Diagonal Relationship between Beryllium and Aluminium

$Be^{2+}$ has an estimated ionic radius of just 31 pm, and its charge-to-radius ratio turns out to be almost identical to that of $Al^{3+}$ — which is precisely why beryllium's chemistry echoes aluminium's in several ways, a pattern known as the diagonal relationship. The key similarities are:

• Like aluminium, beryllium resists attack by acids because a thin oxide film protects its surface.
• Beryllium hydroxide dissolves in excess alkali to give the beryllate ion $[Be(OH)_4]^{2-}$, exactly as aluminium hydroxide dissolves to give the aluminate ion $[Al(OH)_4]^-$. …