the properties of some molecules. Molecular orbital theory, which is also based on
quantum mechanics and was introduced in the late 1920s by Robert Mulliken,
Friedrich Hund, John Slater, and John Lennard-Jones, has proved to be the most
successful theory of the chemical bond: it overcomes all the deficiencies of Lewis’s
theory and is easier to use in calculations than valence-bond theory.
4.8 The Limitations of Lewis’s Theory
In Lewis’s approach and valence-bond theory, the oxygen molecule is somewhat of
a puzzle. To appreciate the puzzle, we need to know that substances can be classified
according to their behavior in a magnetic field: a diamagnetic substance is one that
tends to move out of a magnetic field, and a paramagnetic substance is one that tends
to move into a magnetic field. Diamagnetism indicates that all the electrons in a
molecule are paired; paramagnetism indicates that a molecule possesses unpaired
electrons (BOX 4.2). According to both Lewis’s approach and valence-bond theory,
O2 should be diamagnetic. In fact, it is found to be paramagnetic (FIG. 4.24).