Polar Molecules

When there is a difference in electronegativities in the 2 atoms connected by a bond, then the atom with the higher electronegativity acquires a partial negative charge, the atom with the lower electronegativity acquires a partial positive charge and the bond is called polar. The greater the difference in electronegativity, the greater the polarity of the bond. Bond polarities can be represented by a vector pointing in the direction of the negative partial charge. Consider both CO₂ and H₂O shown below:

For a molecule with polar bonds, these bond dipole vectors add up in a vector sum to produce a molecular dipole. In the case of CO₂, the bond dipoles cancel out leaving no molecular dipole. Therefore CO2 is a nonpolar molecule with polar bonds. In H₂O, the bond dipoles add up to produce a non-zero molecular dipole. Therefore, H₂O is a polar molecule. Polar molecules will orient themselves in an electric field in such a way as to partially counteract the field. Experimental measurements on this property leads to the calculation of a dipole moment for polar molecules.

Q is the charge on each of the two atoms and r is the internuclear distance.

In actuality, the picture of the molecular dipole being the vector sum of the bond dipoles is an oversimplification. Molecular dipoles are very difficult to calculate accurately as can be shown in the table below:

Lone pairs can dramatically effect the size of a dipole moment. In the cases of NH3 and H2O, the lone pairs point in the same general direction as the bond dipoles which enhances the dipole moment. In NF3, the lone pair points in the opposite direction as the bond dipoles which leads to a much smaller dipole moment than expected.

In highly symmetrical structures, the bond dipoles tend to cancel out leaving a nonpolar molecule.

Whether a molecule is polar or not plays an important role in various aspects of its chemistry. Polar molecules tend to have stronger intermolecular forces than nonpolar molecules. This gives them higher melting points and higher boiling points. The general solubility rule of "like dissolves like" is based on polarity of molecules.

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