What do you call atoms of the same element that have different numbers of neutrons?

Atoms of the same element that have different numbers of neutrons are referred to as isotopes. An isotope retains the same number of protons, which defines the element, but varies in the number of neutrons, resulting in different atomic masses. For instance, carbon typically has 6 neutrons, but carbon-14 has 8 neutrons, making it an isotope of carbon.

In contrast, other terms like ions refer to atoms that have gained or lost electrons and thus have a net charge, while molecules are groups of two or more atoms bonded together, regardless of whether they are the same or different elements. An alloy is a mixture of metals, not a variation of neutrons in atoms of the same element. Hence, the term "isotope" specifically describes the phenomenon of varying neutron numbers among atoms of the same element.