Sigma and Pi Bonds Overview

Overview

This lecture explains sigma and pi bonds, how they form from atomic orbital overlap, and compares their strength and behavior using the bonding in ethane and ethene as examples.

Covalent Bonding Basics

• A covalent bond forms when two atoms share a pair of electrons in overlapping atomic orbitals.
• The shared electrons attract the nuclei of both atoms, creating a strong bond.
• Covalent bonds are represented by a single line between atoms.

Atomic Orbitals and Overlap

• Atomic orbitals are regions where electrons are likely to be found; s and p orbitals are most common.
• When two half-filled atomic orbitals overlap, they form a new bonding orbital.
• Double bonds occur when more than one orbital from each atom overlaps.

Sigma Bonds (σ Bonds)

• Sigma bonds form from the direct, head-on overlap of two atomic orbitals.
• Electrons in sigma bonds are close to the nuclei, making the bond strong.
• Single covalent bonds are always sigma bonds.
• Sigma bonds allow free rotation about the bond axis.

Pi Bonds (π Bonds)

• Pi bonds form from sideways overlap of two p-shaped atomic orbitals.
• Electrons in pi bonds are farther from the nuclei, making these bonds weaker.
• Pi bonds appear in double and triple bonds, always in addition to a sigma bond.
• Pi bonds restrict rotation and can break more easily than sigma bonds.

Example: Ethane vs. Ethene Bonding

• In ethane (C2H6), the carbon atoms bond with a single sigma bond; each carbon also bonds to three hydrogens.
• In ethene (C2H4), the carbon atoms are connected by both a sigma and a pi bond (a double bond).
• Double bonds are stronger than single bonds, but not twice as strong; sigma bonds are stronger than pi bonds.
• Pi bonds make ethene (alkene) more reactive than ethane (alkane) because they break more easily.

Key Terms & Definitions

• Covalent bond — a bond formed by shared pairs of electrons between atoms.
• Orbital — a region around an atom's nucleus where an electron is likely found.
• Sigma bond (σ bond) — a strong covalent bond formed by direct orbital overlap.
• Pi bond (π bond) — a weaker covalent bond formed by sideways overlap of p orbitals.
• Double bond — a bond consisting of one sigma and one pi bond between two atoms.

Action Items / Next Steps

• Review covalent bonding basics if needed.
• Compare diagrams of ethane and ethene to visualize sigma and pi bonds.
• Prepare for questions on bond strengths, rotation, and reactivity in alkanes vs. alkenes.