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Why is it that alkenes with greater surface contact have greater London forces? I thought greater London forces were dependent on the size of the molecules, or the number of electrons, rather than the surface contact.

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You are correct that London forces are dependent on the size of a molecule, but think about why this is. One reason is that a larger molecule tends to have a greater surface area than a smaller molecule, and thus there is more surface area available to interact via London (and other) forces.

Also as stated in this Wikipedia article:

London forces become stronger as the atom in question becomes larger, and to a smaller degree for large molecules. This is due to the increased polarizability of molecules with larger, more dispersed electron clouds.

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While yes, London forces do depend on the mass of the molecule, but they also depend on the distance between the two attracting molecules.

London forces are inversely proportional to the sixth power of the separation between them:

$$F_{\text{london}} ∝ \frac{1}{d^6}$$

Now, consider pentane and 2-methylbutane. See how the molecules approach each other and show attractivity:

Pentane:

enter image description here

2-methylbutane:

enter image description here

The former shows attractions of 5 atoms, while the latter shows attractions of only 4 atoms.

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  • $\begingroup$ London forces are proportional to the sixth power of the separation between them or proportional to the sixth power of half the separation between them (radius)? $\endgroup$ – Melanie Shebel May 19 '17 at 2:48
  • $\begingroup$ @MelanieShebel Both. $\endgroup$ – Pritt says Reinstate Monica May 19 '17 at 3:17
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    $\begingroup$ Well yeah, haha. Touché. I mean you state, "London forces are inversely proportional to the sixth power of the separation between them:" and then show an equation with radius. Did you mean to put distance in the equation or radius? Or did you mean to say, "London forces are inversely proportional to the sixth power of half the separation between them." $\endgroup$ – Melanie Shebel May 19 '17 at 3:20
  • $\begingroup$ @MelanieShebel My bad, sorry, I'll fix it $\endgroup$ – Pritt says Reinstate Monica May 19 '17 at 3:37
  • $\begingroup$ Data to supplement this answer: Isopentane has a boiling point nearly 29degC. Pentane instead has a boiling point nearly 36degC. $\endgroup$ – Gaurang Tandon Jan 13 '18 at 8:46

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