In valence bond theory, how is a sigma bond formed?

A sigma bond is formed when there is head-on or end-to-end overlap of atomic orbitals, which results in a bond that has cylindrical symmetry about the bond axis. This type of orbital overlap allows for the electron density to be concentrated along the axis connecting the two bonded nuclei, leading to a strong and stable bond.

In contrast to other types of bonds such as pi bonds, which arise from side-by-side overlapping of p orbitals, sigma bonds can be formed by a variety of orbitals, including s, p, and hybrid orbitals. While hybrid orbitals do contribute to sigma bond formation by providing orbitals with optimal shape and orientation, the fundamental characteristic of a sigma bond is the cylindrical symmetry about the bond axis resulting from the overlap of orbitals.

Understanding this concept is crucial, as it helps to explain the geometric and electronic properties of molecules, making it clear why the formation and orientation of these bonds play a significant role in molecular structure and behavior.