Intermolecular forces > trends in London Dispersion Forces

In summary, the chemistry textbook explains that larger molecules have greater polarizabilities due to their larger number of electrons and distance from nuclei, leading to stronger dispersion forces. This may seem contradictory to the fact that atomic size decreases across a period, but molecular size is not always parallel to atomic weight. This is because adding more atoms to a molecule, not necessarily changing elements, increases molecular size. This is particularly relevant in organic chemistry.
  • #1
CaptainZappo
92
0
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.
 
Last edited:
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  • #2
Where is a molecule located in the periodic table?

The above passage is most likely written in the context of organic molecules. You do not increase molecular size/mass by switching elements as much as by simply having more atoms per molecule. CH3OH is smaller than C4H9OH, because the latter just has more atoms in it.
 
  • #3
Thank you. That makes sense.
 

1. What are intermolecular forces and how do they affect molecules?

Intermolecular forces are attractive forces between molecules that hold them together in a substance. They affect the physical properties of molecules such as boiling point, melting point, and viscosity.

2. What is the trend in London Dispersion Forces?

The trend in London Dispersion Forces is that they increase with increasing molecular size and surface area. This is because larger molecules have more electrons and a larger surface area, resulting in stronger temporary dipoles and therefore stronger London dispersion forces.

3. How do London Dispersion Forces compare to other intermolecular forces?

London Dispersion Forces are the weakest intermolecular force, but they are present in all molecules regardless of polarity. They are typically weaker than dipole-dipole and hydrogen bonding, but can still have a significant effect on the properties of a substance.

4. Can London Dispersion Forces be found in non-polar molecules?

Yes, London Dispersion Forces can be found in non-polar molecules. Even though these molecules do not have permanent dipoles, their electrons are still constantly moving, creating temporary dipoles and allowing London dispersion forces to occur.

5. How do temperature and pressure affect London Dispersion Forces?

As temperature increases, the average kinetic energy of molecules increases, leading to more frequent collisions and stronger temporary dipoles. This results in an increase in London Dispersion Forces. Pressure also affects London dispersion forces by decreasing the distance between molecules, making the temporary dipoles closer and causing the forces to become stronger.

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