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Sigma and Pi Bonds are formed as a result of overlapping orbitals of different atoms. The overlapping is a result of the incomplete orbitals of atoms. This overlapping can be head-on or side-to-side and hence results in the formation of sigma and pi bonds respectively. In this article, we will explore the sigma and pi bonds, their types, strength, and the difference between them.
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Sigma Bond
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“Sigma bonds are the result of head-on overlapping that takes place between atomic orbitals. With respect to rotation about the bond axis, it is symmetrical.”
A sigma bond is represented by the Greek letter σ. It is present in single, double, and triple covalent bonds. Atoms with incomplete s or p orbital with opposite spins overlap and form sigma bonds (the strongest bond). For example, in a covalent molecule C2H4, there are 5 sigma bonds and one pi bond. Sigma bonds are the strongest covalent bonds and they are formed as a result of head-on overlapping of orbitals. H2, HCl, HF, HBr, HI, F2, Cl2, etc. are some examples of molecules having sigma bonds. There are three types of overlapping in sigma bonds:
- s-s overlapping
- s-p overlapping
- p-p overlapping
- s-p sigma bond (s-p overlapping) : s-p sigma bond is formed when two atoms with one having half-filled s orbital and another having half-filled p orbital overlap. Examples of s-p sigma bonds include formation of HF molecule, H-X type molecules (HCl, HBr and HI). In case of HF molecule, electronic configuration of hydrogen atom is H : 1s1 and electronic configuration of fluorine is F : 1s2 2s2 2px2 2py2 2pz1 , which facilitates the formation of s-p sigma bond (provided the spin of electrons in overlapping orbitals is opposite).
- p-p sigma bond (p-p overlapping): p-p sigma bond is formed when two atoms with half-filled p orbitals overlap. The formation of fluorine (F2) molecules is an example of the p-p sigma bond. Here, the electronic configuration of fluorine is F: 1s2 2s2 2px2 2py2 2pz1. When two fluorine atoms having opposite spins approach each other, their p orbitals overlap with each other.
Pi Bond
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”Pi bonds are the result of the pairing of unbound p-orbital electrons between two atoms. Here, side by side overlapping takes place. The overlapping takes place in such a way that the axes of the atomic orbitals remain parallel to each other and perpendicular to the internuclear axis.”
A pi bond is represented by the Greek letter π. It is present in double and triple covalent bonds. Acrylonitrile (C3H3N), Ethylene (C2H4), etc. are examples of molecules having pi bonds. Atoms with incomplete p or d orbitals overlap and form pi bonds.
Types of Pi Bonds
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There is only one type of pi covalent bond formed due to side-wise overlapping of p orbitals. However, their formation can be represented in two ways :
Check out notes CBSE Class 11 Chemical Bonding and Molecular Structure.
Strength of Sigma and Pi Bonds
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The strength of any type of bond depends on the extent of overlapping of orbitals.
- For sigma bonds, the overlapping is to a greater extent.
- For pi bonds, the overlapping is to a comparatively smaller extent.
Thus, the sigma bonds have more strength than the pi bonds. When multiple bonds are formed, in addition to the sigma bonds the pi bonds are also formed.
Difference between Sigma Bond and Pi Bond
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| Sigma Bond | Pi Bond |
|---|---|
| Sigma bonds are formed as a result of head-on-head overlapping of atomic orbitals. | Pi bonds are formed as a result of side-by-side overlapping of atomic orbitals. |
| Sigma bonds can independently exist. | Pi Bonds can not exist without a sigma bond. |
| s and p orbitals are included in the formation of sigma bonds. | p and d orbitals are included in the formation of pi bonds. |
| They have high energies and are stronger. | They are weak compared to the sigma bond. |
| The orbitals that overlap in a sigma bond can either be hybrid or pure. | The orbitals that overlap in a pi bond must be hybridized. |
| Sigma bonds play a role in affecting the shapes of molecules. | Pi Bonds play no role in determining a molecule’s shape. |
Sigma and Pi Bonds
Things to Remember
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- Sigma bonds are formed by head-on overlapping of atomic orbitals.
- Sigma bonds are the strongest covalent bond.
- Pi bonds are formed by side by side overlapping of atomic orbitals.
- Usually p orbital takes part in pi-bond formation.
- Number of sigma and pi bonds in a molecule:
- In the case of a single bond, there is only one sigma bond.
- In the case of a double bond, there is one sigma bond and one pi bond.
- In the case of a triple bond, there is one sigma bond and two pi bonds.
Sample Questions
Ques. How many sigma and pi bonds are there in the benzene (C6H6) ring? (2 marks)
Ans.
Benzene Ring
In a benzene ring, there are 6 C-H single bonds and 6 C-C bonds out of which 3 are single bonds and 3 are double bonds. Hence there are 9 single bonds = 9 sigma bonds and 3 double bonds = 3 sigma + 3 pi bonds. So, the total number of sigma bonds in benzene = 6 + 9 = 12 and the total number of pi bonds in benzene = 3.
Ques. How many pi bonds are present in NH2OH? (2 marks)
Ans.
Structure of NH2OH Molecule
There are two N-H single bonds, one N-O single bond and one O-H single bond, which means there are no pi bonds in NH2OH, there are only sigma bonds in NH2OH. And the number of sigma bonds in NH2OH is 2 + 1 + 1 + = 4.
Ques. How many sigma and pi bonds are present in single, double, and triple covalent bonds? (2 marks)
Ans. There is one sigma bond in single, double and triple covalent bonds, while in single covalent bond there is no pi bond, in double covalent bond there is one pi bond and in triple covalent bond there are two pi bonds.
Ques. Draw diagrams demonstrating the formation of a double bond and a triple bond between carbon atoms in C2H2 and C2H4 molecules. (2 marks)
Ans.
C2H2 Molecule
C2H4 Molecule
Ques. Write the difference between sigma and pi bonds. (5 marks)
Ans. Difference between sigma and pi bond:
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- Formation:
- Sigma bonds are formed as a result of head-on-head overlapping of atomic orbitals.
- Pi bonds are formed as a result of side-by-side overlapping of atomic orbitals.
- Strength:
- Sigma bonds are stronger than pi bonds.
- Pi bonds are comparatively weaker.
- Formation:
- Orbitals Included:
- s and p orbitals are included in the formation of sigma bonds.
- p and d orbitals are included in the formation of pi bonds.
- Pi bonds are present only in double and triple covalent bonds, while sigma bonds can be seen in single, double as well as triple covalent bonds.
- Pi bonds don’t allow the free rotation of atoms, whereas there is no restriction in case of sigma bonds.
- Unsaturated molecules are formed due to pi bonds.
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