All isotopes of a given element must share the same atomic number, which equals the number of protons. An isotope is denoted by its atomic number in the lower left-hand corner next to its chemical symbol and its mass number in the upper left next to the symbol.
The mass of neutrons and protons in an atom makes up the atom's mass. Isotopes have differing numbers of neutrons; therefore, their masses are not the same. Many isotopes of atoms are not stable and are radioactive.
Because isotopes contain the same number of protons, they also have the same number of electrons. Isotopes, therefore, tend to act the same way in chemical reactions. To illustrate, hydrogen has three naturally occurring isotopes: protium, deuterium and tritium. Protium has no neutrons, deuterium has one and tritium has two; however, all forms of hydrogen can bond with oxygen to form water.