CoachA covalent bond shares an electron pair between two atoms and costs to break: the skeletons of molecules are covalent, and only enzymes make or break them in the cell. Non-covalent bonds are the weak, reversible attractions that hold molecules to each other and shape them: the hydrogen bond ( in water); the ionic bond between opposite charges ( or so in water, whose dielectric constant screens charges eightyfold); van der Waals attractions between any two atoms in close contact ( each, but summed over a whole surface); and the hydrophobic effect, which is not a bond but the exclusion of non-polar surfaces by water.
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Example 8.6 (Melting a double helix)
The two strands of DNA are held by two or three hydrogen bonds per base pair plus the stacking (van der Waals) of neighbouring pairs; a single pair would separate at once, but a thousand pairs in a row hold until the temperature reaches about , and reunite, in exactly the same register, when it is lowered (Chapter 11). Specific, strong in number, reversible.