. Elementary biophysics: selected topics. Biophysics. 32 PHYSICAL FORCES AND CHEMICAL BONDS. o Fig. 13. A particle is represented with its charges distributed so that the negative charges tend to be concentrated near one end and the positive charges near the other. The particle is thus equivalent electrically to the dipole indi- cated on the right. together than with single bonds. Indeed, the measured separation ( A) between the carbon atoms in benzene is actually intermediate between the A for a single bond and A for a double bond be- tween carbon atoms. The next kind of bond to
. Elementary biophysics: selected topics. Biophysics. 32 PHYSICAL FORCES AND CHEMICAL BONDS. o Fig. 13. A particle is represented with its charges distributed so that the negative charges tend to be concentrated near one end and the positive charges near the other. The particle is thus equivalent electrically to the dipole indi- cated on the right. together than with single bonds. Indeed, the measured separation ( A) between the carbon atoms in benzene is actually intermediate between the A for a single bond and A for a double bond be- tween carbon atoms. The next kind of bond to be discussed is that called variously the molecular energy or the van der Waals energy. This is a bond between electrically neutral atoms or molecules, or groups of atoms or molecules. Electric neutrality does not necessarily imply zero electric charge; it may also arise from the equality of plus and minus charges. If the particle we are now discussing happens to have some of the negative charges separated from an equal amount of its positive charges, it is still neutral as a whole, but forms what is called a dipole, as in Fig. 13. Here the separation of the charges is indicated schematically by the circled plus and minus signs. Such an entity, composed of plus and minus charges, is called an electric dipole, by analogy with the similar situation with magnetic poles. Now, dipoles can interact electrically, too, although the attraction of one charge is offset to quite an extent by the repulsion exerted by the other charge of the dipole. Thus dipole forces are usually quite weak. We shall not enter into a discussion of these dipole forces other than to say that there are several varieties of them, all contributing to the van der Waals energy, and all decreasing much more rapidly with particle separation than by the regular interaction described at the be- ginning of this chapter. Because these dipoles are involved in the disper- sion of light, the energy involved is also frequen
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