Oo CH;see:^4
of N
CH;
CaffeineOne sees that it, too, is simply a combination of rings of carbon and
nitrogen atoms, together with a couple of oxygen atoms, but witha
distinctly different shape compared with the other molecules drawn
above. Again, these differences in shape determine the different func-
tions these molecules have in living organisms.
Returning to water, a key reason that water is so important in living
systems is that it is so very effective at dissolving things. Why is this
so? The answer lies in a property of water known as polarity. In the
water molecule (H20), the hydrogen atoms, being from the left side of
the periodic table, are prone to giving up their electron and becoming
positively charged. The oxygen, being from the right side of the peri-
odic table, is prone to picking up electrons and becoming negatively
charged. The result of these tendencies is that when the electrons
forming the covalent bonds in water are distributed in the molecular
orbitals describing the bonds, they essentially spend more time in the
vicinity of the oxygen and less time in the vicinity of the hydrogens.
So the oxygen atom in water becomes slightly negatively charged and
the hydrogen atoms in water become slightly positively charged. This
is what is meant by polarity. Polar means separation, and in this case
there is a separation of charge between different parts of the water
molecule. Water is a polar molecule.