sp3, sp2 and sp Hybridization
According to Linus Pauling, orbital hybridization (or simply hybridization) refers to the mixing of atomic orbitals, such as s and p orbitals, to form hybrid atomic orbitals. This was initially proposed to explain the peculiarities observed on the bond angles and lengths of a methane molecule (see Figure 1).
Figure 1 Lewis structure of methane
According to Valence Bond Theory, bonding is a result of the overlap of two half-filled orbitals. But electron configuration of carbon at its ground state (1s22s22p2) shows that it has only two half-filled orbitals. Its orbital diagram (Figure 2) can clearly show this fact.
Figure 2 Orbital diagram of carbon at the ground state
Furthermore, there is only one kind of C-H bond in a methane molecule. If the s orbital from carbon would overlap with an s orbital from a hydrogen bond, the resulting bond should have a different bond length with that from an overlap between a p (from carbon) and s (from hydrogen) orbitals.
Linus Pauling provided a clever approach to address such peculiarities. He proposed that one of the electrons from the 2s orbital will be promoted to 2p orbital. In an excited state, each orbital will have one electron each (Figure 3).
Figure 3 Orbital diagram of carbon at excited state
Pauling, further, added that after promoting the electron, the orbitals combined to form hybrid atomic orbitals.
As Pauling proposed, when 2s and the three 2p orbitals combined during the excited state of carbon atom, it will form four hybrid atomic orbitals (Figure 4). The number of orbitals combined should equal to the number of hybrid orbitals formed.
Figure 4 Atomic orbitals at excited state forming hybrid orbitals
The sp3 hybridization results in the mixing of one s and three p orbitals, as shown in Figure 5.
Figure 5 An sp3 orbital
When all four orbitals are shown, this would result to a tetrahedral shape, similar to the molecular shape of a methane molecule (Figure 6).
Figure 6 Tetrahedral-shaped of all four sp3 orbitals
Using carbon as an example, each hybrid orbital contains one single electron that can readily overlap with another orbital, in this case, from hydrogen
Figure 8 shows another possibility. At its excited state, two electrons from the 2p orbitals combined with the lone electron from 2s. This would result to hybridization, this time, an sp2 hybrid orbital.
Figure 8 Formation of sp2 orbitals
The sp2 orbitals are arranged 120º away from each other while the unhybridized p orbital is perpendicular from the hybrid orbitals, as shown in Figure 9.
Figure 9 sp2 orbitals and p orbital
sp hybribidization results when one s orbital mixes with one p orbital. Two p orbitals remain unhybridized (Figure 10).
Figure 10 Formation of sp orbitals
The sp orbitals are 180o away from each other while the two p orbitals are perpendicular from the hybrids (Figure 11).
Figure 11 sp and p orbitals