the Chemical Bond

the Chemical Bond - he Chemical Bond With our knowledge of...

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he Chemical Bond With our knowledge of the electronic structure of atoms we are now in a position to understand the existence of molecules. Clearly, the force which binds the atoms together to form a molecule will, as in the atomic case, be the electrostatic force of attraction between the nuclei and electrons. In a molecule, however, we encounter a force of repulsion between the nuclei in addition to that between the electrons. To account for the existence of molecules we must account for the predominance of the attractive interactions. We shall give general arguments to show that this is so, first in terms of the energy of a molecule, relative to the energies of the constituent atoms, and secondly, in terms of the forces acting on the nuclei in a molecule. In order to determine what attractive and repulsive interactions are possible in a molecule, consider an instantaneous configuration of the nuclei and electrons in a hydrogen molecule (Fig. 6-1) . When the two atoms are initially far apart (the distance R is very large) the only potential interactions are the attraction of nucleus A for electron number (1) and the attraction of nucleus B for electron number (2). When R is comparable to the diameter of an atom (A and B are close enough to form a molecule) then new interactions appear. Nucleus A will now attract electron (2) as well as (1) and similarly nucleus B will attract electron (1) as well as (2). These interactions are indicated by the four solid lines in Fig. 6-1 connecting pairs of particles which attract one another. Fig. 6-1. One possible set of the instantaneous relative positions of the electrons and nuclei in an H., molecule. The dashed lines represent the repulsive interactions between like charges and the solid lines indicate the attractive interactions between opposite charges. The number of attractive interactions has been doubled from what it was when the atoms were far apart. However, the reduction in R introduces two repulsive interactions as well, indicated by the dashed lines joining charges of like sign in Fig. 6-1 . The two
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electrons now repel one another as do the two nuclei. If the two
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the Chemical Bond - he Chemical Bond With our knowledge of...

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