Basically the basic gas hydrogen has 3 main isotopes. Deuterium is the 2nd most easily available isotope! Isotopes of only 15 deuterium atoms are found in 100,000 hydrogen atoms. And when this isotope combines with oxygen to produce water, it is called heavy water. Heavy water looks like ordinary water. But heavier than ordinary water. It is a compound substance. It has two hydrogen (deuterium) and one oxygen. This heavy water is not readily available.