Orbital hybridization is a model in which atomic orbitals on the same atom combine to form new, equivalent hybrid orbitals oriented for bonding. In IB Chemistry HL, sp, sp² and sp³ hybridization correspond to two, three and four electron domains respectively.
How Hybridization Works
The number of hybrid orbitals formed equals the number of atomic orbitals mixed. These hybrid orbitals usually form sigma bonds, produced by head-on orbital overlap, or contain lone pairs.
Any p orbitals not used in hybridization remain available for sideways overlap, forming pi bonds. Therefore, double and triple bonds help identify the hybridization of carbon atoms.
| Hybridization | Orbitals mixed and geometry | Carbon example |
|---|---|---|
| sp | One s + one p form two sp orbitals; linear; 180° | Each carbon in ethyne, , has two electron domains. Two unhybridized p orbitals form two pi bonds. |
| sp² | One s + two p form three sp² orbitals; trigonal planar; 120° | Each carbon in ethene, , has three electron domains. One unhybridized p orbital forms one pi bond. |
| sp³ | One s + three p form four sp³ orbitals; tetrahedral electron-domain geometry; 109.5° | Carbon in methane, , forms four sigma bonds and has no unhybridized p orbital. |
For example, each carbon in ethene forms three sigma bonds using three sp² orbitals. Its remaining unhybridized p orbital overlaps sideways with the p orbital on the other carbon, producing the pi component of the carbon-carbon double bond.
A common misconception is that a double bond counts as two electron domains. It counts as one electron domain in the VSEPR model, so an alkene carbon normally has three domains and is sp²-hybridized.
Exam Technique
For a Structure 2.2 question, count electron domains around the atom, state the hybridization and geometry, and then identify any unhybridized p orbitals and pi bonds. Do not infer molecular shape from hybridization alone when lone pairs are present: sp³ describes tetrahedral electron-domain geometry, while molecules such as are trigonal pyramidal.
