The isotopes of an element differ from each other in their mass numbers and the number of neutrons they contain. All of the isotopes of an element, however, contain the same number of protons, which is equal to that element's atomic number. The atomic number, because it does not reflect the number of neutrons in an atom, does not distinguish between the isotopes of a given element, which will all occupy the same position in the periodic table.
The total number of protons and neutrons in an atom's nucleus is represented by its mass number, and it differs from the element's atomic number across its varying isotopes. For example, the atomic number of carbon is 6, because its nucleus contains 6 protons. Carbon also has a range of isotopes which include a 6-neutron, 7-neutron and 8-neutron isotope. These three carbon isotopes are called carbon-12, carbon-13 and carbon-14, respectively. The atomic mass of each of these isotopes is derived by adding the number of neutrons in each isotope to the number of protons. The atomic mass of carbon-14, for example, is 14, because 6 protons plus 8 neutrons equals an atomic mass of 14.
The number of neutrons in an isotope has a major effect on its nuclear properties, but an element's chemical properties show only slight variations across its different isotopes. An exception is hydrogen, which can demonstrate significant differences in the ways its individual isotopes behave in certain biochemical reactions.