My chemistry textbook states the following: "In general, larger molecules tend to have greater polarizabilities because they have a greater number of electrons and their electrons are farther from the nuclei. The strength of the dispersion forces, therefore, tends to increase with increasing molecular size. Because molecular size and mass generally parallel each other, dispersion forces tend to increase in strength with increasing molecular weight." This seems to go against what I have previously learned. Atomic size does increases as one moves down a group (due to the addition of extra shells); however, as one moves across a period, atomic size tends to decrease (due to a larger Z effective). Now, atomic weight increases both down a group and across a period. Thus, these two are not always parallels of each other. If dispersion forces are proportional to molecular size, why don't they decrease along a period and increase down a group? Thanks for any insight, -Zachary Lindsey NOTE: Bold words were changed after Gokul43201's response. Thanks for pointing that out, Gokul43201.