The elements of the third period contain d-orbitals also in addition to s- and p-orbitals. The 3d-orbitals are comparable in energy to the 3s- and 3p-orbitals. These d-orbitals are also involved in the hybridization and they explain the geometries of molecules of elements of third period. This results in covalencies of 5, 6 and 7, which are not known amongst the compounds of second period elements.
Due to the availability of d-orbitals, phosphorus and sulphur can exhibit covalency of 5 and 6 respectively whereas the corresponding elements of same group, nitrogen and oxygen of second period cannot extend their octets. The following are some common types of hybridization involving d-orbitals:
a) sp
3d hybridization
b) sp3d2 hybridization
c) sp
3d
3 hybridization
d) dsp2 hybridization
