Structures and Resonance Theory

Electron density surface for hydrogen fluoride depicts overall molecular size and shape. 

Electrostatic potential map for lithium hydride shows negatively-charged regions (in red) and positively-charged regions (in blue). 

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Langmuir s conclusion is correct but, I think, incomplete. Saying that we often choose between two models does not mean that we must, from the time of that choice forward, use only the model that we accept. Instead, we must continually make selections, consciously or subconsciously, among many complementary models. Our choice of models is usually shaped by the need to solve the problems at hand. For example, Lewis electron dot structures and resonance theory provide adequate descriptions of the structures and reactions of organic compounds for some purposes, but in other cases we need to use molecular orbital theory or valence bond theory. Frequently, therefore, we find ourselves alternating between these models. Furthermore, consciously using complementary models to think about organic chemistry reminds us that our models are only human constructs and are not windows into reality. 

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