Hydrogen-2 or 2H, also known as both Deuterium and Heavy Hydrogen, is the second stable isotope of hydrogen. It is not very abundant, although it is naturally occurring in nature, with a rarity of ~154 PPM (Parts per Million). The nucleus of deuterium, called a deuteron, contains one proton and one neutron, whereas the more common hydrogen nucleus contains no neutrons. The isotope is named for the Greek word deuteros, meaning "second", to describe the two particles composing the nucleus.

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