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Molecules have a charge distribution that is never quite even (i.e. it has a directionality or anisotropy), which results from the non-even distribution of the electrons and their negative charge. This uneven electron charge distribution results in a small permanent electric dipole across the length of the molecule.

In Figure 3.11, you can see this schematically illustrated using hydrogen chloride. Chlorine has a higher electronegativity than hydrogen. In other words, it tends to pull electrons toward it. As it tugs hydrogen's electron toward it, it gets a very slight negative charge. This also means that the hydrogen atom develops a slight positive charge (seen from the point of view of the hydrogen atom, the positive charge of its proton is not quite cancelled out by the negative charge of its electron, which is now more associated with the chlorine atom). The result is that the hydrogen atom has a very slight net positive charge. Hence, the molecule now has a polarity or dipole. Now, if two of these molecules meet up, the slightly negatively charged chlorine atom will be attracted to the positively charged hydrogen on the other molecule; they attract one another like small bar magnets (Figure 3.11).

Figure 3.11 The dipoles of two HCl molecules involved in Keesom interactions. They attract each other like tiny bar magnets.