Ci- what element does it stand for?

Halogen group elements, also known as halogens, are the general names of periodic VIIA group elements, such as fluorine, chlorine CI-, bromine br, iodine I and astatine, among which fluorine is the most nonmetallic element. The chemical element symbol of chlorine is CI-, which is a nonmetallic element. It is yellow-green gas at normal temperature and pressure, with strong pungent smell and toxicity. Free chlorine in nature is one of the main simple substances that destroy the ozone layer.