Elements in the first two periods of the periodic table have only s or p orbitals. For these elements, there is a maximum of one filled s and three filled p orbitals in the valence shell. This means there can be at most four sp-type orbitals, as in the case of the sp3 hybrid orbital.
However, when some of the native p orbitals are empty, the hybridization changes. An sp2 hybrid orbital is a hybrid orbital that forms when one s and two p orbitals combine. Consider boron, which has only three valence electrons and therefore two empty p orbitals. Boron can form three covalent bonds, as in boron trifluoride (BF3). For this molecule, boron must have at least three hybrid orbitals. This means the s orbital and two p orbitals combine to form three sp2 orbitals, which have a trigonal planar geometry. The third p orbital is left out of the hybridization.
An sp hybrid orbital is a hybrid orbital that forms when one s and one p orbital combine. Beryllium, for example, has only two valence electrons, both in an s orbital, so it can form only two hybrid orbitals. The s orbital and one p orbital combine to form two sp orbitals, which are linear.
The hybridization theory can be used to describe expanded octets, such as the one observed in an iodine tetrachloride (ICl4–) ion. An sp3d hybrid orbital forms when one s, three p, and one d orbital combine. An sp3d2 hybrid orbital forms when one s, three p, and two d orbitals combine. Remember that a covalent bond requires a half-filled hybrid orbital from each atom. Therefore, in order to achieve the four covalent bonds required by an iodine tetrachloride ion, there must be four half-filled hybrid orbitals. The remaining eight electrons make up the four lone pairs in the filled hybrid orbitals. Similarly, the molecule phosphorus pentachloride (PCl5) requires five unpaired electrons in five half-filled orbitals of the phosphorus atom. In both of these cases, the hybrids are formed by including the d orbitals in the hybridization. For the molecule phosphorus pentachloride, one s, three p, and one d orbital combine to form five sp3d orbitals, which are trigonal bipyramidal. For iodine tetrachloride ion, one s, three p, and two d orbitals combine to form six sp3d2 orbitals, which are octahedral.