Final answer:
An element that has different masses is called an isotope. Isotopes are atoms of the same element that contain different numbers of neutrons, resulting in different mass numbers.
Step-by-step explanation:
An element that has different masses is called an isotope. Isotopes are atoms of the same element that contain different numbers of neutrons, resulting in different mass numbers. For example, carbon-12 and carbon-13 are isotopes of carbon, with mass numbers of 12 and 13, respectively.