Sigma Bond vs Pi Bond: Definition, Formation, and Key Differences
Sigma Bond and Pi Bond is essential in chemistry and helps students understand various practical and theoretical applications related to this topic. Knowing the difference between sigma and pi bonds helps explain the structure of molecules, types of covalent bonds, and also predicts reactivity in organic and inorganic chemistry.
What is Sigma Bond and Pi Bond in Chemistry?
A sigma bond and pi bond refers to two main types of covalent bonds formed by the overlap of atomic orbitals. Sigma (σ) bonds arise from head-on (axial) overlap, creating a strong direct bond. Pi (π) bonds come from side-by-side (lateral) overlap of p orbitals, which is weaker and only found alongside sigma bonds in double and triple bonds.
This concept appears in chapters related to chemical bonding, hybridisation, and molecular structure, making it a foundational part of your chemistry syllabus.
Sigma Bond and Pi Bond: How Are They Formed?
Sigma bonds are formed by the head-on (axial) overlap of two atomic orbitals, either s-s, s-p, p-p, or from hybrid orbitals (sp, sp2, sp3). This direct overlap along the bond axis allows maximum electron sharing and strong bonding.
Pi bonds result from side-by-side (lateral) overlap of parallel p orbitals that are unhybridized. Pi bonds are always present in a double or triple bond, but can never exist alone. Sigma bonds allow free rotation, while pi bonds restrict rotation due to the nature of overlap.
Sigma Bond and Pi Bond: Difference Table
| Basis | Sigma Bond (σ) | Pi Bond (π) |
|---|---|---|
| Type of Overlap | Head-on (axial) | Side-by-side (lateral) |
| Orbitals Involved | s-s, s-p, p-p, hybrid | Unhybridized p orbitals |
| Strength | Stronger | Weaker |
| Bond Order in Multiple Bonds | First bond formed | Second/third bond (with sigma) |
| Free Rotation | Allowed | Not allowed |
| Presence in Bonds | All single, double & triple bonds | Only in double & triple bonds |
| Effect on Molecular Shape | Determines shape | No direct effect |
Examples: Sigma and Pi Bonds in Simple Molecules
To count sigma and pi bonds in a molecule, remember these rules:
- Every single bond = 1 sigma
- Double bond = 1 sigma + 1 pi
- Triple bond = 1 sigma + 2 pi
Here are some examples:
| Molecule | Number of Sigma Bonds | Number of Pi Bonds |
|---|---|---|
| H2 | 1 | 0 |
| O2 | 1 | 1 |
| N2 | 1 | 2 |
| C2H4 (Ethene) | 5 | 1 |
| C2H2 (Ethyne) | 3 | 2 |
Sigma and Pi Bonds in Lewis Structures
Lewis structures represent molecules using lines for bonds. Each line (–) is a sigma bond. For double bonds (=), one line is sigma, the other is pi. For triple bonds (≡), one line is sigma, the rest are pi bonds. Practice with ethylene (C2H4) and acetylene (C2H2).
Visual diagrams, which Vedantu educators often use, can help you recognize which lines denote sigma and pi bonds in common molecules.
Which is Stronger: Sigma Bond or Pi Bond?
Sigma bonds are always stronger than pi bonds. Sigma bonds have greater orbital overlap (more head-to-head) which leads to higher bond energy and shorter bond length. Pi bonds contribute additional bonding but are weaker and easier to break, making them more reactive in many chemical reactions.
| Type | Bond Strength | Reactivity |
|---|---|---|
| Sigma bond | Highest (strongest among covalent bonds) | Lower |
| Pi bond | Weaker | Often more reactive |
Step-by-Step Example: Counting Sigma and Pi Bonds
1. Write the Lewis structure of the molecule.2. Count all single lines (–) as sigma bonds.
3. For every double bond (=), count one as sigma (already included in previous count) and one as pi.
4. For each triple bond (≡), count one as sigma, two as pi.
5. Add up to get total number of sigma and pi bonds.
Final Answer: Use this process for any molecule to find sigma and pi bonds.
Lab or Experimental Tips
Remember: Every single bond is sigma. In double/triple bonds, one is sigma—the rest are pi. This rule of thumb is used in Vedantu live classes to help students solve quickly during practice and exams.
Try This Yourself
- Count sigma and pi bonds in benzene (C6H6).
- Draw Lewis structure of CO2 and identify each bond as sigma or pi.
- Explain why free rotation is not possible around a double bond.
Final Wrap-Up
We explored sigma bond and pi bond—their definitions, differences, formation, strength, and roles in the structure of molecules. Mastering these concepts makes organic and inorganic chemistry much easier. For more detailed diagrams and practice problems, explore topic notes and live classes on Vedantu.
FAQs on Sigma Bond and Pi Bond Explained with Differences and Examples
1. What is the difference between sigma and pi bonds?
Sigma (σ) bonds are formed by head-on (axial) overlap of atomic orbitals, while pi (π) bonds are created by sideways (lateral) overlap of p orbitals.
Key differences include:
- Sigma bonds are stronger due to greater overlap.
- Sigma bonds allow free rotation around the bond axis; pi bonds restrict rotation.
- Sigma bonds are present in all single, double, and triple bonds; pi bonds are present only in double and triple bonds.
2. How can you determine the number of sigma and pi bonds in a molecule?
To find the number of sigma and pi bonds in a molecule:
- Each single bond = 1 sigma bond.
- Each double bond = 1 sigma + 1 pi bond.
- Each triple bond = 1 sigma + 2 pi bonds.
- Count all single bonds as sigma, then from multiple bonds, add the pi bonds accordingly.
Example: In ethylene (C2H4), there are 5 sigma and 1 pi bond.
3. Which is stronger, sigma or pi bond?
Sigma bonds are generally stronger than pi bonds, as the head-on overlap of orbitals is greater than the sideways overlap.
- Greater overlap leads to higher bond strength.
- This results in higher stability for sigma bonds compared to pi bonds.
4. What are real-life examples of sigma and pi bonds in molecules?
Common molecular examples include:
- H2 molecule: 1 sigma bond
- O2 molecule: 1 sigma and 1 pi bond (double bond)
- N2 molecule: 1 sigma and 2 pi bonds (triple bond)
- Ethylene (C2H4): 1 sigma and 1 pi bond between carbons
5. How do sigma and pi bonds affect the rotation of molecules?
Sigma bonds allow free rotation around the bond axis, but pi bonds restrict this rotation.
- Rotation around a pi bond breaks the sideways overlap, which is not energetically favorable.
- This property explains why double bonds are rigid and do not rotate easily.
6. Can a molecule have a pi bond without a sigma bond?
No, a molecule cannot have a pi bond without a sigma bond.
- Pi bonds always accompany an existing sigma bond between the same two atoms.
7. Why do pi bonds restrict rotation while sigma bonds allow rotation?
Pi bonds restrict rotation because rotation would disrupt the parallel orientation of p orbitals, breaking the sideways overlap.
- Sigma bonds maintain their orbital overlap during rotation, permitting free movement.
8. How do you identify sigma and pi bonds in a Lewis structure?
To identify sigma and pi bonds in a Lewis structure:
- Each single line between atoms = 1 sigma bond.
- Additional lines (in double/triple bonds) = pi bonds.
- E.g., a double bond consists of 1 sigma (first line) and 1 pi (second line).
Count lines accordingly for each bond type.
9. How do hybrid orbitals relate to sigma and pi bonds?
Sigma bonds often form from hybrid orbitals (such as sp, sp2, or sp3), allowing maximum head-on overlap.
- Pi bonds form from unhybridized p orbitals with sidewise overlap.
10. Why is it important to distinguish between sigma and pi bonds in organic reactions?
Understanding sigma and pi bonds helps predict reactivity in organic chemistry:
- Pi bonds are more reactive due to weaker overlap and play a key role in reactions such as addition to alkenes and alkynes.
- Recognizing both types guides mechanism analysis and product prediction.
11. Does the presence of pi bonds affect molecular stability?
Yes, pi bonds increase molecular reactivity and reduce bond length, making the molecule less stable compared to those with only sigma bonds.
- Sigma bonds contribute more to molecular stability.
12. What is the difference in bond strength and bond length between sigma and pi bonds?
Sigma bonds are stronger and have longer bond lengths than pi bonds.
- Bond strength order: sigma > pi
- Bond length: Addition of pi bonds (as in double/triple bonds) decreases overall bond length between atoms.
