Which term describes atoms of the same element that have different numbers of neutrons?

The term that describes atoms of the same element that have different numbers of neutrons is isotopes. Isotopes retain the same number of protons, which defines the element, but vary in the number of neutrons, leading to differences in atomic mass. For instance, carbon has isotopes like Carbon-12 and Carbon-14, where both have 6 protons but differ in their neutron count (6 and 8 neutrons, respectively). This principle is fundamental in understanding nuclear chemistry, radioactive decay, and various applications in fields such as medicine and archaeology.

The other options refer to different concepts: ions are atoms or molecules that have gained or lost electrons and thus carry a charge, molecules are combinations of two or more atoms held together by covalent bonds, and the term "elemental variants" is not a standard term used in chemistry to describe different forms of an element.