van der waals

how does van der waals force take place in molecules i understand in atom since electrons are constantly moving they can be one side causing a temporary dipole that that dipole causes another dipole in a neighbouring atom since the slight negative charge causes the repulsion to the electrons so electrons moves to the other side one side is positive so the negative charge is attracted to the positive side . But how would it work in a molecule is it that if one of atom is temporary dipole the other atom in a covalent bond would also be temporary dipole with opposite direction so they can be attracted to each other. if so how does the bonding pair electrons effect it. Or is it that the whole molecule have one slightly positive charge and negative charge sides if so can someone explain it to me on how?

Reply 1

lordaxil

11

That is a very good question, and a full description of van der Waals forces between molecules is actually an active research topic in theoretical chemistry. However, to a good approximation, for most molecules you can assume that the attractive part of van der Waals force (known more formally as "dispersion" interaction) from each atom is independent of its neighbours, so you can calculate the force between two molecules as the sum of forces over pairs of atoms. For some molecules, especially those with delocalised electrons, there are more complex "many-body" dispersion effects, but these require some advanced quantum mechanics to work out.

For atoms in the same molecule, we generally ignore van der Waals forces, as they are negigible compared to bonding interactions (covalent, ionic, etc). For neutral molecules, especially those with large numbers of electrons, then van der Waals forces can contribute significantly to intermolecular bonding.

Reply 2

nnn121337

OP

8

Original post by lordaxil

That is a very good question, and a full description of van der Waals forces between molecules is actually an active research topic in theoretical chemistry. However, to a good approximation, for most molecules you can assume that the attractive part of van der Waals force (known more formally as "dispersion" interaction) from each atom is independent of its neighbours, so you can calculate the force between two molecules as the sum of forces over pairs of atoms. For some molecules, especially those with delocalised electrons, there are more complex "many-body" dispersion effects, but these require some advanced quantum mechanics to work out.

For atoms in the same molecule, we generally ignore van der Waals forces, as they are negigible compared to bonding interactions (covalent, ionic, etc). For neutral molecules, especially those with large numbers of electrons, then van der Waals forces can contribute significantly to intermolecular bonding.

Thank you so am i right to assume that i don't need to know the information on how van der waals forces take place in molecules for a level chemistry aqa. and would you say i have good understanding of van der waals forces for chemistry a level based on the comment.