What is an ion?

• A. An electrically neutral particle formed when an atom gains or loses electrons
• B. A molecule that contains two atoms
• C. A charged entity formed when an atom gains or loses electrons
• D. A gas consisting of ions

Answer: (C)

An ion is a charged particle formed when an atom gains or loses electrons. When electrons are removed, the atom becomes positively charged (a cation); when electrons are added, it becomes negatively charged (an anion). The idea that an ion is neutral is incorrect because gaining or losing electrons changes the balance of charge. A diatomic molecule is simply two atoms bonded together and may be neutral. A gas consisting of ions describes a plasma state, which is a specific condition, not the general definition of an ion. So the essential point is that an ion carries a net electric charge due to electron transfer, such as Na+ or Cl-.