molecular orbital theory, detailed explanation of how electrons are distributed in stable molecules. In the simpler valence theory of the chemical bond, each atom in a molecule is assumed to retain its own electrons. Even when electrons are shared, as in the covalent bond, it is possible to identify which electron came from which atom. The molecular orbital theory, however, treats each electron as associated with the molecule as a whole. Just as a free atom has certain allowed electron orbits and energies, each molecule has its own allowed molecular orbitals. The orbitals give the probability of finding the electron at any point in space. Each orbital can hold a maximum of two valence electrons and the structure of the molecule is built up by filling the lowest energy orbitals first. The calculations involved are extremely complex, and only the simplest molecules can be treated exactly.