CHEMICAL BONDS 29 







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Na 



Fig. 10. A two-dimensional illustration of the interaction of the central Na + 

 atom with its Cl~ neighbors. Most of the 180kcal/mol binding energy comes 

 from interactions with the four nearest CI - neighbors; each of these bonds is 

 shown to be about 30 kcal/mol, so that these four bonds add up to 120 kcal/ 

 mol. The interactions with the eight next nearest CI - neighbors are much 

 weaker because the separations are much greater. In this illustration, each bond 

 is about 8 kcal/mol, so that these eight bonds total to 64 kcal/mol. In prac- 

 tice, the situation is really three-dimensional, and the repulsion of the nearest 

 Na + atoms must also be taken into account. 



where E el is the electrical energy of the two charges of magnitudes q^ 

 and q 2 separated by a distance r. 



The value of r can be deduced in several ways (including x-ray 

 analysis) and the calculated mutual energy turns out to be 120 kcal/mol 

 (Fig. 10). The experimental binding energy is 180 kcal/mol. Thus we 

 see that the sodium chloride binding may be understood fairly well on 

 the basis of what is called a simple ionic bond between the two con- 

 stituent atoms. Electrical attraction of the ions with still other neighbor- 

 ing ions accounts for the other 60 kcal/mol. 



Is it possible to explain all chemical bonds in these ionic terms? As we 

 have remarked above, if the electrical forces didn't exist, there would be 

 no atoms and no chemistry. But the simple picture of atoms giving or 

 receiving electrons and thereby being electrically bonded is too simple. 

 Consider the hydrogen molecule. It is composed of two originally neutral 

 hydrogen atoms. There is no reason to suspect that one of the atoms has 

 given its electron to the other. Perhaps it is reasonable to propose a 

 model in which the two atoms share each other's electron. The state of 

 atoms having two electrons had been found to be one of the stable states 

 described above. So, by sharing each other's electron, both hydrogen 

 atoms can attain this stable state. It is necessary to use quantum physics 



