Anomalous properties of second period elements

Why the first member of a group is different. Elements in the same group generally share similar physical and chemical properties, but the very first element of a group consistently differs somewhat from the rest of its group. For instance, lithium and beryllium form comparatively more covalent compounds, unlike the rest of the alkali and alkaline-earth metals, which predominantly form ionic compounds.

Why: no d orbitals available. Second-period elements have only four valence orbitals available (2s and the three 2p orbitals), which caps their maximum covalence at 4. Elements lower down the same groups have d orbitals available in their valence shell and so can support higher covalences. The classic illustration: boron (second period) can only form $BF_4^-$ (covalence 4, the maximum its orbitals allow), while aluminium (third period, same group) forms $AlF_6^{3-}$ (covalence 6, made possible by its available 3d orbitals). …