Intermolecular Forces

R W Grime Ripon Grammar School

1 – Van der Waals’ Forces

The electrons in an atoms are moving around – at any moment in time they are unlikely to be evenly spread.

This gives the atom or molecule a temporary dipole.

This dipole moment will induce a temporary dipole in a neighboring atom by attracting/repelling its electron charge cloud.

2 – Dipole-dipole forces

Polar molecules (molecules with permanent dipoles) will attract other molecules with permanent dipoles.

3 – H-bonds

These are a special case of dipole-dipole forces.

They occur when H is bonded to N, O or F (which are very electronegative).

As H only has the two electrons in the covalent bond, if they are pulled away from the H atom, the H nucleus is exposed.

The “H-bond” is the attraction between a lone pair on the N, O or F to the + H on a neighbouring molecule.

AQA way of drawing H bonds

from lone pair on N, O, F to + on H, e.g. NH3

H

NH

H

H

NH

H

AQA way of drawing H bonds

H

O

H

H

O

H

from lone pair on N, O, F to + on H, e.g. H2O

AQA way of drawing H bonds

from lone pair on N, O, F to + on H, e.g. HF

FH FH

Also in:

alcohols (O-H bonds)

carboxylic acids (O-H bonds)

amines (N-H bonds)

proteins (N-H bonds)