The Element: Chlorine We researched the chemical element known as chlorine. Chlorine has an atomic number of 17 and an atomic weight of 35.453. It has a valence number of 3. The element has 3 energy levels. Chlorine exists as a greenish-yellow gas at normal temperatures and pressures. Chlorine is second in reactivity only to fluorine among the halogen elements. Chlorine is a nonmetal. It is estimated that 0.045% of the earth’s crust and 1.9% of sea water are chlorine. Chlorine combines with metals and nonmetals and organic materials to form hundreds of chlorine compounds. Chlorine is about 2.5 times as dense as air an