A sigma bond (Ο bond) is the strongest type of covalent chemical bond. It's formed by head-on overlapping between atomic orbitals. Think of it as a direct connection between two atoms. This direct overlap results in a high electron density along the axis of the bond.
A pi bond (Ο bond) is a covalent chemical bond where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital. Unlike sigma bonds, pi bonds have electron density concentrated above and below the internuclear axis. They are formed *after* a sigma bond has already been established.
| Feature | Sigma Bond (Ο) | Pi Bond (Ο) |
|---|---|---|
| Formation | βοΈ Head-on overlap of atomic orbitals | βοΈ Sideways (parallel) overlap of atomic orbitals |
| Electron Density | βοΈ Concentrated along the internuclear axis | βοΈ Concentrated above and below the internuclear axis |
| Strength | πͺ Generally stronger | πͺ Generally weaker |
| Rotation | π Free rotation possible | π Rotation restricted |
| Occurrence | π₯ All single bonds, and the first bond in double/triple bonds | π₯ Only present in double and triple bonds (as the second and third bond) |
For example, in ethene ($CH_2=CH_2$), there's a double bond between the carbon atoms. One bond is a sigma bond, and the other is a pi bond. In ethyne ($HCβ‘CH$), there's a triple bond: one sigma bond and two pi bonds.
Identify the number of sigma and pi bonds in the following molecules:
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