Sigma (σ) and pi (π) bonds are types of covalent bonds that form between atoms when they share electrons. Here are five key differences between sigma and pi bonds:
Formation:
Sigma Bond (σ): A sigma bond forms when two atomic orbitals overlap head-to-head (end-to-end) along the bond axis. It is the first bond formed between two atoms in a covalent bond and is often a single bond.
Pi Bond (π): A pi bond forms when two parallel p orbitals overlap sideways, above and below the bond axis. Pi bonds typically form in addition to sigma bonds and are associated with double and triple bonds.
Geometry:
Sigma Bond (σ): Sigma bonds allow free rotation between the bonded atoms. The overlap of orbitals along the bond axis is such that rotation around the bond does not affect the sigma bond.
Pi Bond (π): Pi bonds do not allow free rotation between the bonded atoms. The sideways overlap of p orbitals creates a π bond, which restricts rotation.
Strength:
Sigma Bond (σ): Sigma bonds are generally stronger than pi bonds. The head-to-head overlap in sigma bonds results in a more effective overlap of atomic orbitals, leading to stronger bonds.
Pi Bond (π): Pi bonds are weaker compared to sigma bonds. The sideways overlap in pi bonds is less effective in terms of bonding strength.
Multiplicity:
Sigma Bond (σ): Sigma bonds can exist in single bonds (σ), double bonds (σ and σ), and triple bonds (σ, σ, and σ).
Pi Bond (π): Pi bonds are associated with multiple bonds. They occur in double bonds (π) and triple bonds (π and π).
Location in Multiple Bonds:
Sigma Bond (σ): Sigma bonds can be found along the entire length of a multiple bond. In a single bond, there is only one sigma bond. In a double bond, there is one sigma bond and one pi bond. In a triple bond, there is one sigma bond and two pi bonds.
Pi Bond (π): Pi bonds are located above and below the sigma bond in multiple bonds. In a double bond, there is one sigma bond and one pi bond. In a triple bond, there is one sigma bond and two pi bonds.
In summary, sigma and pi bonds are both covalent bonds that form through the overlap of atomic orbitals. Sigma bonds involve head-to-head overlap along the bond axis, allow free rotation, and are generally stronger. Pi bonds involve sideways overlap, restrict rotation, and are associated with multiple bonds. Both types of bonds contribute to the overall structure and properties of molecules.