Deuterium (²H, stable, 0.02%)

Deuterium is a stable isotope of hydrogen, represented by the symbol \( \text{D} \) or \( ^2\text{H} \). It contains one proton and one neutron in its nucleus, giving it a mass number of 2, compared to the more common hydrogen isotope, protium, which has no neutrons. Deuterium occurs naturally in small amounts in water, comprising about 0.0156% of all hydrogen in the ocean.