Chlorine-37 is a stable isotope of the element chlorine, which has an atomic number of 17. It contains 20 neutrons in its nucleus, giving it a mass number of 37. Chlorine-37 makes up about 24% of naturally occurring chlorine, with the other major isotope being chlorine-35. This isotope is used in various applications, including nuclear magnetic resonance (NMR) spectroscopy and as a tracer in environmental studies. Its stable nature allows scientists to study chemical processes without the complications that come from radioactive isotopes.