This module is intended to introduce students to the use of the periodic table to customize the properties of materials. The properties of light emitting diodes (LEDs) are controlled by chemistry. LEDs are increasingly used in a variety of lighting venues, including traffic lights, animated billboard displays, vehicle tail lights and dashboard lights. They are comprised of semiconductors whose chemical composition can be tuned using periodic properties, such as isoelectronic principles, electronegativity, and atomic radius. Variations in these properties can be used to control the color of light the semiconductors emit. Solid solutions, in which chemically similar elements like phosphorus and arsenic are substituted for one another in a crystal structure, enable the to light emitted by the semiconductor to be tuned over various regions of the electromagnetic spectrum.