Bonding in compounds of group 13, 14 and 15 elements

The lighter elements of groups 13, 14 and 15 have small atomic radii and high ionization enthalpies, so removing electrons to form simple cations is energetically expensive; instead they form covalent bonds by overlapping their valence-shell orbitals with those of other atoms. Down each group the ionization enthalpy falls and the atomic radius grows because the valence orbitals become more diffuse, so the heavier members of each group increasingly favour ionic bonding instead. A second structural difference follows from period number: the first member of each group (boron, carbon, nitrogen) belongs to the second period and therefore has no d-orbitals available, so B, C and N can never expand their octet beyond four electron pairs. Every subsequent, heavier member of these groups, however, has vacant d-orbitals in its valence shell that it can press into service, letting it expand its octet and form a wider variety of …