Here's why:

* Quantum Mechanical Model: This model describes electrons as wave-like particles that occupy specific energy levels (orbitals) around the nucleus. It uses mathematical equations to predict the behavior and probabilities of finding an electron in a certain region of space.

* Molecular Orbital Theory: This theory extends the quantum mechanical model to molecules. It combines atomic orbitals from individual atoms to create new, delocalized molecular orbitals that span the entire molecule. The electrons in these molecular orbitals determine the bonding and properties of the molecule.

In essence, molecular orbital theory uses the principles of quantum mechanics to explain how atomic orbitals combine to form molecular orbitals and ultimately determine the structure and behavior of molecules.