An isotope is a form of a chemicalelementwhose atomicnucleuscontains a specific number ofneutron's, in addition to the number ofprotons that uniquely defines the element. The nuclei of mostatom's contain neutrons as well as protons. Every chemical element has more than one isotope. For any element, one of the isotopes is more abundant in nature than any of the others, although often multiple isotopes of a single element are mixed.
The isotope of an element is defined by the nucleon number, which is the sum of the number of protons and the number of neutrons in the atomic nucleus. The nucleon number is customarily written as a superscript preceding the chemical symbol for the element.