. Elementary biophysics: selected topics . Fig. 15. A sketch of the water molecule and its electrically equivalent dipole. ing nucleic acid solutions; this effect is, however, primarily associated with increase of the repulsion due to the negatively charged phosphate groups. A second aspect derives from the fact that the water molecules are themselves dipoles; their positive and negative charges are permanently separated from each other, so that the molecule as a whole is electrically neutral, but still can effect electrical interactions. Thus water can act to split molecules bonded appreciabl
. Elementary biophysics: selected topics . Fig. 15. A sketch of the water molecule and its electrically equivalent dipole. ing nucleic acid solutions; this effect is, however, primarily associated with increase of the repulsion due to the negatively charged phosphate groups. A second aspect derives from the fact that the water molecules are themselves dipoles; their positive and negative charges are permanently separated from each other, so that the molecule as a whole is electrically neutral, but still can effect electrical interactions. Thus water can act to split molecules bonded appreciably by electric forces. As an example, salt (sodium chloride) is found to be entirely separated into constituent ions when salt is dissolved in water. Some investigations have yielded results suggesting that, in water, the hydrogen bond strength may be less than lOOOcal/mol. In any event, the strengths of ionic and hydrogen bonds, due to electric forces, are subject to important modifications when the molecules are dissolved in water. BONDS AND ENERGY EXCHANGE Given the existence of the various chemical bonds, it remains to ex- plain the energy exchanges involved in chemical reactions. In order to stick two atoms together in, say, a covalent bonding, we have to bring them close together. As long as they are many atomic diameters apart, there is no force acting between the atoms, but as they approach each other, the outer electrons of each exert a repulsive force on the other, so that we have to do work to push them still closer together. It is not until we have pushed them so close that an electron from one atom experiences the attractive force of the nucleus of the other atom that the electron swap which makes the two atoms exert a net attraction on each other can take place. It turns out that the net attraction is not very great, but as long as it exists, it holds the atoms together. If we now supply the energy to move the electron back where it was in the first place, the repu
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