Final answer:
Option D: The sigma (σ) molecular orbital describes a buildup of electron density along the axis between two atomic nuclei, characteristic of covalent single bonds.
Step-by-step explanation:
The type of molecular orbital used to describe a buildup of electron density along the axis connecting two atomic nuclei to form a bond is a sigma (σ) orbital. This bonding molecular orbital is symmetrical about the internuclear axis, meaning that all cross-sections perpendicular to that axis are circular. The electron density in a sigma bond is concentrated along the internuclear axis and directly between the nuclei, which makes it central to the formation of covalent single bonds. Such orbitals do not contain nodes perpendicular to the internuclear axis, unlike their antibonding counterparts, the sigma star (σ*) orbitals, which contain at least one node where electron probability is zero.
A sigma (σ) orbital is used to describe a buildup of electron density along the axis connecting two atomic nuclei to form a bond. A sigma orbital is a bonding molecular orbital with cylindrical symmetry along the internuclear axis and between the nuclei. It is symmetrical and has no nodes perpendicular to the internuclear axis, making it an effective orbital for creating a strong bond between atoms.