Polarity of molecules

Although a molecule is made up of polar covalent bonds, the molecule itself may, or may not be polar. Remember that bond dipole moments are vector quantities. In order for the molecule to be polar, the molecule's bond dipole moments must add up to give a net dipole moment. Sometimes, the bond dipole moments cancel each other to give a zero net bond dipole moment, and a nonpolar molecule results.

Nonpolar molecule

In some molecules, when all the bond dipole moments are summed together, the overall dipole moment is zero. In such a case, the molecule is said to be nonpolar.

Polarity of carbon dioxide, CO₂

Carbon dioxide, CO₂, is a linear, symmetrical and nonpolar molecule. The bonds between carbon and oxygen are double bonds.

Oxygen is more electronegative than carbon. So, each carbon-oxygen double (C=O) is polar covalent with a partial negative charge on the oxygen atom. Carbon dioxide has two polar covalent bonds and yet it is a nonpolar molecule.

When the two bond dipole moments in carbon dioxide add up, there is no net dipole moment. Therefore, CO₂ is a nonpolar molecule.

Polar molecule

In some molecules, when all the dipole moments are summed up, there is a net dipole moment.

Polarity of water, H₂O

Water, H₂O, is bent-shaped with a bond angle of 104.5°.

Since oxygen is more electronegative than hydrogen, water has two polar covalent H-O bonds. Each hydrogen-oxygen bond (H-O) is polar covalent with a partial negative charge on the oxygen atom.

When all the bond dipole moments (Vector 1 and Vector 2) are added up in the water molecule, there is a net dipole moment. Therefore, H₂O is a polar molecule.

Summary

In summary, to determine whether a molecule is polar or not, we need to sum up all the bond dipole moments of the molecule by adding up all its bond dipole moments (ie - vectors).

• When the molecule does not possess a net dipole moment, the molecule is nonpolar.
• When the molecule possesses a net dipole moment, the molecule is polar.

External link

Comparison between these compounds:

1. SO₂ and BF₃
2. CH₄ and NH₃
3. PF₅ and SF₄
4. SF₆ and XeF₄

• Polarity of Molecules

Content suitability

BCIT courses: CHEM 0011