28 PHYSICAL FORCES AND CHEMICAL BONDS 



gether, rather than about the forces. This has turned out to be more 

 than just a verbal difference, because all the bonds have turned out to be 

 electrical (or, rather, electromagnetic) in origin, so that the study of 

 chemistry using the force concept would have led to impossibly difficult 

 models. 



The picture that we have of atoms is that of miniature solar systems 

 with a central positively charged nucleus and a number of planetary 

 electrons whose total charge is normally equal to the net positive charge 

 of the nucleus. If we consider a crystal of salt, sodium chloride, and ask 

 what holds it together, we have to know something of the conditions 

 within the crystal. If we toss some metallic sodium into water, the chem- 

 ists tell us that there will be a violent reaction, perhaps an explosion and 

 a fire. If we separately bubble some chlorine gas through water, some of 

 the gas seems to dissolve, the rest comes bubbling out. Breathing the 

 chlorine gas or drinking the chlorine water is harmful. Yet when we toss 

 some salt into water, nothing happens and no one is hurt by drinking 

 the water. Clearly, the sodium and chlorine cannot be in their native 

 states while in the salt crystal or in the dissolved state. X-ray analysis 

 of the salt crystal shows us that the centers of the sodium and chlorine 

 atoms are clearly separated from each other. The only difference in state 

 that we can imagine is that sodium and chlorine may have done some 

 swapping of their planetary electrons. Could such an apparently simple 

 swap have accomplished such profound changes in the properties of the 

 substances? The answer is a resounding yes, on purely experimental 

 grounds as sketched above. 



In following up this kind of analysis, physics and chemistry have 

 shown that certain numbers of electrons are much more stable than all 

 others. Further, it was soon noticed that atoms which naturally had one 

 of these stable numbers were quite thoroughly inactive chemically. Thus, 

 the first attempt to explain chemistry from atomic properties utilized the 

 closeness of the atoms concerned to one of these stable states. In our 

 case, sodium turns out to have one electron too many; chlorine has one 

 too few. What could be more natural than for sodium to give its un- 

 wanted electron to the chlorine, thereby transforming both atoms into 

 stable configurations? If we make this hypothesis, we see that we have 

 simultaneously created a reason for the combination of sodium and 

 chlorine. For the former now has a net plus charge and the latter a net 

 minus charge. Electrically, the two atoms are simply pulled together. 



Is it possible actually to explain the bonding by a simple electrostatic 

 interaction? The mutual energy of two charges depends in a very simple 

 way on the charges and their separation: 



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