What is nucleaus of the most common isotope of the hydrogen atom ?
Heavy water , formally called deuterium oxide or or D2O , is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium , ( also known as " heavy hydrogen " , which can be symbolized as 2 H or D ) rather than the common hydrogen-1 isotope ( called protium , symbolized as 1 H ) that makes up most of the hydrogen in normal water .