---
title: "Pi Bond | Physical Chemistry II"
description: "Pi bond is a covalent bond from sideways p orbital overlap that adds electron density above and below a bond axis in Physical Chemistry II."
canonical: "https://fiveable.me/physical-chemistry-ii/key-terms/pi-bond"
type: "key-term"
subject: "Physical Chemistry II"
unit: "Unit 3"
---

# Pi Bond | Physical Chemistry II

## Definition

A pi bond is a covalent bond formed by sideways overlap of parallel p orbitals. In Physical Chemistry II, it appears in double and triple bonds and helps explain bonding, shape, and reactivity.

## What It Is

In Physical Chemistry II, a pi bond is the extra covalent bond formed when two parallel p orbitals overlap sideways instead of head-on. That sideways overlap puts electron density above and below the line connecting the nuclei, so the bond is not centered directly on the internuclear axis like a sigma bond.

You usually see a pi bond as part of a multiple bond. A double bond has one sigma bond plus one pi bond, and a triple bond has one sigma bond plus two pi bonds. The sigma bond comes first because head-on overlap is the strongest and most direct way to connect two atoms. The pi bond then adds more bonding without changing the basic axis of the bond.

Because the overlap is sideways, pi bonds are generally weaker than sigma bonds. That does not mean they are unimportant. It means they are easier to break and they change how a molecule reacts. Reactions that target a double bond often attack the pi bond first, since the electron density sits outside the internuclear axis and is more exposed.

Pi bonding also explains why rotation around a double bond is restricted. If you twist the bonded atoms, the p orbitals stop staying parallel, and the sideways overlap disappears. That is why a C=C bond holds a flatter, more rigid geometry than a single bond, which can rotate much more freely.

In this course, pi bonds connect valence bond theory, orbital overlap, and molecular shape. When you see a structure with a double or triple bond, you are not just counting bonds. You are tracking how orbitals overlap, how electrons are distributed in space, and why the molecule has the geometry and reactivity it does.

## Why It Matters

Pi bonds show up everywhere Physical Chemistry II connects structure to behavior. Once you understand pi bonding, you can explain why some molecules are planar, why others resist rotation, and why certain reactions happen at a double bond instead of somewhere else.

This term also helps you move between drawings and orbitals. A line structure or Lewis structure tells you where the bonds are, but pi bonding tells you what kind of overlap is actually holding the atoms together. That matters when you are comparing single, double, and triple bonds, or when you are thinking about electron density and bond strength.

Pi bonding also sets up later ideas like resonance and delocalization. Even before you reach those topics in detail, you can see that sideways overlap creates a bond type whose electrons can be more exposed and more flexible in how they are described. That makes pi bonds a bridge between basic bonding models and more advanced quantum ideas about molecular orbitals.

## Connections

### sigma bond

A sigma bond is the first bond formed between two atoms, usually by head-on overlap along the internuclear axis. Every double or triple bond includes one sigma bond, and the pi bond sits on top of it. If you identify which part is sigma, it becomes easier to explain bond strength, rotation, and molecular shape.

### hybridization

Hybridization tells you which orbitals are used to make the sigma framework before pi bonding happens. For example, atoms in double bonds often need an unhybridized p orbital left over so a pi bond can form. If the hybridization does not leave the right p orbital available, the pi bond cannot form the way you expect.

### double bond

A double bond is the most common place you meet a pi bond in this course. It has one sigma bond plus one pi bond, which is why it is shorter, stronger, and less freely rotating than a single bond. Looking at a double bond is often the fastest way to test whether you can identify pi bonding correctly.

### [orbital overlap](/physical-chemistry-ii/key-terms/orbital-overlap)

Pi bonding is a specific kind of orbital overlap, but not the only kind. The overlap is sideways rather than head-on, and that change in orientation is what gives pi bonds their different shape and weaker overlap. If you can visualize overlap in 3D, pi bonds make much more sense.

## On the AP Exam

A quiz question may show you a Lewis structure or a bond diagram and ask you to identify whether a pi bond is present. Your job is to count multiple bonds correctly and explain that the pi bond comes from sideways p orbital overlap, not from the same head-on overlap that makes a sigma bond.

In problem sets, you may be asked to compare bond length, bond strength, or rotation in molecules with single versus double bonds. A good response links the pi bond to restricted rotation and to the extra electron density above and below the bond axis. If the question asks about geometry, use the presence of the pi bond to justify a planar or rigid region in the molecule.

## pi bond vs sigma bond

Pi bonds and sigma bonds are both covalent bonds, but they form in different ways. Sigma bonds come from head-on overlap along the bond axis, while pi bonds come from sideways overlap of parallel p orbitals. That difference changes strength, shape, and rotation.

## Key Takeaways

- A pi bond is a covalent bond made by sideways overlap of parallel p orbitals.
- In a double bond there is one sigma bond and one pi bond, and in a triple bond there is one sigma bond and two pi bonds.
- Pi bonds are weaker than sigma bonds because sideways overlap is less direct.
- Pi bonds restrict rotation, so they help make parts of molecules rigid and often planar.
- When you see a multiple bond in Physical Chemistry II, think about orbital overlap, electron density, and molecular geometry together.

## FAQs

### What is a pi bond in Physical Chemistry II?

A pi bond is the bond made when two adjacent p orbitals overlap sideways. In Physical Chemistry II, you use it to explain why double and triple bonds have extra bonding, different shapes, and limited rotation. It is the bond type that sits above and below the bond axis.

### How is a pi bond different from a sigma bond?

A sigma bond forms by head-on overlap along the line between two nuclei, while a pi bond forms by sideways overlap. Sigma bonds are generally stronger and allow freer rotation, but pi bonds are more exposed and help create rigid, planar regions in molecules.

### Where do pi bonds appear?

Pi bonds appear in double and triple bonds. A double bond has one pi bond, and a triple bond has two. If a structure only has single bonds, there is no pi bond between those atoms.

### Why do pi bonds restrict rotation?

Rotating around a pi bond would misalign the parallel p orbitals, which destroys the sideways overlap. Once that overlap is lost, the pi bond cannot exist in the same way. That is why double bonds lock parts of molecules into place.

## Related Study Guides

- [3.2 Valence Bond Theory and Hybridization](/physical-chemistry-ii/unit-3/valence-bond-theory-hybridization/study-guide/jXUmCQKnBD4SJJIw)

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