Which term refers to different versions of the same element that have the same atomic number but different mass numbers?

The term that refers to different versions of the same element that have the same atomic number but different mass numbers is isotopes. Isotopes are variants of a given chemical element that have the same number of protons (which defines the element and gives it its atomic number) but have different numbers of neutrons in their nuclei. This variation in neutron count results in different mass numbers for the isotopes of that element.

For instance, carbon has several isotopes, including carbon-12 (with 6 protons and 6 neutrons) and carbon-14 (with 6 protons and 8 neutrons). Both isotopes are carbon due to having the same atomic number (6) but differ in mass because of the different neutron counts.

Allotropes refer to different structural forms of the same element in the same physical state, such as graphite and diamond, which are both forms of carbon. Compounds are substances formed when two or more elements chemically bond in fixed proportions, and mixtures consist of two or more substances that are combined physically but not chemically.