Mercury (Hg) is a naturally occurring 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. Elemental or metallic mercury is a shiny, silver-white metal and is liquid at room temperature.
It is used in thermometers, fluorescent light bulbs and some electrical switches. When dropped, elemental mercury breaks into smaller droplets which can go through small cracks or become strongly attached to certain materials. At room temperature, exposed elemental mercury can evaporate to become an invisible, odorless toxic vapor. People can be exposed to elemental mercury vapor when products that contain mercury break and expose mercury to the air, particularly in poorly-ventilated spaces.
Inorganic mercury compounds take the form of mercury salts and are generally white powder or crystals, with the exception of mercuric sulfide (cinnabar) which is red. Inorganic mercury compounds have been included in products such as fungicides, antiseptics or disinfectants. Some skin lightening and freckle creams, as well as some traditional medicines, can contain mercury compounds.
Organic mercury compounds, such as methylmercury, are formed when mercury combines with carbon. Microscopic organisms convert inorganic mercury into methylmercury, which is the most common organic mercury compound found in the environment.
Mercury is found throughout our universe. It has been identified on the sun and found in meteorites and moon rocks. On the Earth, natural deposits of mercury are generally found as cinnabar (HgS), a vermilion red ore that is up to 86% mercury. Cinnabar deposits are relatively rare, and are known to occur in association with hot springs and recent volcanic rocks, suggesting a mantle or deep crustal origin. Mercury is 67th in natural abundance in crustal rocks, with concentrations of 0.2 parts per million (ppm) found in granite and less than 0.1 ppm in other crustal rocks.
Like cadmium, zinc and lead, mercury is known as a "heavy metal" and can be toxic to living organisms. The element's atomic mass is 200.59 grams per mole and its specific gravity is 13.5 times that of water. Mercury has a melting point of -38.9°C, a boiling point of 357.3°C, and is the only metal to remain in liquid form at room temperature. Droplets of liquid mercury are shiny and silver-white with a high surface tension, appearing rounded when on flat surfaces. The liquid is highly mobile and droplets combine easily due to low viscosity. The element also combines with other metals such as tin, copper, gold and silver to form mercury alloys known as amalgams. Fortunately, mercury does not form amalgam with iron, which allows for the element to be shipped in standard iron flasks containing 76 pounds, or 34.5 kilograms, of liquid mercury. Mercury has a relatively high vapor pressure and the highest volatility of any metal, vaporizing to become a colorless, odorless gas. The metal is a fair conductor of electricity, but a poor conductor of heat.
Mercury's atomic number is 80. In nature, mercury has 3 possible conditions of electrical charge, or valence states. Elemental mercury (Hg0) has no electric charge. Mercury is also found in two positively charged, or cationic, states, Hg2+ (mercuric) and Hg1+(mercurous). The mercuric cation is more stable and is generally associated with inorganic molecules, such as sulfur (in the mineral cinnabar), chlorine (mercuric chloride), oxygen and hydroxyl ions. Hg2+ is also found in organic (carbon based) substances like dimethylmercury (Me2Hg), which is far more toxic than inorganic forms of mercury and bioaccumulates in the tissues of living organisms. Since mercury can be adsorbed easily onto small particles of matter, some scientists are beginning to use the notation Hg(p) to represent elemental mercury attached onto or absorbed into a particle.
Because it is an element, mercury is not biodegradable. It is converted among its various forms through a range of abiotic and biogeochemical transformations and during atmospheric transportation. Once mercury compounds enter an ecosystem, they do not break down easily and persist in the environment.