154.

Describe, in brief, the types of covalent bonds ?

A covalent bond is classified into two types:
1.    Sigma (σ) covalent bond bond
2.    pi () covalent bond.

1. Sigma (σ) covalent bond (Axial overlap bond): A covalent bond formed by an axial (head on) overlapping of half filled atomic orbital is called sigma (σ) bond. The molecular orbital thus formed is symmetrical about the internuclear axis and is called sigma molecular orbital. Sigma bond may be formed by any one of the following types of overlapping: (i) s-s over-lapping (ii) s–p overlapping (iii) p-p overlapping half filled.
(s-s overlapping: It is the type of overlapping.


in which s-orbital of one atom overlaps with half filled s-orbital of another atom to form the s-s sigma bond. For example, in the formation of H₂ molecule, Is orbitals of two hydrogen atoms mutually overlap along the internuclear axis to form a sigma bond.

(ii) s-p overlapping: It is the type of overlapping in which half filled s-orbital of one atom overlaps with half filled p-orbital of another atom to give s-p



sigma bond. For example, in the formation of hydrogen fluoride molecule (H–F), half filled Is orbital of hydrogen atom overlaps coaxially with half-filled 2p–orbital of fluorine atom to form a sigma bond.

(iii) p-p overlapping: It is the type of overlapping in which half filled p-orbital of one atom overlaps with half filled p–orbital of another atom linearly i.e. along the axis (coaxial overlapping) to give p-p sigma bond. For example, in the formation of fluorine molecule (F – F), half-filled 2p–orbitals of two fluorine atoms mutually overlap coaxially to form a sigma bond. 

2.   A covalent bond formed by the sidewise or lateral overlapping (a non-axial overlapping) of two parallel oriented half filled p-orbitals is called  the bond. The electron density is localised above and below the plane of the bond axis.