Isotopes are atoms of the same element with different numbers of neutrons.

• A. Isotopes
• B. Isotones
• C. Isomers
• D. Ions

Answer: (A)

Isotopes are atoms that belong to the same element, so they have the same number of protons, but they differ in the number of neutrons. That difference in neutrons changes the atom’s mass without changing the element’s identity, which is why carbon-12 and carbon-14 are both carbon even though they have different masses. The chemical properties stay largely the same because those properties are governed mainly by the electron cloud, which is determined by the number of protons.

Other terms don’t fit because isotones are defined by having the same number of neutrons but different numbers of protons, isomers refer to different structural forms or energy states of the same formula, and ions are atoms that have gained or lost electrons, giving a net charge.