What defines an ion?

An ion is defined as an atom that has gained or lost electrons, leading to a net electric charge. When an atom loses electrons, it becomes positively charged and is referred to as a cation. Conversely, when an atom gains electrons, it obtains a negative charge and is called an anion. This alteration in the number of electrons compared to protons—the particles responsible for the positive charge—results in the creation of ions, which play crucial roles in various chemical processes, including reactions in solutions, electrical conductivity, and the formation of compounds.

The other definitions presented do not accurately capture what constitutes an ion. For instance, possessing a full electron shell does not inherently mean an atom is an ion since such atoms are generally neutral. The implication of losing protons is more indicative of a different nuclear process and does not pertain to ionic formation. Additionally, being in a gaseous state does not define an atom as an ion, as both ions and neutral atoms can exist in various states of matter.

Thus, the characteristic that sets ions apart is the change in their electron count, confirming the definition as an atom that has gained or lost electrons.