Why do London forces increase with molecular size?

1 Answer
Aug 14, 2016

The bigger the molecule the more electrons it has..............

Explanation:

And the more electrons it has, the bigger and more distortable and polarizable its electron cloud. Larger electronic clouds offer greater opportunity for intermolecular interaction via dispersion forces.

For the "n-alkane" series, for which only dispersion forces operate as intermolecular forces, we can assess the extent of intermolecular interaction by interrogating the normal boiling points:

"methane" "b.p. = "-161.5 ""^@C

"ethane" "b.p. = "-89 ""^@C

"propane" "b.p. = "-42 ""^@C

"n-butane" "b.p. = "-1 ""^@C

"n-pentane" "b.p. = "36.1 ""^@C

"n-hexane" "b.p. = "68 ""^@C

The longer the hydrocarbyl chain, the more possibility there is for interaction between chains, and this is certainly reflected in the normal boiling points, which is a measure of such interactions.