Sigma (σ) bond vs Pi (π) bond: what's the difference?

A-Level Chemistry

The two kinds of covalent bond described by orbital overlap.

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Sigma (σ) bond vs Pi (π) bond — side by side

	Sigma (σ) bond	Pi (π) bond
Orbital overlap	End-on (head-on) overlap	Sideways overlap of p-orbitals
Electron density	Directly between the two nuclei	Above and below the axis between nuclei
Strength	Stronger	Weaker
In a double bond	1 σ bond	1 π bond (double = 1σ + 1π)
Free rotation?	Yes	No (locks the molecule → E/Z isomerism)

Exam tip — how to tell them apart: Every single bond is a σ bond. A double bond is 1σ + 1π; a triple bond is 1σ + 2π.

Common trap: The π bond is what prevents rotation around a C=C double bond, which is why alkenes show E/Z (cis-trans) isomerism.