The correct answer is isotopes. Isotopes are different forms of an element that have the same number of protons but different numbers of neutrons. Ions, compounds, and molecules are not forms of an element but rather different concepts. Ions are charged particles that form when atoms gain or lose electrons. Compounds are substances made up of atoms of two or more different elements bonded together. Molecules are the smallest unit of a compound that still retains the chemical properties of that compound.
