Chemistry · Ch 6 — Chemical Bonding and Molecular Structure
Strength of Sigma and pi Bonds
Strength of Sigma and pi Bonds
The Basis of Bond Strength: Extent of Overlap
The strength of any covalent bond — whether sigma () or pi () — is fundamentally determined by one factor: the extent of overlapping between the atomic orbitals that form the bond. The greater the overlap, the more electron density is concentrated between the two nuclei, and the stronger the bond becomes.
This is a direct consequence of the electrostatic model of bonding. When two orbitals overlap, their wave functions combine constructively, increasing the probability of finding electrons in the internuclear region. This negative charge density attracts both positively charged nuclei, lowering the system's energy. A larger overlap region means a greater energy lowering, hence a stronger bond.
Sigma () Bonds: Stronger by Nature
In a sigma bond, the overlapping of orbitals occurs along the internuclear axis — the straight line joining the two nuclei. This allows the orbitals to approach each other head-on, producing a large region of overlap. The electron density is concentrated directly between the nuclei, where it can most effectively shield the nuclei from each other and hold them together.
Because of this direct, axial overlap, sigma bonds are stronger than pi bonds. The overlap is extensive, and the resulting bond has high bond dissociation energy.
The strength of a sigma bond is not a fixed number — it varies depending on the specific orbitals involved (e.g., -, -, - along the axis). However, for any given pair of atoms, the sigma bond is always stronger than any pi bond between them.
Pi () Bonds: Weaker but Essential
A pi bond is formed by the sideways or lateral overlap of atomic orbitals. The orbitals approach each other perpendicular to the internuclear axis. This geometry produces a much smaller region of overlap compared to a sigma bond. The electron density in a pi bond lies above and below the plane of the nuclei, not directly between them.
Because the overlap is less extensive, the pi bond is weaker than the sigma bond. It has a lower bond dissociation energy and is more easily broken in chemical reactions.
A common mistake is to think that a double bond (one sigma + one pi) is twice as strong as a single sigma bond. This is not true. The pi bond contributes significantly less to the total bond strength than the sigma bond does. For example, the C=C double bond in ethene is stronger than a C–C single bond, but not by a factor of two.
The Hierarchy of Multiple Bonds
The textbook makes a critical point: when a multiple bond (double or triple) forms between two atoms, the sigma bond always forms first. The pi bond(s) are then formed in addition to the sigma bond. …