What defines an isotope?

An isotope is defined as variations of a particular chemical element that have the same number of protons but different numbers of neutrons. The number of protons in an atom determines which element it is, as this is related to the atomic number. The difference in the number of neutrons leads to different atomic masses for these isotopes, but they still retain the same chemical properties because they are still the same element.

For example, carbon has several isotopes, including carbon-12, which has six protons and six neutrons, and carbon-14, which has six protons but eight neutrons. Both are forms of carbon, hence are isotopes of the same element.

While options that mention the same number of protons but different numbers of electrons relate to ions, and those that refer to the same number of neutrons but different numbers of protons would indicate different elements, the correct definition centered around the relationship of protons and neutrons is what solidifies the concept of isotopes in atomic physics and chemistry.