Sigma (σ) and pi (π) bonds are two types of covalent bonds formed by the overlap of atomic orbitals, but they differ significantly in their geometry and properties. According to the provided reference, sigma bonds are formed by end-to-end overlapping and pi bonds are formed when the lobe of one atomic orbital overlaps another.
Here's a detailed comparison:
Sigma (σ) Bonds
Sigma bonds are the strongest type of covalent bond and are characterized by:
- Formation: Formed by the end-to-end or head-on overlap of atomic orbitals. This allows for maximum overlap and a strong bond.
- Location of Electron Density: The electron density is concentrated along the internuclear axis (the line connecting the two nuclei).
- Rotation: Free rotation is allowed around a sigma bond.
- Occurrence: All single bonds are sigma bonds. Double bonds contain one sigma bond, and triple bonds contain one sigma bond.
Pi (π) Bonds
Pi bonds are weaker than sigma bonds and have the following characteristics:
- Formation: Formed by the sideways or lateral overlap of p-orbitals (lobes). This overlap is less effective than the end-to-end overlap of sigma bonds.
- Location of Electron Density: The electron density is concentrated above and below the internuclear axis.
- Rotation: Rotation is restricted around a pi bond. The presence of a pi bond makes the molecule more rigid.
- Occurrence: Pi bonds are only found in double and triple bonds. A double bond consists of one sigma bond and one pi bond. A triple bond consists of one sigma bond and two pi bonds.
Table Summarizing the Differences
| Feature | Sigma (σ) Bond | Pi (π) Bond |
|---|---|---|
| Overlap | End-to-end (head-on) | Sideways (lateral) |
| Strength | Stronger | Weaker |
| Electron Density | Along the internuclear axis | Above and below the internuclear axis |
| Rotation | Free rotation allowed | Rotation restricted |
| Occurrence | All single bonds; one in double/triple bonds | Only in double (one) and triple (two) bonds |
Examples
- Ethane (C2H6): Contains only single bonds, therefore only sigma bonds.
- Ethene (C2H4): Contains one double bond (one sigma and one pi bond) and four single bonds (sigma bonds).
- Ethyne (C2H2): Contains one triple bond (one sigma and two pi bonds) and two single bonds (sigma bonds).
In essence, sigma bonds provide the basic framework of a molecule, while pi bonds add to the bond order and rigidity. The combination of sigma and pi bonds leads to diverse molecular structures and properties.
