Mercury is a chemical element with the symbol Hg and atomic number 80. It is one of the few elements known since antiquity, used by multiple early civilizations because it occurs naturally in ores (especially cinnabar) and can be obtained by heating them. Archaeological evidence places human use of mercury compounds and liquid mercury thousands of years ago, including in ancient China, India, Egypt, and the Mediterranean world.
In the classical world, mercury was described by authors such as Theophrastus (c. 315 BCE) and later by Pliny the Elder (1st century CE), primarily in connection with cinnabar and pigment production. The element’s unusual liquid state at ordinary temperatures made it a subject of early natural philosophy and alchemy, where it was often treated as a “prime” metallic substance. While no single “discoverer” is credited, systematic study accelerated in early modern chemistry as quantitative experimentation became standard.
The name “mercury” comes from the Roman god Mercury, associated with speed and fluidity, reflecting the metal’s mobile, flowing character. The symbol “Hg” derives from the Latinized Greek name hydrargyrum (“water-silver”), a direct reference to its silvery appearance and liquid form. In the development of the periodic table of elements, mercury’s placement among the heavy metals helped clarify trends in metallic bonding, density, and reactivity across groups and periods.
Mercury played an outsized role in the history of measurement science, notably in the rise of the thermometer and barometer. Daniel Gabriel Fahrenheit popularized mercury-in-glass thermometers in the early 18th century, and mercury barometers became standard in physics and meteorology. By the 20th century, awareness of mercury’s toxicity changed its status from a “miracle liquid metal” to a tightly regulated hazardous substance.
Mercury is a dense, silvery, reflective metal that is liquid at standard conditions, a defining trait among metals. At standard temperature and pressure (STP), it is a liquid with a melting point of −38.83 °C and a boiling point of 356.73 °C. Its density is about 13.534 g/cm³ at 20 °C, making it more than 13 times as dense as water and denser than lead.
Mercury’s high density and surface tension produce distinctive behavior: droplets bead up and roll, and the liquid forms a convex meniscus in glass. It has a relatively low vapor pressure for a liquid metal, but mercury vapor is still significant enough at room temperature to pose inhalation risks. The metal’s coefficient of thermal expansion is large and predictable across a useful range, which historically supported its role in precision thermometry.
Electrical and thermal conductivities of mercury are lower than those of many solid metals, partly because it is liquid and because of its electronic structure in the heavy-atom regime. Its electrical resistivity at 20 °C is about 9.6 × 10−7 Ω·m, higher (worse conductivity) than copper by roughly an order of magnitude. Mercury’s liquid state also means it can form smooth electrical contacts and was once used in switches and relays where arcing needed to be controlled.
Mercury’s liquid phase at room temperature is linked to strong relativistic effects in its electron shells, which stabilize the 6s electrons and weaken metallic bonding compared with lighter congeners. The metal can wet some metals and amalgamate with them, affecting mechanical properties and corrosion behavior. These physical traits drive both its applications (e.g., dense working fluid) and its hazards (e.g., mobility and vapor formation).
Mercury exhibits chemistry dominated by oxidation states 0, +1, and +2, with +2 being most common in stable compounds. Its ground-state electron configuration is [Xe] 4f14 5d10 6s2, and the filled 5d shell and relativistically stabilized 6s electrons help explain its low tendency to oxidize compared with many metals. In air at room temperature, bulk mercury is relatively unreactive, though it can slowly form surface films in the presence of ozone or reactive sulfur species.
In aqueous and inorganic chemistry, Hg(II) forms many complexes and salts, including mercury(II) chloride (HgCl2), mercury(II) sulfide (HgS), and mercury(II) oxide (HgO). Hg(I) commonly exists as the dimeric cation Hg22+, as in calomel (Hg2Cl2), rather than as isolated Hg+. The strength and geometry of chemical bonds in mercury compounds often reflect soft-acid behavior, with strong affinity for soft bases such as sulfide (S2−) and thiol groups in organic molecules.
Mercury reacts readily with sulfur to form HgS, a reaction exploited for stabilizing spills using sulfur powder and for ore formation in nature. It also forms amalgams with many metals, including gold, silver, tin, and sodium, which can change hardness, melting behavior, and electrical properties; iron is a notable exception under many conditions, which historically enabled storage in iron containers. Mercury can be oxidized by strong oxidizing agents, and its salts can participate in redox and precipitation reactions important in analytical chemistry.
Organic mercury compounds, especially methylmercury (CH3Hg+), are chemically and biologically significant due to their ability to cross membranes and bind to proteins. Methylation can occur in aquatic environments through microbial processes, converting inorganic mercury into bioaccumulative forms. This environmental chemistry links mercury’s reactivity directly to long-term ecological and health impacts.
Mercury has been used in scientific instruments because it is dense, opaque, and remains liquid over a wide temperature range (−38.83 °C to 356.73 °C). Classic applications include mercury thermometers, barometers, manometers, diffusion pumps, and mercury seals, where its low wetting of glass and predictable expansion were advantageous. In metrology and laboratory practice, mercury columns enabled pressure measurement with high precision for centuries.
In electrical and industrial contexts, mercury was used in switches, tilt relays, and rectifiers (mercury-arc rectifiers) for converting AC to DC, particularly in the early and mid-20th century. Mercury-vapor lamps and fluorescent lighting rely on mercury to produce ultraviolet light that excites phosphors; typical fluorescent tubes historically contained on the order of a few milligrams of mercury, with many modern designs reducing content to meet regulations. Mercury also served as a cathode in the chlor-alkali process (the Castner–Kellner process) to produce chlorine and sodium hydroxide, though membrane and diaphragm cells have largely replaced it to reduce emissions.
Mercury’s ability to form amalgams once made it central to gold extraction in small-scale and artisanal mining: mercury binds gold, and heating drives off mercury to leave the metal behind. This practice can release large quantities of mercury vapor and contaminate waterways, prompting widespread efforts to phase it out. Historically, mercury amalgams were also used in dentistry for fillings; modern dental amalgam is typically about 50% mercury by mass combined with silver, tin, and copper, though usage has declined in some regions.
Specialized applications persist where mercury’s properties are uniquely useful, such as in certain reference electrodes (e.g., the calomel electrode) and in some high-precision scientific setups. However, due to toxicity and regulatory restrictions, substitutes (digital sensors, alcohol thermometers, aneroid barometers, and non-mercury lighting technologies) now dominate consumer and many professional markets. Contemporary use is therefore more niche, with strong emphasis on containment, recycling, and end-of-life handling.
Mercury is highly toxic, and the most significant risk for elemental mercury is inhalation of mercury vapor, which can be absorbed efficiently through the lungs. Acute exposure can cause respiratory irritation, cough, chest tightness, and chemical pneumonitis, while chronic exposure is associated with neurological symptoms such as tremor, irritability, memory problems, and sensory disturbances. Liquid mercury is poorly absorbed through intact skin, but spills are dangerous because they can produce invisible vapor and disperse into hard-to-clean droplets.
Inorganic mercury salts (such as HgCl2) are corrosive and toxic if ingested, affecting kidneys and the gastrointestinal tract. Organic mercury compounds, particularly methylmercury, pose severe neurodevelopmental risks and can bioaccumulate in fish and marine mammals, making dietary exposure a major concern. Pregnant people and young children are especially vulnerable, which is why public health guidance often limits consumption of high-mercury fish species.
Environmentally, mercury cycles between air, water, and soils, with both natural sources (volcanic activity, weathering) and anthropogenic sources (coal combustion, mining, industrial processes). Once deposited into aquatic systems, microbial methylation can convert inorganic mercury into methylmercury, which biomagnifies up food chains. This pathway was tragically illustrated by Minamata disease in Japan (identified in the 1950s), where industrial discharge led to severe neurological illness in local communities.
Safe handling involves minimizing vapor release, using sealed containers, adequate ventilation, and mercury spill kits designed to bind or capture droplets. Heating mercury or using household vacuums on spills is dangerous because it can dramatically increase airborne concentrations. Many countries restrict mercury in consumer products and require specialized hazardous-waste disposal; international coordination is addressed through agreements such as the Minamata Convention on Mercury (adopted in 2013) to reduce emissions and phase out specific uses.