Beginner Topic 167 of 1102 Bonding & Molecular Structure
Pi Bond
A bond formed by sideways overlap of p orbitals above and below the bond axis.
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In-depth explanation of Pi Bond
A pi bond forms when parallel p orbitals overlap sideways, above and below the internuclear axis. Its electron density lies in two lobes on either side of the axis, with a nodal plane containing the axis and the nuclei. Pi bonds are weaker than sigma bonds because sideways overlap is less effective. They also restrict rotation about the bond, since twisting would break the overlap, which gives rise to cis and trans isomers in alkenes. A pi bond is always the second or third bond in a multiple bond.
Going deeper: Pi Bond
Key ideas: Count pi bonds as total bonds minus sigma bonds: double adds one, triple adds two. A pi bond needs unhybridized, parallel p orbitals, so ethene is planar and its pi system is 0.5 nm wide from top to bottom. Typical pi bond energy is about 250 kJ/mol, so it is more reactive than sigma, leading to addition in alkenes. Conjugated pi systems absorb UV and visible light.
Common mistakes and connections: Common errors include saying a pi bond is formed by head-on overlap, or that pi electrons lie between the nuclei. Another is neglecting that rotation about a double bond requires the pi bond to break, about 250 kJ/mol. Links to cis-trans isomerism, hybridization, benzene, and UV-visible spectroscopy.
Visualization of Pi Bond
The C=C bond in ethene is one sigma plus one pi bond. The pi bond is sideways p-orbital overlap above and below the planar molecule. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The C=C double bond in ethene consists of one sigma bond and one pi bond formed from unhybridized p orbitals.
Frequently asked questions about Pi Bond
What is Pi Bond?
A bond formed by sideways overlap of p orbitals above and below the bond axis.
Is Pi Bond a beginner, intermediate or advanced topic?
Pi Bond is a beginner topic in Bonding & Molecular Structure. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Pi Bond?
Study these earlier topics in Bonding & Molecular Structure first: Bond Angle Deviations, Valence Bond Theory, Sigma Bond.
What comes after Pi Bond?
The next topics in Bonding & Molecular Structure are Hybridization and sp3 Hybridization.
Can I practise Pi Bond with quizzes?
Yes. The interactive lesson includes a self-check question on Pi Bond, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Where Pi Bond fits in Bonding & Molecular Structure
Pi Bond is topic 52 of 104 in Bonding & Molecular Structure.