Isotopes of chlorine. Illustration showing the two principal stable isotopes of chlorine: chlorine-35 and chlorine-37. Isotopes are forms of an element that contain different numbers of neutrons in the atomic nucleus. Chlorine-35 has 17 protons (red) and 18 neutrons (blue) in the nucleus, and chlorine-37 has 17 protons and 20 neutrons. Both isotopes also have 17 electrons (black) orbiting the nucleus.