Deuterium (symbol: Hydrogen-2, D or 2H), also known as heavy Hydrogen. is one of the two stable isotopes of hydrogen. The other one is Protium (symbol: Hydrogen-1, 1H). The nucleus of Deuterium, called Deuteron, contains one proton and one neutron, whereas the far more common, Protium has no neutron in the nucleus. Deuterium has a natural abundance in Earth’s oceans of about 1 atom to 6420 atoms of hydrogen. Thus deuterium accounts for approximately 0.0156% (or 0.0312%, on a mass basis) of all the naturally occurring hydrogen in the oceans, while Protium accounts for more than 99.98%. The abundance of deuterium changes slightly from one kind of natural water to another (see Vienna Standard Mean Ocean Water).