SPL Substance Indexing Terminology Element Isotope A form of a chemical element containing a specific number of neutrons and a specific atomic mass. isotope C47933 A form of a chemical element in which the atoms have the same number of protons (part of the nucleus of an atom) but with a different number of neutrons (part of the nucleus of an atom). For example, carbon 12, carbon 13, and carbon 14 are isotopes of carbon. They all have six protons in the nucleus, but each has different number of neutrons. Isotopes may be used in certain medical tests and procedures. Isotope ISOTOPE FDA C0022262 Element, Ion, or Isotope Isotope Isotope FDA Structured Product Labeling Terminology