4. Electron Sharing and      Covalent Bonds  
     Oxygen and Water

The attractions between water molecules are more than just the attractions of one tiny dipole for another. Each hydrogen atom with its partial positive charge is attracted to one of the lone pairs on an oxygen of a neighbor molecule, by a weak ionic attraction known as a hydrogen bond (right). Hydrogen bonds can be formed whenever a hydrogen atom with a partial positive charge is near a small N, O, or F atom carrying an excess of negative charge. Such bonds, although weak, are important in holding molecules such as proteins together because there are so many of them. We will discuss the occurrence of hydrogen bonds in living organisms in Chapter 22. For example, the most central process in all living organisms, the coding of genetic information in molecules of DNA, also depends on hydrogen bonds for the preservation of the message.

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