An ion is best defined as an atom or a group of atoms that have an electrical charge. This occurs when an atom gains or loses one or more electrons, resulting in a positive charge (cation) if electrons are lost, or a negative charge (anion) if electrons are gained.
The other descriptions you provided are not accurate for defining an ion:
- An atom that can only be found in a gaseous state does not define ions, as ions can exist in gaseous, liquid, or solid states.
- An atom that has the same number of electrons as protons is a neutral atom, not an ion.