# When do molecules become polar?

Aug 6, 2017

A molecule is polar when its structure contains polarity, i.e. significant charge separation between bound atoms. This is a necessary but not a sufficient condition.

#### Explanation:

A polar molecule contains functional groups that enforce charge separation.....Alcohols are the classic polar group.....and we may represent this polarity by a diagram such as H_3C-stackrel(""^+delta)CH_2-stackrel(delta^-)O-stackrel(delta^+)H.

While an INDIVIDUAL bond may be polar, it is the GEOMETRIC SUM of ALL the bond dipoles that determines MOLECULAR POLARITY. Methylene chloride, ${\text{H"_2"CCl}}_{2}$, and $\text{chloroform}$, $C H C {l}_{3}$, are thus BOTH polar molecules....but ${\text{CCl}}_{4}$, with FOUR $C - C l$ bonds is NON-POLAR because the vector sum of the four individual $\stackrel{+ \delta}{C} - \stackrel{\delta -}{C} l$ dipoles is CLEARLY ZERO.....

Of course the intermolecular hydrogen bonding that results is a special case of bond polarity, but it might also occur in non-hydrogen bonding molecules such as ${H}_{3} \stackrel{{\text{^+delta)C-stackrel(delta^-)O-stackrel(}}^{+} \delta}{C} {H}_{3}$ or stackrel(""^(+)delta)CH_2stackrel(delta^-)X_2.

This bond polarity gives rise to greater intermolecular interaction, and decreased volatility.