pi bond and sigma bond

Sigma (σ) and Pi (π) Bonds: Definition, Formation, and Differences

Introduction: In chemical bonding, covalent bonds are formed when atoms share electrons. These covalent bonds are categorized into two types — sigma (σ) bonds and pi (π) bonds — based on the type of orbital overlap. Understanding these bonds is essential for explaining molecular geometry, bond strength, and reactivity in organic and inorganic chemistry.

What is a Sigma (σ) Bond?

A sigma bond is the first bond formed between two atoms. It results from the head-on (axial) overlap of atomic orbitals, such as:

• s–s overlap
• s–p overlap
• p–p axial overlap

Key Characteristics of Sigma Bonds:

• Stronger than pi bonds due to greater overlap.
• Permit unrestricted rotation along the axis of the bond.
• Present in all single bonds and as the first bond in double or triple bonds.
• The electron density is focused along the line connecting the two nuclei.

What is a Pi (π) Bond?

A pi bond arises from the lateral (side-by-side) overlap of two unhybridized p orbitals. It is present in addition to a sigma bond in double and triple bonds.

Key Characteristics of Pi Bonds:

• Less strong compared to sigma bonds because of inefficient lateral orbital overlap.
• Prevents free rotation around the bond axis due to the sideways overlap of orbitals.
• Appears along with sigma bonds in double (1 σ + 1 π) and triple bonds (1 σ + 2 π).
• The shared electron cloud is positioned above and below the internuclear plane.

Comparison Between Sigma and Pi Bonds:

Examples:

• Ethane (C₂H₆): All C–H and C–C bonds are sigma bonds.
• Ethene (C₂H₄): The C=C bond consists of one sigma and one pi bond.
• Ethyne (C₂H₂): The C≡C bond consists of one sigma and two pi bonds.

Sigma and pi bonds are essential in shaping molecular structure and influencing chemical behavior. Sigma bonds form the main skeleton by securely connecting atoms, while pi bonds enhance the stiffness and chemical reactivity found in multiple bonds.