An ion is defined as an atom that has had a(n) _____ added or removed.

An ion is an atom that has acquired a positive or negative charge due to the loss or gain of electrons. When an atom loses one or more electrons, it becomes a positively charged ion, known as a cation. Conversely, if it gains electrons, it becomes a negatively charged ion, called an anion.

The role of electrons is crucial in determining the electrical charge of an atom, whereas protons reside in the nucleus and define the element itself; changing the number of protons would result in a different element entirely. Neutrons also do not affect the charge of the atom, as they are neutral and primarily influence the atom's stability and isotopic identity. The term "atom" refers to the entire entity, which includes protons, neutrons, and electrons, but does not refer to the process of forming ions. Therefore, adding or removing electrons is what specifically defines an ion.