Understanding Valence Bond and Molecular Orbital Models: Contingency at Work

Abstract

A historical reconstruction of the origin of the covalent bond shows that the idea of sharing electrons (covalent bond) posed considerable constraints for the scientists. When G.N. Lewis first introduced the idea of sharing electrons in 1916 based on his cubic atom, it was completely out of tune with established belief and was considered as bizarre and absurd. Pauli’s exclusion principle was the next step in understanding the covalent bond. Later in the 1930s, these ideas were developed further by L. Pauling, in his valence bond (VB) model. Just at about the same time, a rival model (molecular orbital (MO)) was developed by R. Mulliken. Both VB and MO models have drawn upon quantum mechanics and have been rivals in their explanation of the chemical bond. However, the VB model emphasizes the pictorial aspects of the model and helps visualization, whereas the MO model is more mathematical and complex. According to the contingency thesis, both models have theoretical and empirical backing and provide alternative interpretations (Gavroglu and Simoes, Neither physics nor chemistry: A history of quantum chemistry. Cambridge, MA: Massachusetts Institute of Technology Press; 2012). Even today, the rivalry between the two models continues (Hoffmann et al., Accounts of Chemical Research, 36(10), 750–756; 2003). The objective of this study is to evaluate the degree to which general chemistry textbooks recognize the importance of the contingency thesis in the development of the two models. Results obtained show that a majority of the textbooks evaluated do present both models. However, the rivalry between the two models is ignored. This study shows that there is much to learn about how atoms combine in molecules and that different theories can be used to explain the same concept. Lack of a definitive solution to a research question shows that different theories combine to provide the “truth,” thus providing students with an opportunity to do further research. Discussion of the VB and MO models illustrates that no theory can provide a complete and literally correct picture of the world (Giere 2006a, b). Furthermore, the transition from Lewis’s cubic atom to Pauli’s exclusion principle to the valence bond model (Pauling) to the molecular orbital model (Mulliken) to what’s next clearly shows the tentative nature of our understanding of valence.