The addition or removal of which subatomic particle results in the formation of an ion?

An ion is formed when there is a change in the balance of electrons in an atom. Electrons carry a negative charge, and when electrons are added to an atom, it becomes negatively charged, resulting in an anion. Conversely, when electrons are removed, the atom loses its negative charge and becomes positively charged, forming a cation. Since ions are defined by their charge, and this charge change directly results from the addition or removal of electrons, this makes it clear that the correct answer involves electrons.

Protons, while also subatomic particles, are found in the nucleus and contribute to the overall positive charge of the atom. Changing the number of protons alters the type of element and its identity, rather than simply forming an ion. Neutrons, being neutral particles also reside in the nucleus, and their change results in isotopes, which do not affect the charge of the atom. Photons are not subatomic particles in the sense of contributing to the structure of an atom; they are quantum of light and interact with electrons but do not cause the formation of ions. Hence, the role of electrons in creating ions is crucial and directly relevant to understanding atomic structure and charge.