What are different atoms of the same element with varying numbers of neutrons called?

Atoms of the same element that have differing numbers of neutrons are referred to as isotopes. This term specifically describes variations in the atomic mass of an element while maintaining the same number of protons, which defines the element itself. For example, carbon has isotopes such as carbon-12 and carbon-14; the numbers indicate the total number of protons and neutrons in the nucleus, leading to different atomic masses.

In contrast, compounds consist of two or more different elements chemically bonded together, and therefore do not apply to variations within a single element. Allotropes refer to different forms of the same element in the same physical state, such as graphite and diamond in the case of carbon. Lastly, the term "elements" simply categorizes substances on the periodic table, and does not imply any variation in neutron count. Thus, isotopes are the correct terminology for atoms of the same element that differ in neutron number.