Chlorine, periodic table Cl, is a gas with 17 protons. It's most common isotope is CL35 with an occurrence of 76% and with CL37 making up the other 24%.
Chlorine is abundant on the Martian surface where is is combined in many minerals, notably sodium chloride, or table salt. Chlorine is a precursor required for the production of polyvinyl chloride plastics, or PVC.
Chlorine is not available as a gas from the Martian atmosphere. However, it could be produced in situ from Sodium chloride as a co-product in the preparation of caustic soda (NaOH). Chlorine is also a constituting element of the Perchlorate family of chemicals. These perchlorates may provide a significant hazard in the establishment of a Martian settlement. So treating Martian soils could provide a source of chlorine for the chemical industry.