Final Answer:
The term used to describe the increased electron density along the internuclear axis is 1) Sigma bond.
Step-by-step explanation:
A sigma (σ) bond is formed when two atomic orbitals overlap along the internuclear axis, resulting in an increased electron density between the nuclei. In molecular orbital theory, the combination of atomic orbitals gives rise to molecular orbitals. When two s orbitals overlap head-on, a sigma bond is formed. The sigma bond is denoted by the symbol σ. It's essential to understand that sigma bonds can also be formed by the overlap of different types of orbitals, such as the overlap between an s orbital and a p orbital or between two p orbitals.
The formation of a sigma bond involves constructive interference, where the wave functions of the overlapping atomic orbitals add up, leading to an increased electron density along the internuclear axis. The mathematical representation of a sigma bond involves the addition of wave functions, and the resulting molecular orbital is lower in energy compared to the original atomic orbitals.
In simple terms, the sigma bond is a strong and stable bond that plays a fundamental role in the structure of molecules. Its formation is a crucial step in building molecular structures, and it contributes significantly to the overall stability and properties of compounds. Understanding sigma bonds is essential in comprehending the basics of chemical bonding and molecular structure.