The answer to the crossword puzzle is 'IONS'. Ions are atoms or molecules that have a net electric charge due to the loss or gain of one or more electrons. They are formed when an atom or molecule gains or loses an electron, creating a charged particle. Ions can be either positively charged (cations) or negatively charged (anions). Ions play an important role in many chemical and biological processes, such as the transport of nutrients and waste products in cells.