Atoms of the same element with different mass numbers are called isotopes. Isotopes are atoms of the same element that have the same number of protons (which determines the element), but they differ in the number of neutrons in their nuclei. As a result, isotopes have different mass numbers, which is the sum of protons and neutrons in an atom's nucleus. Isotopes of an element exhibit similar chemical properties but may have different physical properties due to their varying atomic masses.