4.5.4 Types of Overlapping and Nature of Covalent Bonds The covalent bond may be classified into two types depending upon the types of overlapping: (i) Sigma( σ ) bond, and (ii) pi( π ) bond (i) Sigma(σσ σσ σ) bond : This type of covalent bond is formed by the end to end (head-on) overlap of bonding orbitals along the internuclear axis. This is called as head on overlap or axial overlap. This can be formed by any one of the following types of combinations of atomic orbitals. • s-s overlapping : In this case, there is overlap of two half filled s-orbitals along the internuclear axis as shown below: