CBSE Class 11-science Answered

The shapes of the molecules affect the strength of dispersion forces.

Long thin molecules can develop bigger temporary dipoles due to electron movement than the short molecules containing same number of electrons.

Long thin molecules also lie closer together.

For example butane and 2-methyl propane has a molecular formula C₄H₁₀ but the atoms are arranged differently in both. In butane the carbon atoms are arranged in a single chain, but 2-methyl propane is a shorter chain with a branch.

So butane has a higher boiling point because the dispersion forces are greater.

The molecules are larger so they set up temporary dipole and can lie closer together than the shorter and fatter 2-methyl propane molecules.