What is polarity?
When two atoms with the same electronegativity form a bond, the electrons are shared equally between the two atoms. However, if atoms with differing electronegativity form a bond, the electrons are not shared equally and more of the negative charge is found closer to one of the atoms. In the O2H bonds in water, oxygen is more electronegative than hydrogen, so there is a higher probability that the bonding electrons are closer to the oxygen. The difference in electronegativity between oxygen and hydrogen gives rise to a partial positive and negative charge, usually pictured asd1andd2, respectively (Figure 2.1). Bonds such as this are called polar bonds. In situations in which the electronegativity difference is quite small, such as in the C-H bond in methane (CH4), the sharing of electrons in the bond is very nearly equal, and the bond is essentially nonpolar.
The bonds in a molecule may be polar, but the molecule itself can still be nonpolar because of its geometry. Carbon dioxide is an example. The two C=O bonds are polar, but because the CO2 molecule is linear, the attraction of the oxygen for the electrons in one bond is cancelled out by the equal and opposite attraction for the electrons by the oxygen on the other side of the molecule.
Water is a bent molecule with a bond angle of 104.3° (Figure 2.1), and the uneven sharing of electrons in the two bonds is not cancelled out as it is in CO2. The result is that the bonding electrons are more likely to be found at the oxygen end of the molecule than at the hydrogen end. Bonds with positive and negative ends are called dipoles.