The melting point of a substance is determined by the strength of the intermolecular forces holding the particles together. In the case of argon, the intermolecular forces are very weak, known as van der Waals forces. These forces are caused by the temporary fluctuations in the electron clouds of neighboring atoms, which create instantaneous dipoles. These dipoles can then interact with each other, creating a weak attractive force.

The strength of van der Waals forces depends on the polarizability of the atoms involved. Polarizability is a measure of how easily the electron cloud of an atom can be distorted. The more polarizable an atom, the stronger the van der Waals forces will be.

Argon atoms are very non-polarizable, meaning that their electron clouds are not easily distorted. This results in very weak van der Waals forces between argon atoms. Consequently, the melting point of argon is very low, at -189.4oC.

In contrast, substances with stronger intermolecular forces, such as hydrogen bonding or ionic bonding, have higher melting points. For example, water has a much higher melting point than argon (0oC), due to the presence of hydrogen bonding between water molecules.