For liquid argon the strongest intermolecular interaction is?

London dispersion forces exist between all molecules including atoms of argon.

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More about intermolecular forces.

1. London dispersion forces .... forces between all molecules. In the past London dispersion forces were characterized as the attraction between temporary dipoles. There may be more to it than that. Dispersion forces are a quantum interaction. All molecules exhibit London dispersion forces, LDF's. The strength of LDF's is proportional to the polarizability of the molecule, which in turn, depends on the number of electrons and the surface area of the molecule. Contrary to what some teachers and even some authors say, in many cases London dispersion forces can be stronger than Keesom or Debye forces and are second only to hydrogen bonding.

2. Keesom forces (dipole-dipole attraction) ... the attraction of one polar molecule for another. Oppositely charged ends of the molecules undergo electrostatic attraction.

3. Debye forces (induced attraction) .... the attraction between a polar molecule and one which is nonpolar, where the polar molecule induces a charge separation in the nonpolar molecule. Debye forces can also exist between two polar molecules, even the same kind of molecule.

4. Hydrogen bonding .... The weakly covalent bonds found between the hydrogen atom of one molecule where it is bonded to N, O or F, and the N, O, or F of an adjacent molecule. The hydrogen atom functions as a bridge, forming a bond between two molecules. Hydrogen bonding is more than an electrostatic attraction between molecules, instead there is evidence of orbital overlap and covalent bond formation.

Note: There are three van der Waals forces (Keesom forces, Debye forces and London dispersion forces). Some textbooks and some teachers' dusty old notes erroneously equate van der Waals forces only with London dispersion forces.

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There is nothing more than a coincidental connection between molecular weight and the strength of London dispersion forces. The strength of London dispersion forces is directly proportional to the polarizability of the molecule, which it turn is determined by the total number of electrons and the area over which they are spread. Molecules with greater molecular weight may have more electrons (but not necessarily) and then may have more surface are (again, not necessarily) so there can be a coincidental increase in the molecular weight and the strength of London dispersion forces. But you would never, never use molecular weight as a reason or in an explanation referencing London dispersion forces.

This is an odd question, as argon exists as single atoms, not molecules. Liquid argon exists at only very low temperatures (87.3 K is the boiling point according to Wikipedia). With this information, we know that the "interatomic" forces between argon atoms must be weak. We know this because the temperature (average kinetic energy of atoms) must decrease in order for the argon gas to condense into a liquid. The motion of the atoms need to be slowed down substantially in order for the "interatomic" forces to be able to overcome that motion. Once that happens, the atoms come together and a liquid is formed. Back to the question, the forces between liquid argon atoms would probably be weak dispersion forces (instantaneous dipoles), which creates a weak attraction between atoms. That would be the main one, but there also may be weak electrostatic attractions between the nucleus of one argon atom and the electrons of another atom.

Dispersion forces...This is the only possible force between single atoms of nonmetals.