Ions are electrically-charged particles that are formed when atoms or molecules gain or lose electrons. They are the smallest particles of an element that still retain its chemical properties. Ions can be either positively charged (cations) or negatively charged (anions). Positively charged ions are formed when atoms or molecules lose electrons, while negatively charged ions are formed when atoms or molecules gain electrons. Ions play an important role in many biological processes, such as cell signaling, muscle contraction, and nerve impulse transmission.