Anomalous Behaviour of Beryllium

As the first member of Group 2, beryllium — like lithium in Group 1 — behaves quite differently from the rest of its group, and for the same underlying reason: its atom and ion are exceptionally small. It also shows a diagonal relationship, in beryllium's case to aluminium (examined next in §10.8.1). Three concrete anomalies stand out:

(i) Beryllium's exceptionally small atomic and ionic size means it simply does not fit the trend set by the rest of the group. Combined with its comparatively high ionization enthalpy, this small size pushes beryllium towards forming compounds that are largely covalent — and, being covalent rather than ionic, these compounds hydrolyse easily in water. …