Sigma and Pi Bonds

Sigma (σ) bond. When two atomic orbitals overlap LINEARLY, along the axis joining the two nuclei, the resulting bond is a sigma bond -- also called 'head-on overlap' or 'axial overlap'. Any overlap that involves an s orbital (i.e. s-s or s-p overlap) ALWAYS results in a sigma bond, because the s orbital is spherically symmetric and so overlaps head-on no matter the direction of approach. Overlap between two p orbitals ALONG the molecular axis also gives a sigma bond -- for instance, if the x-axis is taken as the molecular axis, then a pₓ-pₓ overlap results in a σ bond.

Pi (π) bond. When two atomic orbitals overlap SIDEWAYS (laterally) -- their lobes lying parallel to each other but perpendicular to the internuclear axis -- the resulting covalent bond is a pi bond. Taking the x-axis as the molecular axis, p_y-p_y and p_z-p_z overlaps (both perpendicular to x) result in π-bond formation. …