Sigma bonds and Pi Bonds

Science > Chemistry > Physical Chemistry > Nature of Chemical Bond > Sigma and Pi Bonds

According to Valence Bond Theory, covalent bonds are of two types a) Sigma bond (σ) and b) Pi bond (π)

Sigma bond (σ):

The covalent bond formed as a result of an end to end overlap i.e. head-on collision of atomic orbitals Or A covalent bond formed by collinear or coaxial i.e. in a line of internuclear axis overlapping of an atomic orbital is known as a sigma bond. Example: In hydrogen molecule, there is a sigma bond between two overlapping ‘s’ orbitals.

Formation of Sigma Bond:

A covalent bond formed by collinear or coaxial i.e. in a line of internuclear axis overlapping of an atomic orbital is known as a sigma bond. S orbitals are non-directional hence they can overlap in any side. Thus s-s overlap always forms a sigma bond. In order to form sigma bond p orbitals must lie along the internuclear axis. They are formed by s-s overlap e.g. H-H, s-p overlap e.g. H-F, p-p overlap e.g. F-F

Characteristics of Sigma Bond:

• It is a covalent bond formed by a coaxial overlap of bonding orbitals.
• It is a very strong bond, due to a greater extent of overlapping.
• It is possible between any two orbitals s-s, p-p or s-p and also hybrid orbitals.
• Bond energy is more.

Pi Bond (π):

A covalent bond is formed by lateral or sideway or collateral overlapping of pure orbitals is known as a pi bond. Example: Ethylene has one pz – pz  π bond. Acetylene has two π bonds.

Formation of pi Bond:

A covalent bond is formed by lateral or sideway or collateral overlapping of pure orbitals is known as a pi bond. A pi bond is formed by a lateral overlap of two p orbitals oriented mutually parallel but perpendicular to the internuclear axis.

Characteristics of Pi Bond:

• It is a covalent bond formed by a lateral or sideways overlap of atomic orbitals.
• It is a weak bond due to the lower extent of overlapping.
• It is possible between two ‘p’ orbitals only.
• Bond energy is less.
• It is to be noted that the pi bond is formed when the sigma bond already exists. The formation of only a pi bond is not possible.

Sigma Bond is Stronger Than a pi Bond:

The extent of overlapping of orbitals along the same axes is always greater than the extent of overlapping at an angle. Also, the larger the overlapping of orbitals, the stronger is the covalent bond.

A sigma bond is formed by co-axial overlapping of atomic orbitals. Due to the large overlap of atomic orbitals, the bond involves more evolution of energy than Pi bond. Due to the large overlap of atomic orbitals, the bond involves more evolution of energy than Pi bond. In this bond, the electron density between two nuclei on the internuclear axis is very high.

A pi bond is formed by collateral or sidewise overlapping of atomic orbitals. Due to the less overlap of atomic orbitals, the bond involves less evolution of energy than the sigma bond. In the case of a pi bond, the electron density is higher above and below the internuclear axis and not on the nuclear axis.

Hence the sigma bond in which overlapping of orbitals take place along the same axis is stronger than the pi bond in which overlapping of orbitals takes place at an angle (laterally).

Sr. No.	Sigma Bond	Pi Bond
1	A covalent bond formed by collinear or coaxial i.e. in a line of internuclear axis overlapping of an atomic orbital is known as a sigma bond.	The bond is formed by a lateral overlap of two p orbitals oriented mutually parallel but perpendicular to the internuclear axis is called the pi bond.
2	It is stronger as overlapping takes place to a greater extent.	It is a weak bond because very little overlapping takes place.
3	Bond energy is more.	Bond energy is less
4	It results in high electron density between two nuclei on internuclear axis.	It results in high electron density above and below the internuclear axis and not on nuclear axis.
5	The bond is rotationally symmetrical about the internuclear axis.	The bond is not rotationally symmetrical about the internuclear axis.
6	it can be formed between any two orbitals i.e. s-s, s-p or p-p etc.	It can be formed only between ‘p’ orbitals.
7	It determines the direction of the bond, internuclear distance, and shape of the molecule.	It does not affect the direction of the bond, internuclear distance, and shape of the molecule.
8	Pure and hybrid orbitals can form this type of bond.	Only pure orbitals can form this type of bond.

Science > Chemistry > Physical Chemistry > Nature of Chemical Bond > Sigma bonds and Pi Bonds