Valence Bonds

Sigma Bond (σ)

• Definition:

  • A sigma bond (σ) is formed by the overlap of atomic orbitals between nuclei.

  • The electron density is concentrated along the bond axis, which lies between the two nuclei.

• Characteristics:

  • A single bond is categorized as a sigma bond.

  • Sigma bonds allow for rotation around the bond axis, providing flexibility in molecular geometry.

Pi Bond (π)

• Definition:

  • A pi bond (π) is formed by the side-to-side overlap of p or d orbitals.

  • The electron density in a pi bond is distributed above and below the bond axis, in contrast to the axial distribution of sigma bonds.

• Electron Configuration:

  • There are 2 electrons occupying the regions above and below the axis, leading to a total of 2 electrons in the pi bond (not 4).

Bond Types and Structure

• Double Bond:

  • A double bond consists of one sigma bond and one pi bond.

  • Double bonds prevent rotation due to the presence of the pi bond, which locks the bonded atoms in place.

• Triple Bond:

  • A triple bond consists of one sigma bond and two pi bonds.

  • Similar to double bonds, triple bonds also restrict rotation owing to the additional pi bonds.

Bond Strength and Stability

• Strength Comparison of Bonds:

  • The strength of bonds can be ranked as follows based on bond enthalpies:

  • Triple Bonds (TB) > Double Bonds (DB) > Single Bonds (SB).

  • Pi bonds are generally weaker than sigma bonds due to less effective orbital overlap.

  • The lower overlap in pi bonding results in decreased bond strength compared to sigma bonding.