For gases the particle unit is the molecule.
Chlorine, and other gases, like the other halogens, the oxygen, nitrogen, sulfur, among other, are diatomic. That means that the molecule is formed by two atoms (the prfiex di means two).
So, the particle unit for those gases is a molecule of two atoms.
Answer:The particle unit for chlorine gas is the molecule Cl2 (2 is a subscript that indicates two atoms of Cl)