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).