What distinguishes an ion from a neutral atom?

An ion is distinguished from a neutral atom by the number of electrons. In a neutral atom, the number of electrons is equal to the number of protons, resulting in a net charge of zero. However, when an atom loses or gains electrons, it becomes charged, thus forming an ion. If an atom loses one or more electrons, it has more protons than electrons, becoming positively charged (a cation). Conversely, if an atom gains electrons, it has more electrons than protons, resulting in a negative charge (an anion). The fundamental difference lies in this electron imbalance, making the correct distinction the number of electrons.