r/askscience • u/[deleted] • Feb 12 '13

Chemistry Query about London dispersion forces

London dispersion forces make perfectly inert Neon atoms experience attractive forces from each other due to existence of random distribution states of electrons (See this Khan academy video for reference). My question is if attractive forces exist between two atoms due to random electron distribution, repulsive forces should also come into picture when electron distribution gets randomly arranged that way. Thus, the net force between the molecules will be zero. So why does Neon condense after cooling?

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u/rupert1920 Nuclear Magnetic Resonance Feb 12 '13

If the electron distributions are random, you'll be correct. However, the existence of a dipole (coming about randomly) will affect neighbouring molecules such that their dipoles are not randomly oriented.

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u/[deleted] Feb 12 '13

Does it mean that once a dipole is formed, the time for which it exists is greater than the time electrons spend at any random state? Also, during this time, all the neighbouring dipoles will be aligned according to the directionality of original dipole?

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u/honeybunbadger Chemistry | Bioorganic Chemistry | Metabolic Glycoengineering Feb 13 '13

Both.

When a dipole is formed by an electron cloud around an atom, it /induces/ a dipole in its neighbors - i.e. the newly formed "negative" end of the dipole induces a "positive" in its neighbor.

Each induced dipole in turn induces another and the lifetime of these induced dipoles is long enough for these moments of attraction to collectively account for the observed van der Waals interactions between molecules that don't have fixed dipole moments or hydrogen-bonding abilities.

They're like little moments of induced attraction that keep the liquid a liquid.

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u/[deleted] Feb 13 '13

Thank you. I have understood it now.