When atoms of the same element have different numbers of neutrons, what are they called?

When atoms of the same element have different numbers of neutrons, they are identified as isotopes. Isotopes share the same number of protons and thus belong to the same element but differ in their atomic mass due to the variation in neutron count. This difference can influence certain physical properties, such as stability and nuclear behavior, but it does not change the chemical identity of the element itself.

The other terms provided refer to different concepts in chemistry. Elements are pure substances consisting of only one type of atom, defined by their atomic number. Molecules are groups of atoms bonded together, which can be from the same or different elements. Compounds are specific types of molecules that contain at least two different elements bonded together chemically. Therefore, isotopes specifically address the variations in neutron numbers among atoms of the same element, making it the correct term for this question.