London Dispersion Forces

Why is I₂ a solid? There are no hydrogen-bonds nor are there any dipole-dipole interactions to attract one molecule of I₂ to another molecule of I₂. Yet molecules of I₂ are attracted to each other. Afterall, I₂ is a solid; if there was no attraction between I₂ molecules then I₂ would be a gas.

The forces responsible for the attraction of one I₂ to another are called London Dispersion Forces. All molecules are attracted to other molecules via London Dispersion Forces.

We continually say that electrons are distributed evenly in the orbitals around an atom, ion, or molecule. If we average the positions of the electrons over time the electrons are distributed evenly.

Look at He and H2 

However, at any given moment in time the electrons might be at one side of the atom or molecule. For the moment in time during which more electrons are congregated on one side of the molecule an uneven distribution of charge exists. That is, a momentary dipole exists!

 

	On average an He atom resembles atom A; the electrons are distributed evenly. However, the electrons could be distributed unevenly for a moment in time. Since the electrons are on one side of the nucleus there is a positive side and a negative side to He atom B.
	The positive side of the polar He atom B attracts the electrons of He atom A. A dipole-dipole interaction is now occuring.

another example

The bigger the atoms or molecules involved, the larger the force of attraction is going to be. A big atom has many electrons moving around a large volume, so at any instant in time it is more likey to have an uneven distribution of electrons; thus, it is more likely to have a dipole. As was previously stated, I₂ is more polarizable than F₂. I₂ is more likely to form an instantaneous dipole; therefore, there will be more attraction between I₂ molecules than there is between F₂ molecules. The London Dispersion Forces in I₂ are strong enough to keep I₂ solid at room temperature; where as, F₂ is a gas at room temperature.

In general London Dispersion Forces are considered to be the weakest intermolecular force; however, London Dispersion Forces become very important for larger molecules. We will get back to this.