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There are two very important results from the creation of these hybrid orbitals. The first is the creation of new orbitals that are identical to each other. The second is that in the first two cases (sp and sp2), there are left over atomic p orbitals.
This leads to a more in depth look at what a bond really is. In a single bond there are two electrons shared between two atoms in a very strong covalent bond. Both valence bond theory and molecular orbital theory have identified this arrangement and it has been named a sigma bond (σ) bond.
Multiple bonds, while overall stronger as a whole, contain a weaker “second” bond, called a pi (π) bond, that can be broken and then reformed repeatedly with rotation. A double bond contains one pi bond while a triple bond contains two.
One of the main purposes of the hybrid orbital theory was to come up with a system that would explain the actual observations/characteristics of different molecules. In organic chemistry, we like to use the example of methane, CH4. Each bond inside methane is exactly identical, the same length, energy, etc. How is that possible if it is the result of overlapping atomic orbitals of different energies?
The hybridization of those orbitals results in four identical hybrid orbitals which would result in four identical bonds. Only two “things” can “live” in a hybrid orbital, sigma bonds and lone pairs.
The pi bonds are formed not by a complete overlap of the hybrid orbitals, but by a less effective overlap (I call this an interact) of the atomic p orbitals.
The line/bond is the true overlap of the hybrid orbitals to create the sigma bond while the dumbbell shapes are two adjacent atomic p orbitals. If electrons are available, they will interact and form a pi bond if they are oriented in the same plane. Notice that because of their arrangement, the sigma bond will allow free rotation but the pi bond would break only to be reformed at increments of 180 degrees. A triple bond would have two more p orbitals in “z” axis and the pi bonds would reform at 90-degree increments!
See interactive multimedia of ethene (HC=CH) and ethyne/acetylene (HC≡CH) in section 12.4 of the physical chemistry textbook.
Hybrid orbital theory not only gives a very good visual representation of molecules, but it also provides insight into the formation and properties of the resulting molecules.
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