Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. It only takes a minute to sign up.
Sign up to join this community
As per different websites on internet van der Waals force are of
While an expert website also explained H-bonds to be a van der Waals force.
I'm confused. What is originally correct?
The definition of van der Waals forces according to IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8., is as follows:
The attractive or repulsive forces between molecular entities (or between groups within the same molecular entity) other than those due to bond formation or to the electrostatic interaction of ions or of ionic groups with one another or with neutral molecules. The term includes: dipole–dipole, dipole-induced dipole and London (instantaneous induced dipole-induced dipole) forces. The term is sometimes used loosely for the totality of nonspecific attractive or repulsive intermolecular forces.
The phrase "other than those due to bond formation or to the electrostatic interaction of ions or of ionic groups with one another or with neutral molecules" would seem to imply that according to the IUPAC Gold Book, if a force is not an ionic or covalent chemical bond or electrostatic interaction, then it is a van der Waals force.
There are too many definitions for something that seems to be incompletely understood and it is best to look at it operationally. There are forces of attraction between molecules. We know this because it is observed. Different types of molecules have different forces. I will only talk about the weakest of these, the "pure" van der Waals forces between simple molecules that shouldn't attract each other, but do, such as the inert gases especially He and Ne and the nonpolar diatomics such as H2 and the Halogens and possibly to the alkanes, N2 and O2. London dispersion forces arise because of the temporary dipoles in molecules by "motions" of the electrons about the nuclei. I have a problem with this explanation probably because I just don't understand it and have trouble understanding why it is only attractive. The explanation I was taught was that as molecules approached each other the electron clouds were displaced more or less tangentially exposing the nonvalence electrons to the nucleus of the other molecule The distortion of the orbitals is of slightly lower energy than the attraction hence a positive attraction until the molecules got too close. A simple conversion of kinetic to potential energy. This has a problem with H2 and He because no inner electrons but I can imagine a distance where there is an attractive force. With alkanes the slight bond polarity will give some attraction and perhaps more induced dipoles, with actual polar bonds the dipole dipole attraction is evident. Hydrogen bonding is a great leap upwards in attraction and I wonder why these are considered types of van der Waals forces when they can stand on their own. So the various definitions seem, to me at least, a bit arbitrary.
