Isotopes

    Nuclides of the same element possess the same number of protons and electrons but may have different mass numbers; the number of protons and electrons defines the element but the number of neutrons may vary. Nuclides of a particular element that differ in the number of neutrons and, therefore, their mass number, are called isotopes .

Isotopes of some elements occur naturally while others may be produced artificially.  Elements that occur naturally with only one nuclide are monotopic and include phosphorus, ³¹₁₅P, and fluorine, ¹⁹₉F. Elements that exist as mixtures of isotopes include C (¹²₆C and ¹³₆C) and O (¹⁶₈O, ¹⁷₈O and ¹⁸₈O). Since the atomic number is constant for a given element, isotopes are often distinguished only by stating the atomic masses, e.g. ¹²C and ¹³C.