Mercury (Hg on the periodic table) is a naturally occurring chemical element that is found in air, water, and soil. It exists in several forms: elemental or metallic mercury, inorganic mercury compounds, and organic mercury compounds. Mercury circulates through the environment in different chemical forms and different physical states. In its inorganic form, it exists in three oxidation states: elemental mercury (Hg0), monovalent mercury (Hg+1), or divalent mercury (Hg+2).
Inorganic mercury compounds take the form of mercury salts (powder or crystal form) and are usually white, except for cinnabar, which is red. Inorganic mercury is mined as cinnabar ore, which contains mercuric sulfide. By heating the cinnabar ore to temperatures above 1,000 °F (538 °C), the mercury in the ore is vaporized to its metallic form, or elemental mercury. The vapors are then cooled to form liquid elemental mercury.
In its elemental form, mercury is a very dense, shiny, silvery metal that is liquid at room temperature. Elemental mercury conducts electricity and expands and contracts in response to changes in temperature or pressure. These unique properties of mercury have made it useful in a variety of products (e.g., thermometers, barometers, switches, and lamps).
Organic mercury compounds are formed when inorganic mercury combines with carbon. Methylmercury (CH3Hg+) is the most common form of mercury in the environment, and it is the form that is found in fish. Methylmercury is formed by bacteria in soil or water.