Which one of these compounds listed below is least likely to exist, and why? NaBr HgCl2 ArCl Mg3N2 C3H8 CaO

Argon chloride, ArCl is least likely to exist.

Argon belongs to the noble gas family. It has an atomic number of 18 and a valence electron configuration: 3s²3p⁶. Since the valence shell is completely filled i. e. it already has an octet configuration, this makes it stable and chemically inert. When approached closely by another element, Cl in this case, the fully filled valence shell of Ar would repel the incoming electron cloud. The only bond that could exist is the extremely weak, long-range Vander-Waals bond between Ar and Cl.