Polarity

The phenomenon of polarity helps explain many of the properties of various molecules in living systems. Whole molecules can be polar as a result of the polarity of bonds within them. Two examples are the water molecule (below left) and the ammonia molecule (below right). In water, two hydrogen molecules are bonded covalently to one oxygen atom, but the shared electrons are pulled closer to the oxygen because of its higher electronegativity. The charge distribution is asymmetrical, with two partial negative charges concentrated at the oxygen end of the molecule, and a partial positive at each hydrogen end. As a result, the whole molecule is polar. Ammonia, NH₃, is similar: nitrogen has a higher electronegativity than hydrogen and pulls electrons closer to itself, resulting in a concentration of negative charge at the nitrogen end and positive charge at the hydrogen ends. The whole molecule is therefore polar.

Polar molecules: water (left) and ammonia (right)