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Physical chemistry 268

Polyatomic molecules

The principle of spin pairing of electrons in singly occupied orbitals to f form bonds may
be extended to molecules with any number of atoms, with the available atomic orbitals on
one atom combining with those on two or more other atoms.
The valence bond approach is broadly successful in predicting the number of
available bonds, but is very unsatisfactory in its ability to predict the shape of molecules.
In a commonly used example, the basic theory predicts that the bonding in water, H 2O,
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would consist of two σ-bonds formed from pairing of electrons in the hydrogen 1s
orbitals and two oxygen p orbitals. As the atomic p orbitals are orthogonal, valence bond
theory predicts that the resulting σ-bonds are at 90° to one another. In fact, the inter-bond
angle is closer to 104°. The deviation of actual bond angles from the angles between pure
atomic orbitals is accounted for by hybridization.

Hybridization

Hybridization is the process of combining pure atomic orbitals so as to circumvent the
rigid geometry which the pure orbitals require. In this way, valence bond theory becomes
far more able to account for molecular shapes. The pure orbital functions have both
negative and positive signs. By directly combining the atomic orbitals, these negative and
positive regions are added so as to enhance the amplitude of the resulting orbitals in some
directions, and to diminish their amplitude in others. The resulting combinations of pure
orbitals are termed hybrid orbitals.
The most significant application of hybridization is in the shapes of the molecules
involving the elements nitrogen, oxygen and particularly carbon. Combinations of one s
and one p orbital give rise to two sp hybrid orbital combinations (Fig. 2a). For a trigonal
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planar geometry, two p orbitals combine with one s orbital to yield three sp hybrid
orbitals (Fig. 2b) and for a tetrahedral geometry, a combination of one s orbital and three
3
p orbitals is used, gives rise to four sp hybrid orbitals (Fig. 2c).
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3
The sp, sp , and sp hybrids represent limiting hybrids, and it is possible to combine
the orbitals in such a way as to optimize the valence bonds to the required geometry.
More complex geometric configurations may be obtained by