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CHAPTER 9: Covalent Bonding: Orbitals
9.1 Hybridization and the Localized Electron Model
In molecular orbital theory, electrons in a molecule are viewed as occupying orbitals which are not
necessarily constricted to the volume of space between any single pair of atoms, but instead, may extend
over several atoms, or even over the entire molecule. The success of molecular orbital theory indicates that
this concept is probably an accurate picture of the behavior of electrons in real molecules, and that the
simplified valence bond theory should therefore be expected to give erroneous results at times. (One such
erroneous result is the description of "resonance".) Hybrid orbital theory is an attempt to bring to valence
bond theory some of the advantages of molecular orbital theory without adding too many complications.
The premise is that, prior to overlapping, the valence orbitals can "hybridize", or mix, to form a new set of
hybrid atomic orbitals that have different spatial characteristics than the atomic orbitals from which they
were derived. For the main group elements, there are three types of hybrid orbitals:
sp hybrid orbitals, formed by mixing one s orbital with one p orbital, leaving two p orbitals
unhybridized. The two new sp hybrid orbitals are 180° apart (linear), and perpendicular to the two
remaining p orbitals.
hybrid orbitals, formed by mixing one s orbital with two p orbitals, leaving one p orbital
unhybridized. The three new sp
hybrid orbitals are 120° apart (trigonal), and perpendicular to the
remaining p orbital.
hybrid orbitals, formed by mixing one s orbital with all three p orbitals (leaving no unhybridized
valence orbitals). The four new sp
hybrid orbitals are 109.5° apart, in a nonplanar tetrahedral
The type of hybridization can be determined by considering the properties of each type of orbital.
Since hybrid orbitals are part s and part p, they are going to be directional, but with one fat end, which is
effective for forming sigma bonds better than the slim unhybridized p orbitals. However, the other end of a
hybrid orbital is stunted, making it ineffective for forming pi bonds. The energy of a hybrid orbital is lower
than an unhybridized p orbital, and therefore more stable for lone pairs.