Isotopes Since it is known that a given element (identified by its atomic number Z ) can have a varying number of neutrons altering the atomic mass A , a notation is required to distinguish these isotopes . Hydrogen, for example, has three main isotopes. All of them have just one proton ( Z = 1 ) as this is what makes hydrogen what it is. The most common form, technically called protium has no neutrons, so its atomic mass is A = 1 . It is therefore also called hydrogen-1 and is denoted by the symbol 1 1 H . The upper is the mass number and the lower is the atomic number. Far more rare is an isotope called deuterium or heavy hydrogen , so named because water made from this isotope, while chemically identical to regular water in nearly every reaction, is measurably heavier than regular water, and ice cubes made of it was sink in ordinary tap water. The high weight occurs because this hydrogen contains a neutron. It is therefore hydrogen-2 since A = 2 , and its symbol is 1 2 H . The third kind of hydrogen, called
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