What is an isotope?

An isotope refers specifically to variants of a chemical element that have the same number of protons but differ in the number of neutrons within their atomic nuclei. This means that isotopes of an element share the same atomic number, which defines the element itself, but will have different mass numbers due to the variation in neutron count.

For instance, both carbon-12 and carbon-14 are isotopes of carbon. They both contain six protons (defining them as carbon) but differ in their neutron counts—carbon-12 has six neutrons, while carbon-14 has eight. This differentiation is significant because it affects the stability and radioactive properties of the isotopes, leading to various applications in fields such as medicine, archaeology (carbon dating), and nuclear science.

The other choices do not accurately define isotopes. Atoms of different elements do not qualify as isotopes because they have different numbers of protons, thus different atomic identities. Atoms with no neutrons cannot be isotopes, as isotopes require at least one neutron. Atoms with varying protons and electrons represent different elements and ions rather than isotopes. Thus, the definition of isotopes is exclusively related to the number of neutrons while keeping the proton count constant.