Intermolecular Forces. R W Grime Ripon Grammar School. 1 â€“ Van der Waalsâ€™ Forces. The electrons in an atoms are moving around â€“ at any moment in time they are unlikely to be evenly spread. This gives the atom or molecule a temporary dipole.
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R W Grime Ripon Grammar School
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.
Polar molecules (molecules with permanent dipoles) will attract other molecules with permanent dipoles.
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.
from lone pair on N, O, F to + on H, e.g. NH3
from lone pair on N, O, F to + on H, e.g. H2O
from lone pair on N, O, F to + on H, e.g. HF
alcohols (O-H bonds)
carboxylic acids (O-H bonds)
amines (N-H bonds)
proteins (N-H bonds)