In chemistry, a polar bond is a type of bond between two atoms in which electrons are shared unequally. Because of this, one end of the bond has a fractional negative charge and the other a positive charge.
- Non-polar bonds occur when the difference in electronegativity between the two atoms is less than 0.5
- Polar bonds occur when the difference in electronegativity between the two atoms is between 0.4 and 2.0
- Ionic bonds occur when the difference in electronegativity between the two atoms is greater than 2.0
If the bond dipole moments of the molecule do not cancel, the molecule is polar. For example, the water molecule (H2O) contains two polar O-H bonds in a bent (nonlinear) geometry. The bond dipole moments do not cancel, so that the molecule forms a molecular dipole with its negative pole at the oxygen and its positive pole midway between the two hydrogen atoms. In the figure each bond joins the central O atom with a negative charge (red) to an H atom with a positive charge (blue).
However, not every molecule with polar bonds is a polar molecule. Carbon dioxide (CO2) has two polar C-O bonds, but the geometry of CO2 is linear so that the two bond dipole moments cancel and there is no net molecular dipole moment; the molecule is non-polar.