Which term describes an ion that has a negative charge?

The term that describes an ion with a negative charge is "anion." Anions are formed when an atom or molecule gains one or more electrons, resulting in a net negative charge. This is in contrast to cations, which carry a positive charge because they lose one or more electrons. The process of gaining electrons allows anions to achieve a more stable electronic configuration, often resembling that of noble gases.

The other terms listed have different meanings. Isotopes refer to atoms of the same element that have different numbers of neutrons, leading to different mass numbers, but they do not relate to charge. Radicals are species that contain unpaired electrons, making them highly reactive, but they can be neutral, positively charged, or negatively charged depending on the specific species. The accurate identification of "anion" for an ion with a negative charge comes from understanding these basic definitions in chemistry.