Which intermolecular force is most responsible in allowing xenon gas to liquefy?

We begin by recalling that atoms or molecules in the gas phase are kept together in the liquid phase only because of weak attractive forces that operate between them. These attractive forces are collectively called intermolecular forces.

There are several recognised categories of intermolecular forces, each arising from a different physical origin:

• Ionic interactions involve the full positive charge of a cation being attracted to the full negative charge of an anion.
• Ion-dipole interactions involve an ion (bearing full charge) attracting the partial charge on the ends of a polar molecule.
• Dipole-dipole interactions arise when two permanently polar molecules align so that the partially positive end of one molecule is near the partially negative end of another.
• London dispersion forces (also called instantaneous dipole-induced dipole forces) arise even in atoms or molecules that have no permanent charge separation. They are caused by moment-to-moment fluctuations in the electron cloud that produce a temporary dipole. This temporary dipole can then induce a dipole in a neighbouring atom or molecule, and the two temporary dipoles attract each other.

Now we analyse the specific substance mentioned in the question: xenon, written as $$\text{Xe}$$.

Xenon is a noble-gas atom. Its outer electron shell is completely filled, so the atom is electrically neutral and non-polar; there is no permanent dipole moment in a xenon atom. Because of this:

• Xenon atoms cannot engage in ionic interactions with each other: there are no oppositely charged ions present.
• Xenon atoms cannot engage in ion-dipole interactions for the same reason: there is no ion and no permanent dipole.
• Xenon atoms also cannot engage in dipole-dipole interactions, because a dipole-dipole force requires each partner to possess a permanent dipole moment, which xenon does not have.

Therefore, the only class of intermolecular force that is available to xenon atoms is the one that operates even between completely non-polar species, namely the London dispersion force. These London forces, though individually weak, are numerous when many atoms are close together, and they are sufficient to hold xenon atoms in the liquid state when the gas is cooled and/or compressed.

Hence, the force that is most responsible for allowing xenon gas to liquefy is the London dispersion force.

Hence, the correct answer is Option A.