Chemical Bonding and Molecular Structure
11.0 Sigma and Pi Bonds ($\sigma $ and $\pi $ Bonds)
11.0 Sigma and Pi Bonds ($\sigma $ and $\pi $ Bonds)
A covalent bond is formed by the overlapping of atomic orbitals. Covalent bonds formed are of two types depending upon the way the orbitals overlap each other.
$1.$ Sigma bond ($\sigma $ bond):
The bond formed by the overlapping of two half filled atomic orbitals along their axis is known as sigma bond. $\sigma $ bond is a strong bond because overlapping in it takes place to large extent. The hybrid orbitals always from $\sigma $ bond.
- $s – s$ overlapping:
- $s – p$ overlapping:
- $p – p$ overlapping:
$2.$ Pi bond ($\pi $ bond):
The bond formed by the lateral overlapping of half filled atomic orbitals is known as pi bond. The side-wise overlapping takes place to less extent.
Therefore, $\pi $ bond formed is a weak bond. $\pi $ bond overlapping takes place only at the sides of two lobes. A $\pi $ bond is formed when a $\sigma$ bond already exists between the combining atoms.
Example:
- In $A - B$ molecule the bond formed is $\sigma$ bond.
- In $A\mathop = \limits_\pi ^\sigma B$, molecule there is one $\sigma$ and one $\pi$ bond.
- In $A\mathop \equiv \limits_{2\pi }^\sigma B$, molecule there is one $sigma$ and two $\pi$ bonds.
Thus, all the single bonds are $\sigma$ bonds. Double bond has one $\sigma$ and one $\pi$ bond. Triple bond has one $\sigma$ and two $\pi$ bonds.
