In the book that I'm reading it is mentioned higher molar mass results in stronger van der Waals force, but didn't continued why.

I wonder why is that so? molar mass is actually dependable on the number of protons and neutrons, how can it be effective on the van der Waals force?

  • $\begingroup$ Just to make sure, by Van der Waals forces do you mean the induced dipole - induced dipole force? Since it's wrong to think that way for all of them. $\endgroup$ – M.A.R. Apr 10 '15 at 12:32
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    $\begingroup$ @MARamezani Yes. it is said "The van the waals forces" so I think it include induced dipole-induced dipole too. could it be wrong? $\endgroup$ – Mobin Apr 10 '15 at 12:38

From chemwiki.Ucdavis:

The more electrons a molecule contains, the higher its ability to become polar. Polarizability increases in the periodic table from the top of a group to the bottom and from right to left within periods. This is because the higher the molecular mass, the more electrons an atom has. With more electrons, the outer electrons are easily displaced because the inner electrons shield the nucleus' positive charge from the outer electrons which would normally keep them close to the nucleus.

This seems totally legitimate. However, (as chemwiki subtly points out) the main factor in work here is the volume occupied by electrons. Polarizability increase is mainly due to a volume increase 1. So, all you need to take into consideration is whether a molecule is bigger than the other in terms of comparing the stronger London dispersion force.

Simply put, the ability of a molecule to get polarized is called polarizability. See Wikipedia for more info.

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  • $\begingroup$ Thanks. So it is something like that: higher molar mass >> larger shell >> more Polarity >> Higher van der waals forces , right? $\endgroup$ – Mobin Apr 10 '15 at 13:30
  • $\begingroup$ I would say "more polarizability". "Polarity" is a different concept. And this statement is generally true. $\endgroup$ – M.A.R. Apr 10 '15 at 13:32

Somewhat related to: Hydrogen and Helium ideal gas behavior in room temperature

The induced dipole-dipole interactions operate through the polarizability of the molecules. Generally speaking heavier atoms will have more electrons and they will have some of those in the outer, higher radius shells. These higher radius elements will be more polarizable, hence stronger Van der Waals forces.

Though I can see where the statement comes from, I doubt it is a completely true statement. I would think one can have higher molar mass compounds that are less polarizable(though I cannot come up with an easy example right now)

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  • $\begingroup$ Depends on what we call "true". I rather call it a statement teachers teach to students in order to make the comparison easier for them. $\endgroup$ – M.A.R. Apr 10 '15 at 13:28

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