Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. It is a naturally occurring, slightly radioactive metal that is found in soil, rocks, water, plants, and animals. Thorium is light silver and tarnishes olive gray when exposed to air, forming thorium dioxide. It is moderately soft and malleable and has a high melting point. Thorium is an electropositive actinide whose chemistry is dominated by the +4 oxidation state; it is quite reactive and can ignite in air when finely divided. Thorium is much more abundant in nature than uranium and can be used as a nuclear fuel through breeding to uranium-233 (U-233). Thorium oxide (ThO2), also called thoria, has one of the highest melting points of all oxides (3300°C) and has found applications in light bulb elements, lantern mantles, arc-light lamps, welding electrodes, and heat-resistant ceramics. Glass containing thorium oxide has both a high refractive index and wavelength dispersion and is used in high-quality lenses for cameras and scientific instruments. Thorium has been used in lantern mantles for the brightness it imparts and in welding rods, which burn better with small amounts of added thorium. Thorium dioxide was used as a contrast agent (called Thorotrast) in medical radiology until the 1950s. Thorium is a fertile material, which means it can be used to fuel a nuclear chain reaction that can run a power plant and make electricity. When exposed to neutrons, thorium undergoes a series of nuclear reactions until it eventually emerges as an isotope of uranium called U-233, which will readily split and release energy next time it absorbs a neutron.