Which aspect makes molecular orbital theory an improvement over valence bond theory?

Molecular orbital theory represents a significant advancement over valence bond theory due to its ability to account for the behavior and properties of polyatomic molecules. While valence bond theory typically focuses on the formation of bonds between pairs of atoms, largely through the concept of overlapping atomic orbitals and hybridization, molecular orbital theory takes a more comprehensive approach.

In molecular orbital theory, atomic orbitals from all the atoms within a molecule combine to form molecular orbitals that are delocalized over the entire molecule. This allows for the description of bonding and anti-bonding interactions across multiple atoms, which is especially important in polyatomic molecules where multiple bonds and geometries can influence the properties of the substance. It effectively captures the electronic structure and characteristics of the molecule as a whole rather than just as individual pairs of atoms.

This holistic approach is particularly useful in explaining phenomena such as resonance and the behavior of electrons in complex bonding situations, which cannot be easily described using valence bond theory, where the focus on localized bonds can overlook the symmetry and the overall molecular structure that affects reactivity and stability. Thus, the capability to include and accurately describe polyatomic molecules is what makes molecular orbital theory a valuable improvement over valence bond theory.