Hg0

Hg0 denotes elemental mercury in the zero oxidation state. In chemical notation, Hg0 refers to neutral mercury atoms; in atmospheric chemistry, Hg0 designates gaseous elemental mercury (Hg(g)), the dominant form of mercury in the air under typical conditions. Elemental mercury is released by the evaporation of mercury-containing materials and by various industrial processes, and it can occur as a liquid metal at room temperature or as mercury vapor at ambient temperatures. Its volatility enables Hg0 to be transported over long distances before oxidation and deposition remove it from the atmosphere.

Physical properties and behavior: Mercury has atomic number 80 and atomic weight 200.59 g/mol. It melts at -38.83 °C and boils at 356.73 °C. The liquid metal is dense and an excellent conductor of electricity. In air, Hg0 is relatively persistent but can be oxidized to divalent mercury species (Hg2+), either in the gas phase or on particle surfaces, which then deposit to soils and waters.

Environmental and health significance: Inhalation of Hg0 vapor is toxic, particularly to the nervous system, with chronic exposure risks. As a transport form, Hg0 carries mercury from emission sources to distant ecosystems, where microbial processes can convert it to methylmercury, a more toxic, bioaccumulative compound that concentrates in fish and wildlife.

Uses and regulation: Historically used in thermometers, barometers, dental amalgams, and gold-mining practices. Due to health and environmental concerns, intentional use of mercury is increasingly restricted and regulated, and monitoring of atmospheric Hg0 is part of air-quality programs.