Sigma Bond and Pi bond

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  • 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 is called sigma bond.   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 sigma bond between two overlapping ‘s’ orbitals.

Sigma Bond and Pi Bond 01

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 pi bond
  • Example: Ethylene has one pz – pz  π bond. Acetylene has two π bonds.

Pi Bond

Formation of pi Bond:

  • A covalent bond is formed by lateral or sideway or collateral overlapping of pure orbitals is known as pi bond.
  • 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 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 pi bond is formed when sigma bond already exists. Formation of only 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. A pi bond is formed by collateral or sidewise overlapping of atomic orbitals.
  • Due to large overlap of atomic orbitals sigma bond involves more evolution of energy than Pi bond
  • In sigma bond, the electron density between two nuclei on internuclear axis is very high. In the case of a pi bond, the electron density is higher above and below the internuclear axis and not on nuclear axis.
  • Hence the sigma bond in which overlapping of orbitals takes place along the same axis is stronger than the pi bond in which overlapping of orbitals takes place at an angle (laterally).


Distinguish between sigma bond and pi bond:

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 a this bond. Only pure orbitals can form this bond.
Science > Chemistry > Nature of Chemical BondYou are Here
Physics Chemistry  Biology  Mathematics

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