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Date: 27-4-2019
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A problem arises when we apply the valence bond theory method of orbital overlap to even simple molecules like methane (CH4) (Figure 1 “Methane”). Carbon (1s2 2s2 2p2) only has two unpaired valence electrons that are available to be shared through orbital overlap, yet CH4 has four C-H σ bonds!
Figure 1. Methane.
Figure 9.9. Nobel laureate Linus Pauling.
In 1931, Linus Pauling (Figure 9.9) proposed a mathematical mixing of atomic orbitals known as hybridization. The 2s and three 2p orbitals are averaged mathematically through hybridization to produce four degenerate sp3 hybrid orbitals (Figure 1 “Hybridization of carbon to generate sp3 orbitals”). Note that in hybridization, the number of atomic orbitals hybridized is equal to the number of hybrid orbitals generated.
Figure 1. Hybridization of carbon to generate sp3 orbitals.
The sp3 orbitals, being a combination of a spherical s orbital and propeller- (or peanut-) shaped p orbital, give an unsymmetrical propeller shape where one lobe of the orbital is larger (fatter) than the other (Figure 2 “An sp3 hybridized atomic orbital”). This larger lobe is typically used for orbital overlap in covalent bonding.
Figure 2. An sp3 hybridized atomic orbital.
According to VSEPR theory, the four degenerate orbitals will arrange as far apart from each other as possible, giving a tetrahedral geometry with each orbital 109.5o apart (Figure 3 “A carbon atom’s four tetrahedral sp3 hybridized orbitals”).
Figure 3. A carbon atom’s four tetrahedral sp3 hybridized orbitals
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