By John Brennan, Updated Mar 24, 2022
Every atom of an element shares the same number of protons, but its isotopes differ in the number of neutrons within the nucleus. For instance, hydrogen’s nucleus contains one proton, while the isotope deuterium adds a neutron. The mass number of an isotope—the sum of its protons and neutrons—provides a quick estimate of its mass. Because nuclear binding energy slightly reduces the true mass, the exact atomic weight is only obtainable through experiment.
Write down the element’s proton number, which equals its atomic number on the periodic table. Carbon, for example, has an atomic number of 6, so its nuclei contain six protons.
Neutron numbers vary between isotopes. Carbon‑13, for instance, has seven neutrons.
Sum the protons and neutrons to obtain the isotope’s mass number. Carbon‑13’s mass number is 13. While the binding energy causes a minute deviation from this nominal value, the mass number is sufficient for most calculations.
For the exact atomic weight, consult the National Institute of Standards and Technology (NIST) Atomic Weights database: NIST Atomic Weights. This experimentally determined figure is the most reliable source.
