CHEMICAL BONDS 27 



and each other. Stones from Magnesia, however, have no effect. The 

 mutual gravitational forces of the amber and fur are inappreciable. 

 Therefore, there is a third force in Nature, called the electrical force. 



Except for the nuclear force (which does not concern us here), these 

 are the only forces now known in Nature, and the explanations of natural 

 phenomena must necessarily involve these three forces in various com- 

 binations. With them, science has explained the consequences of a ball's 

 hitting a bat, light being absorbed by leaves, the trajectories of stars, and 

 the properties of molecules interacting with each other. 



For completeness, it is worth mentioning that Einstein showed that the 

 electric and magnetic forces are really only two special aspects of a 

 single, unified, electromagnetic force. 



There are other situations in which we speak of forces acting. For 

 example, when particles move through a solution, or an airplane moves 

 through the air, we speak of the drag or viscous forces acting on the 

 particle or airplane. In fact, on the molecular level there is no such thing 

 as a viscous force. There is something properly called viscous energy, 

 perhaps, but even this terminology would be unacceptable to some 

 scientists. 



When a particle tries to move through a solution, it meets a resistance 

 deriving from the fact that in the liquid state the solvent molecules 

 attract each other strongly (generally by the van der Waals "force" to 

 be explained later). Thus it takes work for the particle to move between 

 the solvent molecules, since it must separate these molecules. Therefore, 

 when the particle has pushed the solvent molecules apart, and then 

 moves farther on, the molecules come back together with sufficient 

 energy (imparted by the particle and thus lost by that particle) that 

 they bounce off each other. This oscillational energy is communicated 

 by collision to the other molecules of the solvent, with the final result 

 that the average oscillation (thermal vibrational) energy is increased 

 somewhat. In this fashion, the energy required to move a particle through 

 the solvent produces heat, which our usual manner of speaking ascribes 

 to the work done against frictional forces. 



Similar explanations are available for other apparent forces. 



CHEMICAL BONDS 



The accent on forces lasted more than 200 years, until quantum 

 physics shifted the emphasis to energies. In the meantime, chemists had 

 become increasingly occupied with energies, as thermodynamics developed 

 and was able to make assertions about energies involved in reactions 

 while saying nothing directly about forces. In this spirit, the chemists 

 talked about the energies of the bonds holding atoms and molecules to- 



