The Science of Existence (68-2) Polarity


When atoms with different electronegativity join by covalent bonding, electrons may not be equally shared: electrons are pulled more toward one atom than another. When this happens, the force of alignment causes one end of a molecule to have an overall negative charge, leaving the other end to assume a positive charge. A molecule with such an unequal charge distribution is polar.

Polarity means that a molecule has positive and negative poles; in other words, the molecule possesses an electric dipole moment. Polarity underlies several physical properties, including surface tension, solubility, and critical thermodynamic points (melting and boiling). Water (H2O) is the poster child of polar molecules.

Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. A polar molecule with multiple polar bonds must have an asymmetric geometry so that the bond dipoles do not cancel each other.

Nonpolar molecules share electrons equally, rendering them electrically neutral. Oxygen (O2) and methane (CH4) are nonpolar, as are lipids: fat not being so easily excited makes it a relatively contented energy storage medium.