There must be at least two atoms of chlorine in a chlorine molecule. This produces what is called a diatomic molecule. The gas hydrogen also appears as a diatomic molecule.
Chlorine is a greenish-yellow gas at room temperature and has an atomic number of 17, which means there are 17 protons in the nucleus. It is a highly reactive element and is toxic if ingested or inhaled, and chlorine gas was actually used as a weapon during World War I.
Chlorine can also be very beneficial when small amounts are used or when it's combined with other elements. Sodium hypochlorite, or bleach, is an excellent disinfectant, and sodium chloride, or salt, is necessary for life.