa = 5.0).
• Be and Al both react with aqueous alkali, liberating H₂; Mg does not react with aqueous alkali.
• Be(OH)₂ and Al(OH)₃are amphoteric, reacting with both acids and bases the hydroxides of the later group 2 metals are basic.
• BeCl₂ and AlCl₃ fume in moist air, reacting to give HCl.
• Both Be and Al form complex halides, hence the ability of the chlorides to act as Friedel–Crafts catalysts.