Glossary

Mercury (chemical symbol Hg) is a heavy, silvery-white metallic element that is unique among metals because it remains liquid at room temperature. It has an atomic number of 80, an atomic mass of 200.59, and belongs to Group 12 of the periodic table, along with zinc (Zn) and cadmium (Cd). The name “Hg” comes from its Latin name hydrargyrum, meaning “liquid silver.”

Mercury is dense, lustrous, and highly mobile, with a melting point of −38.83°C (−37.89°F) and a boiling point of 356.7°C (674°F). It is a poor conductor of heat compared to most metals but an excellent conductor of electricity, which historically made it useful in electrical switches, relays, and thermometers. Because of its ability to form amalgams—alloys with many metals such as gold, silver, and tin—mercury was once widely used in mining, dentistry, and manufacturing.

In the past, mercury had many industrial and scientific applications. It was used in thermometers, barometers, fluorescent lamps, and electrical components, and its vapor was key in early mercury-arc rectifiers and vacuum tubes. However, mercury and most of its compounds are now known to be highly toxic, capable of causing severe neurological and organ damage even at low levels of exposure. For this reason, most of its traditional uses have been phased out or tightly controlled.

Mercury naturally occurs in the Earth’s crust, primarily as the mineral cinnabar (mercury sulfide, HgS), which has a deep red color. It is extracted by heating cinnabar ore in air, which causes the sulfur to combine with oxygen and release elemental mercury vapor, later condensed into liquid form. Small amounts of mercury are also released naturally from volcanic activity and weathering of rocks.

Chemically, mercury is relatively unreactive. It does not oxidize readily in air at room temperature but forms compounds like mercuric oxide (HgO) and mercuric chloride (HgCl₂) under heat or chemical reaction. Mercury can exist in two common ionic states: Hg⁺¹ (mercurous) and Hg⁺² (mercuric), leading to a variety of inorganic and organic compounds—many of which are environmentally persistent and biologically harmful, such as methylmercury (CH₃Hg⁺).

Today, mercury use is heavily restricted due to its toxicity and environmental persistence. Modern technology has largely replaced it with safer alternatives in thermometers, electrical switches, and batteries. However, it still finds limited use in specialized applications such as scientific instruments, dental amalgams, and fluorescent lighting—though even these are declining.