9.5. Hybridization and Molecular Orbitals http://www.ck12.org
FIGURE 9.24
The trigonal pyramidal ammonia molecule also results from sp^3 hybridiza-
tion of the central (nitrogen) atom. Of the four groups of electrons sur-
rounding the nitrogen atom, three form single covalent bonds to hydrogen
atoms, while one group is a lone pair.sp^2 Hybridization
Boron trifluoride (BF 3 ) is predicted to have a trigonal planar geometry by VSEPR. First, a paired 2s electron is
promoted to an empty 2p orbital.
This is followed by hybridization of the three occupied orbitals to form a set of three sp^2 hybrids, leaving the 2pz
orbital unhybridized. The choice of which p orbital to leave unhybridized is arbitrary, but 2pzis conventionally
chosen in the case of sp^2 hybrids.
The geometry of thesp^2 hybrid orbitals is trigonal planar, with the large lobe of each orbital pointing towards one
corner of an equilateral triangle (Figure9.25). The angle between any two of the hybrid orbital lobes is 120°. Each
can bond with a 2porbital from a fluorine atom to form the trigonal planar BF 3 molecule.
Other molecules with a trigonal planar electron domain geometry also form sp^2 hybrid orbitals. For example, the
electron domain geometry of ozone (O 3 ) is trigonal planar, although the presence of a lone pair on the central oxygen
atom makes the molecular geometry bent. The hybridization of the central O atom of ozone is sp^2.
sp Hybridization
A beryllium hydride (BeH 2 ) molecule is predicted to be linear by VSEPR. The beryllium atom contains only paired
electrons, so it must also undergo hybridization. One of the 2s electrons is first promoted to an empty 2p orbital.