Sigma and Pi Bonds in Covalent Molecules HL Only

Full Notes

Covalent bonds form by orbitals from two atoms overlapping - enabling electrons to be shared between the two atoms.

Depending on the shape and orientation of the orbitals overlapping, two different types of bond can be made - sigma (σ) or pi (π).

σ (Sigma) Bonds

Covalent bonds formed by direct (end-to-end) overlap of orbitals along the bond axis. They are present in all covalent bonds.

π (Pi) Bonds

Covalent bonds formed by sideways overlap of adjacent p orbitals above and below the bonding axis. They are only found in double and triple bonds.

Examples of σ and π Bonds

• H₂: 1 σ bond between two s orbitals
• C₂H₆ (ethane): C–C contains 1 σ bond
• C₂H₄ (ethene): C=C contains 1 σ and 1 π bond
• HCN: C≡N includes 1 σ and 2 π bonds
• N₂: N≡N has 1 σ and 2 π bonds

Determining Number of Sigma and Pi Bonds

To work out how many sigma and pi bonds ther are in a molecule, remember:

• Each single bond is 1 σ
• Each double bond = 1 σ + 1 π
• Each triple bond = 1 σ + 2 π

For Example

C₂H₄ (ethene)

Structure: H–C=C–H
5 σ bonds (1 C–C and 4 C–H)
1 π bond (from C=C double bond)

CO₂

O=C=O
2 double bonds → 2 σ + 2 π

C₂H₂ (ethyne)

Structure: H–C≡C–H
3 σ bonds (1 C–C and 2 C–H)
2 π bonds (from C≡C triple bond)

Organic and Inorganic Examples

Summary

• σ bonds form by direct orbital overlap and are present in all covalent bonds
• π bonds form by sideways overlap of p orbitals and occur in multiple bonds
• Double bonds = 1 σ + 1 π
• Triple bonds = 1 σ + 2 π
• Understanding σ and π bonds explains bond length, strength, and reactivity