A crystalline bond in which one or more electrons are transferred between the component atoms, allowing them to become ionized. The crystal retains overall charge neutrality, and the ions are arranged by electrostatic attraction in a regular array to minimize the energy of the system of ions and form the highest packing density. An example is sodium chloride, NaCl, in which the sodium atoms lose a valence electron to achieve a stable electronic configuration, and become positive ions, Na⁺, and the chlorine atoms gain an extra valence electron to achieve a stable electronic configuration, and become negative ions, Cl⁻; the crystal contains equal parts of sodium and chlorine to retain charge neutrality, and the ions are arranged in a regular array.