What defines an isotope of an element?

An isotope of an element is defined by having the same number of protons but a different number of neutrons. This distinction is crucial because protons determine the identity of the element—meaning that all isotopes of a particular element share the same atomic number and thus exhibit the same chemical properties. However, the variation in the number of neutrons results in different atomic masses for the isotopes of that element. For example, carbon has isotopes like carbon-12 and carbon-14, where both have six protons, but carbon-12 has six neutrons while carbon-14 has eight. This difference in neutrons is what defines them as isotopes.

Other choices do not accurately describe what constitutes an isotope. Variations in the number of protons signify different elements altogether, while atomic mass is not the same across isotopes because it is influenced by the number of neutrons. Therefore, the core characteristic of isotopes lies in the variation of neutrons while maintaining the same number of protons.