KEY2CHEM

Polar Molecules

Polar molecules contain permanent dipole moments (uneven distribution of electrons). Oppositely charged portions of these neighboring dipole moments interact with one another via electrostatic attraction. When a molecule contains a hydrogen atom directly connected to a small, highly-electronegative atom ($$\require{mhchem}\ce{N}$$, $$\require{mhchem}\ce{O}$$, or $$\require{mhchem}\ce{F}$$), the bond is strongly polarized, leaving a partial positive on $$\require{mhchem}\ce{H}$$ and partial negative on the other atom. These polarized bonds interact strongly with neighboring molecules (also polarized), resulting in strong dipole-dipole attractions called hydrogen bonds. Note that the term "hydrogen bond" refer to interparticle interactions and is not a covalent bond.

Example 1.

Which compound will have hydrogen bonding as the strongest type of intermolecular force between its molecules?

A. $$\require{mhchem}\ce{CHF3}$$

B. $$\require{mhchem}\ce{NH3}$$

C. $$\require{mhchem}\ce{NF3}$$

Solution

B. $$\require{mhchem}\ce{NH3}$$

Hydrogen bonding is a strong subtype of dipole-dipole interaction, in which a hydrogen atom is directly connected to an atom of $$\require{mhchem}\ce{N}$$$$\require{mhchem}\ce{O}$$, or $$\require{mhchem}\ce{F}$$ (small, highly-electronegative atoms). This creates a highly polarized bond with uneven charge distribution that interacts strongly with neighboring molecules.

Example 2.

Which compound will have dipole-dipole interactions as its strongest type of intermolecular force between its particles?

A. $$\require{mhchem}\ce{NaCl}$$

B. $$\require{mhchem}\ce{CH4}$$

C. $$\require{mhchem}\ce{CHCl3}$$

Solution

C. $$\require{mhchem}\ce{CHCl3}$$

Polar molecules display dipole-dipole interactions as the strongest type of intermolecular force. Ionic compounds are held together by ionic bonds and nonpolar compounds show London dispersion forces as the strongest type of intermolecular force.

Example 3.

Which diagram best describes the interparticle forces in $$\require{mhchem}\ce{HCl}$$?

A.

B.

C.

Solution

B.

$$\require{mhchem}\ce{HCl}$$ is a polar molecule that will have dipole-dipole interactions as its strongest type of intermolecular force. $$\require{mhchem}\ce{Cl}$$ is the larger atom and is more electronegative so will bear a partial negative charge. $$\require{mhchem}\ce{H}$$ is the smaller atom and also has lower electronegativity, so it will bear a partial positive charge. The dipoles will align to maximize electrostatic attraction (opposite charges attract) and minimize electrostatic repulsion (like charges repel).