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 and moderately solu