What defines an ion?

An ion is defined as an atom or molecule that carries a net electrical charge, which occurs when the number of protons and electrons in it is not equal. When an atom loses one or more electrons, it becomes a positively charged ion (cation), while gaining electrons leads to a negatively charged ion (anion). This phenomenon is foundational in understanding chemical reactivity and bonding, as the presence of charged particles allows for interactions with other ions and molecules, thereby influencing the behavior of substances in a chemical system.

The other choices do not capture the defining feature of an ion. A balanced number of protons and electrons signifies a neutral atom, while the specific number of neutrons pertains to isotopes and does not relate to charge. The concept of sharing electrons is part of covalent bonding, not ion formation. Therefore, the choice that correctly identifies an ion is the one that emphasizes the presence of a net electrical charge.