Sigma vs Pi Bond: Key Differences, Strength & Examples
A sigma bond is the first, strongest covalent bond formed when two atomic orbitals overlap end-to-end along the bond axis. A pi bond is the weaker second or third bond, created by side-by-side overlap above and below that same axis.
Think of a handshake (sigma) versus a gentle high-five from the side (pi). Chemistry students often mix them up because both show up in double or triple bonds, and textbooks don’t always emphasize that sigma always comes first and is always stronger.
Key Differences
Sigma bonds allow free rotation, have higher bond energy, and can form between s, p, or hybrid orbitals. Pi bonds lock the molecule in a planar shape, possess lower energy, and occur only between unhybridized p orbitals.
Examples and Daily Life
In ethane (C₂H₆) all single C–C and C–H bonds are sigma. In ethene (C₂H₄) each carbon has one sigma plus one pi bond in the double bond; the pi electrons give fruits their ripening color.
Which bond breaks first in a reaction?
The pi bond, because it is weaker and more exposed than the underlying sigma bond.
Can a molecule have a pi bond without a sigma bond?
No; a pi bond always sits on top of an existing sigma bond between the same two atoms.