Salt crystal: NaCl or sodium chloride. Salt forms an ionic crystal consisting of vast arrays of positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). Each ion is surrounded by 6 ions of opposite charge. The oppositely charged ions lie on axes at right angles to each other (like the x, y, and z axes in geometry). This arrangement maximises the contact between oppositely charged ions and minimises the contact between similarly charged ions. It is a clear crystalline substance that dissolves readily in water. Also known as: common salt, rock salt, halite, evaporite (when it is left after salt water has evaporated), and brine (when dissolved in water).