Ionization
Ionization, formation of electrically charged atoms or molecules. Atoms are electrically neutral; the electrons that bear the negative charge are equal in number to the protons in the nucleus bearing the positive charge. When sodium combines with chlorine, for example, to form sodium chloride, each sodium atom transfers an electron to a chlorine atom, thus forming a sodium ion with a positive charge and a chloride ion with a negative charge. In a crystal of sodium chloride the strong electrostatic attraction between ions of opposite charge holds the ions firmly in place and close together. When sodium chloride is melted, the ions tend to dissociate because of their thermal motion and can move about freely. If two electrodes are placed in molten sodium chloride and an electric potential is applied, the sodium ions migrate to the negative electrode and the chloride ions migrate to the positive electrode, causing a current of electricity to flow. When sodium chloride is dissolved in wate