Atoms, Bonds, and Water: The Molecule of Life

Living things are built almost entirely from six elements -- carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur, sometimes remembered by the acronym CHNOPS -- linked mostly by covalent bonds, in which atoms share electrons rather than transferring them outright the way ionic bonds do (as in table salt, sodium chloride). Water's two covalent bonds are not shared evenly: oxygen pulls the shared electrons closer to itself than hydrogen does, giving the oxygen end of the molecule a slight negative charge and each hydrogen end a slight positive charge. A molecule with this kind of lopsided charge is called polar, and water's bent, polar shape is the single fact that makes life as we know it possible.

Because water is polar, one molecule's slightly positive hydrogen is attracted to a neighboring molecule's slightly negative oxygen, forming a hydrogen bond -- weaker than a covalent bond, but powerful in huge numbers. Hydrogen bonding produces cohesion (water molecules cling to each other, creating surface tension), adhesion (water clings to other polar surfaces, driving capillary action up a plant stem), a high specific heat (water resists rapid temperature swings, which is why large bodies of water moderate nearby climates and why your own mostly-water body maintains a stable temperature), and a solid form, ice, that is less dense than liquid water, which is why ice floats and insulates the water beneath it instead of sinking and freezing a lake solid from the bottom up. Water's polarity also makes it an excellent solvent for other polar and ionic substances, which is why it is often called the universal solvent of biology.