Polyatomic molecules valence-bond description

This shows no unpaired electrons, so it predicts that O2 is diamagnetic. Experiments show, however, that O2 is paramagnetic therefore, it must have unpaired electrons. Thus, the valence bond description is not consistent with experimental fact, so it cannot be accepted as a description of the bonding in O2. Molecular orbital theory accounts for the fact that O2 has two unpaired electrons. This ability of MO theory to explain the paramagnetism of O2 helped to establish it as a major theory of bonding. We will develop some of the ideas of MO theory and apply them to some molecules and polyatomic ions. 

Actually, the recent studies of transition-group spectra have shown that two different models cherished by many theorists— the electrostatic ligand field model and the valency-bond description (more specifically, the Pauling hybridization theory)—cannot be applied to the observed distribution of energy levels. On the other hand, the molecular orbital (M.O.) configurations give an excellent classification of all energy levels of all complexes and a unified description is obtained of all polyatomic molecules. 

Section 21.5 The Valence-Bond Description of Polyatomic Molecules... 

We learned in Chapter 4 that an isolated atom has its electrons arranged in orbitals in the way that leads to the lowest total energy for the atom. Usually, however, these pure atomic orbitals do not have the correct energies or orientations to describe where the electrons are when an atom is bonded to other atoms. When other atoms are nearby as in a molecule or ion, an atom can combine its valence shell orbitals to form a new set of orbitals that is at a lower total energy in the presence of the other atoms than the pure atomic orbitals would be. This process is called hybridization, and the new orbitals that are formed are called hybrid orbitals. These hybrid orbitals can overlap with orbitals on other atoms to share electrons and form bonds. Such hybrid orbitals usually give an improved description of the experimentally observed geometry of the molecule or ion. In fact, the concept of hybrid orbitals was developed specifically to explain the geometries of polyatomic ions and molecules. 

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