Which statement describes isotopes?

• A. Atoms with the same number of protons but different numbers of neutrons
• B. Atoms with the same number of neutrons but different numbers of protons
• C. Atoms with different numbers of protons
• D. Atoms with the same number of protons and neutrons

Answer: (A)

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. The number of protons defines what element it is, so both atoms are the same element, but changing the neutrons changes the mass while leaving the charge (and therefore the electron arrangement) the same. That’s why carbon-12 and carbon-14 are isotopes of carbon: each has 6 protons, but carbon-12 has 6 neutrons and carbon-14 has 8 neutrons, giving different masses and sometimes different stability. The other ideas would describe different elements (different protons) or the exact same nucleus (same protons and neutrons), which aren’t what isotopes are.