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Intermolecular Forces

Intermolecular forces are weak attractive forces that contribute to many of the physical properties exhibited by liquids. From the MCAT point of view, you have to be familiar with the main three types of attractive forces:
  1. Hydrogen bonding
  2. Dipole-dipole interactions
  3. London forces

Hydrogen Bonding

Hydrogen bonding is an intermolecular attractive force that exists between a hydrogen atom covalently bonded to an electronegative atom (fluorine, oxygen, nitrogen), and another electronegative atom (fluorine, chlorine, nitrogen) in the neighboring molecule. Hydrogen bonding in water is shown below:

Hydrogen bonding in water

As you can see in Figure 7-1, the hydrogen which is covalently bonded to the electronegative atom (in this case it is oxygen), can undergo hydrogen bonding with an electronegative atom (the oxygen of another water molecule) present in the neighboring molecule.

Dipole-Dipole Interactions

Polar molecules have negative and positive polarity. This is because of the difference in electronegativities of the bonded atoms in such molecules. This polarity results in the formation of attractive forces among molecules. Such interactions are called dipole-dipole interactions. Consequently, the positive end of one molecule is attracted to the negative end of another. Figure 7-2 illustrates the dipole-dipole interactions between hydrogen chloride molecules.

Dipole-dipole interaction between HCl molecules

London Forces

London forces are weak attractive forces that exist between instantaneous dipoles. What is an instantaneous dipole? In order to understand this, let us talk about atoms. Atoms have a central positively charged nucleus surrounded by electrons. These electrons are constantly moving and the electronic cloud may not be always perfectly distributed. In other words, the electrons at any given moment may be distributed more on one side and less on the other, creating an instantaneous dipole. The side which has more electrons at any moment will have a slight negative charge, whereas the side which is electron deficient will have a slight positive charge.
The polarity that results from such charge separations in a molecule can induce polarity in other molecules when they are close enough to the instantaneous dipole molecule. For this reason, London forces are often called instantaneous dipole-induced dipole interactions. London forces are more pronounced in bigger molecules, because they are more polarizable.
Let us talk about the importance of these ideas. It is more likely that a polar molecule will exist as a liquid or solid than a nonpolar molecule. Why is it so? It is because of the intermolecular interactions that are possible among polar molecules.

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