Yes, I did note the use of elemental chlorine as a chemical warfare agent. It was found to be too light, and not poisonous enough ... So the Powers went on to using Sulfur Mustard compounds, Lewisite (organo-arsenic compound), and a whole bunch of other organic compounds involving chlorine and sulfur or arsenic in the molecule. Generally, they were either severe irritants, or blister agents. Organo-phosphate nerve agents were developed later.