anomalous behavior of beryllium

Beryllium is the lithium of Group 2 — the small, contrary member that refuses to behave like the rest of its family. Where calcium and barium are firmly metallic and ionic, beryllium is harder, higher-melting, and its compounds are largely covalent. Its oxide, BeO, dissolves in both acids and bases (it is amphoteric), unlike the firmly basic oxides of magnesium and calcium, and beryllium chloride is a covalent, polymeric solid that sublimes, not a typical salt.

The reason is the same one that makes lithium odd, only stronger: the beryllium ion Be2+ is extraordinarily small and carries a double charge, giving it the highest charge density of any cation in the s-block. Such a fierce little ion polarizes the electron clouds of its partners so strongly (Fajans' rules again) that its bonds become predominantly covalent rather than ionic — Be2+ as a naked ion is essentially never found. Beryllium also keeps a low coordination number, typically four, and in beryllium chloride it forms electron-deficient bridges, the gas-phase molecule being a chain in which chlorine atoms bridge between beryllium centres. Its amphoterism follows: a small, polarizing centre can act as a Lewis acid, accepting hydroxide to form beryllate ions, [Be(OH)4]2-, while also reacting with acids.

Beryllium's anomaly matters because it is the textbook case of the diagonal relationship: Be2+ resembles the aluminium ion Al3+ (down one, across one) far more than it resembles its own group. Both have amphoteric oxides, covalent and easily hydrolysed chlorides, and a tendency to form complexes — the charge-to-size ratio, not the column, dictating the chemistry. The honest practical note is that beryllium and its compounds are seriously toxic and must be handled with great care; its lovely chemistry is best admired from behind safety equipment.