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a al ad UT, ee a ow The basis of a van der Waals interaction is that the distribution of electronic charge around an atom fluctuates with time. At any instant, the charge distribution is not perfectly symmetric, leading to a dipole. This dipole acts through dipole-dipole interactions to induce a complementary dipole in its neighboring molecules, causing an attraction between the molecules. This attraction increases as the two atoms come closer to each other, until they are separated by what is termed the van der Waals contact distance (Figure 1.11). At distances shorter than the van der Waals contact distance, very strong repulsive forces become dominant because the outer electron clouds of the two atoms overlap. | S 5 'a i a van der Waals 2 J contact distance Distance Bo c I a I c I 8 ; G i g i x FIGURE 1.11 The energy of a van der Waals interaction varies and is most favorable at the van