A sigma bond is a type of covalent bond formed by the axial overlapping of two half filled atomic orbitals. The atomic orbitals undergoing sigma bond formation overlap along the internuclear axis and involve head on head overlap. The electron cloud is thus cylindrically symmetrical about the internuclear axis.
Three types of these axial overlaps are possible. These are between the s and the pz orbitals and are as under. s-s overlap: It involves the overlap of two half filled s-orbitals approaching towards each other to form a bond. The bond so formed is called as s-s sigma bond. s-p overlap: It involves the mutual overlap of half filled s-orbital of one atom with half filled pz -orbital of another atom. The bond so formed is called as s-p sigma bond. p-p overlap: It involves the mutual overlap of half filled pz -orbital of one atom with half filled pz -orbital of another atom. The bond so formed is called as p-p sigma bond. A pi (π) bond is formed by sideways overlapping as the axes of the orbitals remain parallel to each other, and perpendicular to the inter-nuclear axis. A px- px and a py- py bond are pi (π) bond.
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