Chlorine-35 is a stable isotope of the element chlorine, which has an atomic number of 17. It contains 18 neutrons in its nucleus, giving it a mass number of 35. Chlorine-35 is one of the two naturally occurring isotopes of chlorine, the other being chlorine-37. This isotope is commonly found in nature, making up about 75% of all chlorine isotopes. Chlorine-35 plays a significant role in various chemical reactions and is often used in the production of chlorine gas and other chlorine compounds, which are important in water treatment and sanitation.