Chlorine has an atomic number of 17, but its atomic mass is approximately 35.45. The atomic mass of an element is the weighted average mass of all of its naturally occurring isotopes. Chlorine has two stable isotopes, chlorine-35 and chlorine-37, which occur in approximately equal amounts in nature. Chlorine-35 has a mass of 34.96885 Da, while chlorine-37 has a mass of 36.96590 Da. The weighted average mass of these two isotopes is approximately 35.45 Da.