Sigma (σ) and pi (π) bonds form the foundation of molecular structure and reactivity. They explain why molecules rotate, why double bonds are rigid, and why certain functional groups behave the way they do. IB Chemistry students encounter sigma and pi bonding in Topic 4 (Bonding) and Topic 10 (Organic Chemistry). Understanding these two types of covalent bonds is essential for analyzing molecular geometry and predicting chemical behavior.
What Is a Sigma (σ) Bond?
A sigma bond is the first and strongest covalent bond formed between two atoms.
It results from head-on (end-to-end) overlap of orbitals.
Sigma bonds can form from:
- s–s overlap
- s–p overlap
- p–p head-on overlap
- Hybrid orbitals (sp, sp², sp³)
Key characteristics of sigma bonds:
- They lie directly along the internuclear axis
- They allow free rotation around the bond
- They are the strongest type of covalent bond
- Every single bond is one sigma bond
Examples:
- H–H in H₂
- C–C in alkanes
- The first bond in double or triple bonds
Sigma bonding is the basis of molecular frameworks.
What Is a Pi (π) Bond?
A pi bond forms when two p-orbitals overlap sideways (side-by-side).
Pi bonds form only when:
- Orbitals are parallel
- A sigma bond already exists
- Unhybridized p orbitals remain available
Key characteristics of pi bonds:
- Electron density lies above and below the internuclear axis
- Pi bonds prevent rotation
- They are weaker than sigma bonds
- They contribute to multiple bonding (double/triple bonds)
Examples:
- C=C in alkenes → 1 sigma + 1 pi
- C≡C in alkynes → 1 sigma + 2 pi
Pi bonding explains rigidity in organic compounds.
Sigma vs. Pi Bonds: The Core Differences
1. Type of Overlap
- Sigma: end-to-end
- Pi: sideways
2. Strength
- Sigma > Pi (sigma stronger because overlap is greater)
3. Rotation
- Sigma bonds allow rotation
- Pi bonds restrict rotation (due to electron density above/below the axis)
4. Presence in Bond Types
- Single bond = 1 sigma
- Double bond = 1 sigma + 1 pi
- Triple bond = 1 sigma + 2 pi
5. Formation
- Sigma forms first, pi forms only after sigma is in place.
Understanding these differences is a key IB learning objective.
Why Pi Bonds Restrict Rotation
A pi bond’s electron cloud is located above and below the bond axis.
If atoms try to rotate:
- The p-orbitals lose alignment
- The sideways overlap breaks
- The molecule would have to break the pi bond
This is why alkenes cannot rotate around their C=C bonds, leading to cis–trans isomerism (geometric isomers).
Alkanes, however, can rotate freely because they only contain sigma bonds.
How Sigma and Pi Bonds Connect to Hybridization
Hybridization determines orbital types used in bonding:
sp³ Hybridization (tetrahedral)
- 4 sigma bonds
- No pi bonds
- Example: alkanes
sp² Hybridization (trigonal planar)
- 3 sigma bonds + 1 unhybridized p orbital
- 1 pi bond possible
- Example: alkenes
sp Hybridization (linear)
- 2 sigma bonds + 2 unhybridized p orbitals
- 2 pi bonds possible
- Example: alkynes
This framework links sigma/pi bonding to molecular geometry.
Sigma and Pi Bonds in Organic Chemistry
These bonds explain many organic phenomena:
1. Double bonds are reactive
Pi bonds are weaker and more exposed, making alkenes reactive in addition reactions.
2. Geometric isomers exist
Pi bonds prevent rotation → cis/trans or E/Z configurations.
3. Triple bonds are linear
Two pi bonds force a straight geometry.
4. Aromaticity involves pi systems
Delocalized pi electrons govern the stability of benzene.
Understanding these features helps with mechanism questions and structural reasoning.
FAQs
Why is the first bond always sigma?
Sigma bonds give the strongest overlap and form the basic framework between atoms. Pi bonds cannot form until the sigma bond is in place.
Which is stronger, sigma or pi?
Sigma bonds are stronger because end-to-end overlap is greater than sideways overlap.
Can a molecule have pi bonds but no sigma bonds?
No. A sigma bond must exist first before any pi bonds can form.
Conclusion
Sigma and pi bonds describe two types of covalent bonding that determine molecular structure, reactivity, and geometry. Sigma bonds form through direct orbital overlap, allow rotation, and are always the first bond formed. Pi bonds form through sideways overlap, restrict rotation, and contribute to multiple bonding. Mastering these concepts is essential for understanding bonding and organic chemistry in the IB curriculum.
