²H, also known as deuterium, is a stable isotope of hydrogen. It contains one proton and one neutron in its nucleus, giving it a mass approximately twice that of regular hydrogen, which has no neutrons. Deuterium is naturally occurring and makes up about 0.0156% of all hydrogen found in nature. Deuterium is often used in scientific research and various applications, including nuclear fusion and as a tracer in chemical reactions. It can also be found in heavy water, or D2O, which is used in certain types of nuclear reactors and in some specialized laboratory experiments.