Why do London forces increase with molecular size?

1 Answer
anor277
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.

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