What a Bonding Theory Should Do

The hydrogen molecule is a simple model for discussing bonding theories. Imagine bringing together two H atoms that are initially very far apart. When the H atoms are infinitely far apart, the two H atoms do not interact with each other, and by convention the net energy of interaction between the H atoms is zero. As the two H atoms approach each other, three types of interactions occur (1) each electron is attracted to the other nucleus (illustrated by a red dashed line in Fig. 11-1) (2) the electrons repel each other (illustrated by a blue dashed line in Fig. 11-1) and (3) the two nuclei repel each other (illustrated by a black dashed line in Fig. 11-1). 

A FIGURE 11-1 Type of interactions between two hydrogen atoms 

The types of interactions that occur as two hydrogen atoms, infinitely separated, approach each other. The dashed lines represent the types of interactions (red for attractive interactions and blue and black for repulsive interactions). The solid black line represents the internuclear distance, r, between the two hydrogen atoms. 

Energy of interaction of two hydrogen atoms plotted for internuclear separations from zero to infinity 

However, neither method yields quantitative information about bond energies and bond lengths, and the Lewis theory has problems with odd-electron species and situations in which it is not possible to represent a molecule through a single structure (resonance). 

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