Science glossary film: Isotopic mass

The mass of 1 atom of a specific isotope of an element. Each isotope of an element has a different number of neutrons, and therefore a different relative atomic mass. The atomic mass numbers quoted in the periodic table are a weighted average of these different isotopic masses. For example, copper's relative atomic mass is given as 63.546 because copper has two main naturally occurring isotopes – copper-63 and the less abundant copper-65.

Each isotope of an element has a different number of neutrons, and therefore a different relative atomic mass. The atomic mass numbers quoted in the periodic table are a weighted average of these different isotopic masses. For example, copper's relative atomic mass is given as 63.546 because copper has two main naturally occurring isotopes – copper-63 and the less abundant copper-65.