Types Of Covalent Bond:

There are two types of covalent bond.

1.     Sigma Bond

2.     Pi-Bond

Let’s discuss the difference between sigma and pi-bond.      

Sigma bond

Pi bond

·         A molecular orbital that is symmetrical about the line joining two nuclei is called a sigma molecular orbital and the linkage established is called a sigma bond. ·         A bond that is formed by the head-to-head overlap of atomic orbital between two nuclei is called a sigma bond. ·         Probability of finding an electron is maximum. ·         A straight line joining two nuclei of a bonded atom is called the bond axis. ·         Max and Min no. of sigma bond between two atoms in a molecule is 1. ·         It is formed by overlapping of S.S, S.P, P.P, and P.d atomic orbital. ·         Reactivity of compounds is inversely proportional to the no. of sigma bonds. ·         It determines the shape and geometry of a molecule. ·         In the formation of HF, the half-filled 1s orbital of the H-atom overlaps with the half-filled 2px orbital of the F-atom and this allows the pairing of an electron to form a sigma bond. ·         Sigma molecular orbital obtained when ++ or +- portions of two atomic orbital overlap with each other along the inter-nuclear axis. ++ overlap gives a sigma bonding molecule while +- overlap gives a sigma antibonding molecular orbital. ·         Sigma bonding as well as sigma anti-molecular orbitals has cylindrical symmetry around the inter-nuclear axis. It is for this reason that these molecular orbitals are called sigma molecular orbitals. ·         Sigma pi-x and sigma*x molecular orbital do not have the same energy or they are not degenerate. ·         Sigma molecular orbital can be rotated freely about the bond axis. ·         Sigma molecular orbital Is a molecular orbital since it results from the greater overlap of orbitals along the nuclear axis. ·         The formation of the sigma molecular orbital decides the function and length of the bond. ·         It is a strong bond.

A pi bond is formed by side-wise overlapping of two half-filled, parallel p-orbital in which the region of electron density is above and below the inter-nuclear axis is called pi-bond.            It is a weak bond.            It is formed after the formation of the sigma bond.            The minimum no. of the pi bond between two atoms is 0 and the maximum no. of the pi bond between two atoms is 2.            The reactivity of compounds is directly proportional to the no. of pi-bonds.            Can’t determine the shape and geometry of a molecule.            Pi molecular orbital involves an overlap of p-orbital.            The probability of finding an electron decreases in the region between the nuclei and becomes zero at the mid-point of inter-nuclear distance. Now the two nuclei will repel each other and hence no bond is formed. The middle point at which the electron density is zero is called the node point.            Pi molecular orbital contains a nodal plane containing nuclei and dividing orbital into two hales.            Pi-MO possesses a sausage-like region parallel to the plane passing through the two nuclei.            Sigma-MO is obtained when ++ or +- portions of two atomic orbital overlap with each other along a line perpendicular to the antinuclear axis. ++ Overlap gives a pi-bonding molecular orbital while +- overlap gives pi ABMO.            Pi-y and pi-z molecular orbitals have the same energy and hence are degenerate. Similarly, pi*y and pi*z are also degenerate.            Pi-MO is a weak molecular orbital since it results from the lesser overlap of orbitals along a line perpendicular to the molecular axis.            The formation of pi-MO does not decide the direction of the bond but shortens the bond distance. The formation of two pi-MO shortens the bond distance to a greater extent than the formation of one pi-molecular orbital (as in a double bond).            Pi MO is not symmetrical about the molecular axis and hence is not sigma molecular orbitals.