Page 119 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:37 9in x 6in b985-ch10 Elementary Physical Chemistry

Chapter 10

Quantum Theory. The Chemical Bond

Concepts developed in previous chapters, especially those involving orbitals,
can be extended to describe electronic structures of molecules.
There are two quantum mechanical theories of electronic structure of
molecules: Valence Bond Theory and Molecular Orbital Theory.

1. Valence Bond Theory. The starting point is the concept of shared
electron pairs. The theory introduces the concepts of σ and π bonds,
promotion, hybridization.
2. Molecular Orbital Theory. The idea is that there are orbitals (wave-
functions) that spread over all atoms in the molecule. As in atoms,
the Aufbau Principle is used to determine the electron configuration of
molecules.

Preceding the quantum mechanical description of molecular structures,
it is well to mention treatments based on ideas introduced by G.N. Lewis
in 1916. Among these was the Octet Rule, stating that an atom in a
molecule likes to be surrounded by 8 electrons, normally denoted by dots.
But there are exceptions to the octet rule — sometimes there are more
than 8 electrons, sometimes less. Furthermore, the original theory could
not account for the shape of molecules, but when coupled with VSEPR
(Valence Shell Electron Pair Repulsion), the theory is remarkably successful
in predicting shape.
Lewis already recognized before the advent of quantum theory that
there are two major types of bonds: covalent bonds (in which electrons are
shared) and ionic bonds (in which cohesion is due to electrostatic attraction
between positive and negative ions).

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