Deuterium is one of the three forms (called isotopes) of the first element, hydrogen. The first hydrogen isotope is named protium (the most abundant by far and the lightest), consisting of a proton and an electron. Protium is commonly expressed by the symbol 1H. The second isotope is named deuterium (much less abundant than protium but twice as heavy due to the addition of a neutron). Deuterium is commonly expressed by the symbol D or 2H. The most important fact to know is that in nature, both protium and deuterium combine with oxygen to form water molecules.