An ion is

• A. A neutral particle
• B. Formed by changing the number of protons
• C. Formed by gaining or losing electrons
• D. Cannot be formed from metals

Answer: (C)

The main idea is that ions are charged particles formed when the electron count is changed, not when the number of protons changes. Electrons carry negative charge, so removing electrons leaves a positive charge (a cation) and adding electrons leaves a negative charge (an anion). The identity of an element is set by the number of protons, so changing protons would create a different element entirely, not an ion. That’s why an ion is formed by gaining or losing electrons. A neutral particle describes a neutral atom or molecule, which isn’t an ion. And metals can indeed form ions by losing electrons (cations), so saying ions can’t be formed from metals isn’t correct. For example, sodium can lose an electron to become Na+, and chlorine can gain an electron to become Cl−.