Polarity of a Molecule

The polarity of a molecule tells whether the electron cloud is equally distributed across the atoms within the molecule, or whether an electronegative atom is affecting the electron density. The distribution of the electrons will affect the behavior and reactivity of the molecule. For example, you can predict which solvents will be most effective with a given chemical if you know its polarity.

Water and oil do not mix well, despite the fact that they are both homogenous solutions on their own. This behavior can be explained through a deeper look at the polar forces affecting the two solutions.

When atoms with differing electronegativity are bonded together, the electrons may spend more time around one atom than the other, creating an unequal distribution of charge and a polar bond. In a polar bond, the electron-rich atom has a partial negative charge ($\delta^-$) and the electron-poor atom has a partial positive charge($\delta^+$).

The dipole moment is the result of asymmetrical charge distribution in a polar substance. Mathematically, it is the product of the partial charge on the bonded atoms and the distance between them.

The vector sum of the dipole moments determines the polarity of a molecule.

The vector sum of the dipole moments of a non-polar molecule will be zero. It is possible for a molecule to have polar bonds, but be a non-polar molecule. This occurs when the three-dimensional shape of the molecule is symmetric. For example, carbon dioxide $\left(\ce{CO2}\right)$ and methane $\left(\ce{CH4}\right)$ are non-polar because their symmetric shapes cancel out the dipole moments to zero, as shown in the figure below.

On the other hand, water $\left(\ce{H2O}\right)$ is a polar molecule because the overall dipole moment points toward the oxygen atom (indicating the oxygen atom is the most electron-rich).

Understanding the electron pair and molecular geometries described in the valence shell electron pair repulsion (vsper) model is useful in predicting the polarity of molecules.

Polarity is important because it determines whether a molecule is hydrophilic (from the Greek for water-loving) or hydrophobic (from the Greek for water-fearing or water-averse).

Molecules with high polarity are hydrophilic, and mix well with other polar compounds such as water. Molecules that are non-polar or have very low polarity tend to be hydrophobic, and mix well with other non-polar (or nearly non-polar) compounds such as oil.

Polarity also affects the strength of intermolecular forces. The only intermolecular force that non-polar molecules exhibit is the van der Waals force. Polar molecules can bond with each other via dipole-dipole interactions, which are generally stronger than van der Waals forces. Thus, if two molecules are similar in size and one is polar while the other is non-polar, the polar molecule will have higher melting and boiling points compared to non-polar molecule.

Determine the polarity of a boron trifluoride molecule $\left(\ce{BF3}\right)$.

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The above left shows the Lewis dot structure of a $\ce{BF3}$ molecule. Boron trifluoride is an exception of the octet rule, where the boron atom only has 3 electron pairs. For this reason, boron trifluoride has a trigonal planar shape, which is symmetric. Thus, although the $\ce{B-F}$ bond is polar, the dipole moments cancel out and the overall dipole moment sums to zero, as shown in the above right figure. Therefore, boron trifluoride is nonpolar. \