Sodium chloride. Computer graphic of crystallised common salt (sodium chloride), showing a crystal plane (at left) which forms part of sodium chloride's cubic crystal (at right). Small spheres represent positively-charged sodium ions, and large spheres are negatively-charged chloride ions. Sodium chloride crystals consist of rows of such planes, each arranged so that the chloride ion in one plane fits over the sodium ion in the one beneath it. Each positive ion therefore has six negatively-charged nearest neighbours, and vice versa. The electrostatic attraction holding these nearest neighbours together makes the crystal strong but brittle.