Chlorine Chlorine is (at room temperature) a greenish-yellow gas that can be readily liquefied at 5170 Tarr or 6.8 atmospheres, at 20 C (68 F), and has a very disagreeable odor. It’s Element Symbol is Cl, atomic number is 17, and atomic mass is 35.453. Chlorine’s melting point is -101 C or 149.8 F. The boiling point is -34.05 C or -29.29 F, at one atmosphere pressure. Chlorine is a member of the halogen group. Chlorine was discovered by Swedish scientist Karl Wilhelm in 1784, but he first thought it was a compound, rather than an element. In 1810, Sir Humphrey Davy named it Chlorine, from the Greek word meaning "greenish-yellow". Chlorine is used in bleaching agents, disinfectants,