What are isotopes?

Isotopes are defined as atoms of the same element that have different numbers of neutrons. This means that while the atomic number (which is the number of protons and defines the element) remains constant, the mass number (which is the total number of protons and neutrons) varies due to the differing neutron counts. For instance, carbon has isotopes such as carbon-12 and carbon-14; both have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8. This variation in neutrons leads to differences in stability and radioactivity among isotopes of the same element, making them significant in fields like nuclear physics and medicine.

In contrast, the other options refer to different concepts that do not accurately define isotopes. Atoms of different elements would have different atomic numbers, directly contradicting the definition of isotopes. Atoms with the same mass number could be either isotopes of the same element or different elements, so this description is not specific enough. Atoms with an equal number of protons and electrons refer to neutral atoms, which does not encompass the concept of isotopes.