Isotopes. Illustration of the structures of the nuclei of two isotopes each of carbon (bottom) and hydrogen (top). The nuclei are made up of nucleons (balls). There are two types of nucleon: protons (grey) and neutrons (white). Isotopes, chemically- identical variants of an element, vary only in the number of neutrons in their nuclei, and are named for their nucleon number. Hydrogen-1 (upper left) and carbon-12 (lower left) each make up over 98% of these elements. Carbon-13 (lower right) has an overall nuclear spin (unlike carbon-12), and so is used in nuclear magnetic resonance spectroscopy. Hydrogen-2 (deuterium, upper right) occurs in the heavy water used to shield nuclear reactors.