One of two or more varieties of a chemical element whose atoms have a common number of protons and electrons (i.e. their atomic number is the same) but which vary in the number of neutrons in their nucleus (i.e. their atomic weight, signified by their mass number, is different). For example, hydrogen exists in the forms ¹₁H (one proton, no neutron), ²₁H deuterium: one proton, one neutron), and ³₁H (tritium: one proton, two neutrons). Water in which ²₁H replaces the more common ¹₁H is known as ‘heavy water’. There are 300 naturally occurring isotopes, but only 92 naturally occurring elements, and in nature elements often occur as a mixture of isotopes, with one form being the most common. Isotopes may be produced by various nuclear reactions and the products are frequently radioactive. There are three different ways of specifying an isotope; for example ²³⁵U, U-235, and uranium 235 all indicate the isotope of uranium with a mass number of 235. See also isotopic dating.