KEY2CHEM

London Dispersion Forces

London dispersion forces are a type of intermolecular (or interparticle) force that exists between all atoms and molecules. London dispersion forces rely on the movement of electrons within a particle to create a temporary (not permanent) dipole moment. This temporary dipole moment then induces, or creates, a dipole moment in a neighboring particle to avoid electron-electron repulsions. Since formation of temporary dipoles relies on movement of the electron cloud, species that have greater polarizability (ability to move the electron cloud around) form temporary dipoles easier and will have stronger London dispersion forces. Polarizability increases as the number of electrons in a species increases; most often this correlates with increased molar mass. Species having more electrons have an easier time shifting a few of them to create a temporary dipole moment. Because this type of electrostatic attraction relies on the interaction of temporary and induced dipoles, London dispersion forces are generally the weakest type of intermolecular force. 


Example 1.

Which hydrocarbon is expected to have the strongest London dispersion forces?

 

A. \(\require{mhchem}\ce{C8H18}\)

B. \(\require{mhchem}\ce{CH4}\)

C. \(\require{mhchem}\ce{C3H8}\)

 

Solution 

A. \(\require{mhchem}\ce{C8H18}\)

London dispersion forces are stronger in species with more electrons; this generally corresponds to species having more mass. Species with more electrons are more polarizable and form temporary dipole moments more easily.


Example 2.

London dispersion forces are present between ______.

A. only polar molecules

B. only nonpolar molecules

C. all atoms and molecules

 

 

Solution

C. all atoms and molecules

London dispersion forces are attractive forces present between all atoms and molecules. London dispersion forces are Coulombic attractions that arise from temporary dipole moments inducing dipole moments in neighboring particles.


Example 3.

Which statement about London dispersion forces is true?

A. London dispersion forces result when the permanent dipole moments of polar molecules are attracted to one another.

B. London dispersion forces are attractive forces that result when the temporary uneven distribution of electrons around nuclei in a molecule create a dipole moment in a neighboring molecule, resulting in attraction between the molecules.

C. London dispersion forces are stronger in species with fewer electrons, since it is easier to move all the electrons at one time.

 

 

 

 

Solution

B. London dispersion forces are attractive forces that result when the temporary uneven distribution of electrons around nuclei in a molecule create a dipole moment in a neighboring molecule, resulting in attraction between the molecules.

London dispersion forces are attractive electrostatic interactions that result when a temporary dipole (resulting from uneven distribution of electrons in a particle) induces a dipole moment in a neighboring particle, resulting in an attractive interaction between the particles.