Chlorine is a greenish yellow poisonous gas with a disagreeable suffocating odour. Chlorine is made of hypochlorite salt, which is a compound that contains CIO.
1 Additional Answer
Chlorine is an element, which means it is not made up of anything else. The chlorine atom has seventeen each protons and electrons and has an atomic mass of 35.4527. You can find more information here: http://en.wikipedia.org/wiki/Chlorine