Mercury (Hg) is a naturally occurring element, historically nicknamed “quicksilver.” It is the only metal found in liquid form at standard temperature and pressure. This fluidity, high density, and metallic luster made it valuable in industrial and medical applications for centuries. Its volatility and toxicity pose significant environmental and health concerns.
Elemental Mercury: The Liquid Metal Identity
Elemental mercury is a heavy, silvery-white substance with an atomic number of 80. Unlike most metals, mercury’s melting point is exceptionally low, freezing at approximately \(-38.8^{\circ}\text{C}\). This unusual characteristic is explained by the principles of special relativity. For heavy elements, the innermost electrons move rapidly, causing a relativistic effect that holds the outer electrons more tightly to the nucleus. These outer electrons are less available to form the strong metallic bonds typical of solid metals. The weak forces remaining between mercury atoms are easily overcome by ambient thermal energy, resulting in its liquid state at room temperature.
Unique Physical and Chemical Characteristics
The primary physical characteristic of elemental mercury is its extreme density, measuring around \(13.5 \text{ g/cm}^3\). For perspective, heavy materials like iron or lead will float on its surface. Mercury also exhibits high surface tension, causing small amounts to bead up into nearly perfect spheres.
Chemically, mercury forms alloys called amalgams with many other metals, including gold, silver, and zinc. This property was historically used in dentistry for fillings, where dental amalgam is typically about 50% elemental mercury by weight. While it is a fair conductor of electricity, mercury is a relatively poor conductor of heat compared to most other metals.
A property contributing to its danger is its high vapor pressure, meaning it evaporates easily even at room temperature. Invisible mercury vapor is constantly released from exposed liquid mercury, which is the primary route by which elemental mercury becomes a toxic threat.
Primary Health Risks and Toxicity
The health risks associated with mercury depend entirely on its chemical form and the route of exposure. Elemental liquid mercury (Hg°) is poorly absorbed through the skin or the digestive tract, with less than \(0.01\%\) being absorbed if swallowed. The primary danger comes from inhaling the invisible mercury vapor (Hg°), which is absorbed by the lungs at a rate of approximately \(80\%\).
Once inhaled, the vapor is lipid-soluble and rapidly crosses the blood-brain barrier and the placenta. Inside the body, it converts into an oxidized, ionic form that accumulates in the brain and kidneys. Chronic exposure, even at low levels, leads to neurological damage, causing symptoms such as tremors, memory loss, and coordination problems.
A different, more potent form is methylmercury (MeHg), an organic compound formed by bacteria in aquatic environments. MeHg is the most toxic form because it is readily absorbed after ingestion and easily crosses the blood-brain barrier and the placenta. This form is particularly concerning for developing fetuses and children, as it causes severe, permanent damage to the central nervous system.
Modern Sources of Exposure
Exposure to elemental mercury remains possible despite reduced usage. Historical sources like old thermometers, barometers, and thermostats still contain liquid mercury in sealed glass containers. When these devices break, they release the liquid mercury and its toxic vapor into the immediate environment.
The largest environmental releases come from industrial activities, particularly coal-fired power plants and certain manufacturing processes. These facilities release mercury into the atmosphere, where it eventually deposits into water bodies. Once in the water, it converts to methylmercury and bioaccumulates in the food chain, making contaminated fish and shellfish the main source of exposure for the general public.
Spilled liquid mercury must be treated as hazardous waste and never vacuumed or swept, as this increases the release of toxic vapor. Specialized mercury-absorbing sponges or powders are used for cleanup, and all resulting waste must be collected and disposed of through appropriate governmental or commercial hazardous waste programs.