Deuterium (symbol: hydrogen-2, D or 2H), also known as heavy hydrogen. It is one of the two stable isotopes of hydrogen. The other is protium (symbol: Hydrogen-1, 1H). The nucleus of deuterium, called the deuteron, contains a proton and a neutron, while the protium, which is much more common, has no neutrons in the nucleus. Deuterium has a natural abundance in Earth's oceans of about 1 atom per 6420 hydrogen atoms. Thus, deuterium accounts for about 0.0156% (or 0.0312%, by mass) of all natural hydrogen in the oceans, while protium accounts for more than 99.98%. Deuterium abundance changes slightly from one kind of natural water to another (see Vienna Standard Mean Ocean Water).