Mercury (element)

History

Mercury has been found in Egyptian tombs dating from 1500 BC, and cinnabar has been used since the Neolithic Age. In China and Tibet, mercury was believed to prolong life and heal fractures, though it is now known to cause serious health effects. The first emperor of a unified China, Qín Shǐ Huáng Dì, was reportedly killed by drinking a mercury and powdered jade mixture intended as an elixir of immortality. Khumarawayh ibn Ahmad ibn Tulun, a Tulunid ruler of Egypt, reportedly built a basin filled with mercury on which he would lie on air-filled cushions to be rocked to sleep.

Quantities of liquid mercury ranging from 90 to 600 g have been recovered from a chamber 60 feet below the Temple of the Feathered Serpent at Teotihuacan, Mexico. In Lamanai, a Maya city, a pool of mercury was found under a marker in a ballcourt, possibly used in bowls as mirrors for divination. Aristotle recounts that Daedalus made a wooden statue of Aphrodite move by pouring quicksilver inside it. The ancient Greeks used cinnabar in ointments, and the Egyptians and Romans used it in cosmetics.

By 500 BC, mercury was used to make amalgams with other metals. Alchemists considered mercury the First Matter from which all metals were formed, believing that varying the quality and quantity of sulfur within it could produce different metals, with gold being the purest.

Properties

Mercury is a dense, silvery-white metal that stays liquid at standard temperature and pressure, a property unique among metals. It conducts heat poorly but electricity reasonably well. Its melting point is -38.83 °C and its boiling point is 356.73 °C, the lowest for any stable metal, giving it the smallest liquid range under standard conditions. This happens because lanthanide contraction and relativistic contraction pull the outermost electrons closer, weakening the metallic bonds.

When mercury freezes, its volume shrinks by 3.59%, and its density jumps from 13.69 g/cm³ as a liquid to 14.184 g/cm³ as a solid. Its volume expansion coefficient ranges from 181.59 × 10⁻⁶ per °C at 0 °C to 182.50 × 10⁻⁶ per °C at 100 °C. Solid mercury is soft enough to cut with a knife and can be hammered or drawn into wires. Near its critical point, around 1750 K and 1720 bar, it shifts from metal to nonmetal, with sudden changes in how it conducts electricity and interacts with light.

Mercury was the first superconductor ever discovered, found in 1911 by Heike Kamerlingh Onnes when he chilled it below 4 K.

Industrial, technological, and scientific uses

Mercury is primarily used to manufacture industrial chemicals and for electrical and electronic applications. It is used in some liquid-in-glass thermometers, especially for high temperatures, and increasingly as gaseous mercury in fluorescent lamps. Thermometers containing mercury were invented in the early 18th century by Daniel Gabriel Fahrenheit, though earlier quicksilver-filled instruments were described in the 1650s. Historically, mercury sphygmomanometers, barometers, diffusion pumps, and coulometers took advantage of its dense, opaque, and nearly linearly expanding properties.

Since the early 21st century, mercury thermometers have been declining in use, and many jurisdictions have banned mercury-containing instruments following the 1998 Protocol on Heavy Metals. Gaseous mercury is used in mercury-vapor lamps, fluorescent lamps, and some neon signs, emitting narrow spectral lines used for calibration in optical spectroscopy. It is also found in electron tubes such as ignitrons, thyratrons, and mercury arc rectifiers, and in specialist medical care lamps for skin tanning and disinfection. Gaseous mercury is added to cold cathode argon-filled lamps to increase ionization and electrical conductivity.

Toxicity and safety

Liquid mercury is absorbed very poorly through intact skin and the gastrointestinal tract due to its physical properties and chemical inertness. Mercury vapor is the primary hazard of elemental mercury, so containers are securely sealed to prevent spills and evaporation. Heating mercury or its compounds should be done with adequate ventilation. The most toxic forms are organic compounds such as dimethylmercury and methylmercury, which can cause both chronic and acute poisoning.

Preindustrial mercury concentration in ice unaffected by volcanic activity was between 1 and 4 ng per liter. Volcanic eruptions and related natural sources account for about half of atmospheric mercury emissions. Outdoor urban air at the start of the 21st century had mercury concentrations of 0.01–0.02 μg/m³, while indoor levels in a New York study ranged from 0.0065 to 0.523 μg/m³, with an average of 0.069 μg/m³. Half of mercury emissions are attributed to human activities, with stationary combustion (especially coal-fired power plants) being the largest source at 65%.

Other sources include gold production (11%), non-ferrous metal production (6.8%), cement production (6.4%), waste disposal (3.0%), caustic soda production (3.0%), pig iron and steel production (1.4%), and mercury production (1.1%).

Did You Know?

• The chemical symbol Hg comes from hydrargyrum, a romanized form of the ancient Greek name hydrargyros, meaning 'water-silver'.
• Mercury is an extremely rare element in Earth's crust, with an average abundance of only 0.08 parts per million, making it the 66th most abundant element.
• All known mercury compounds exhibit oxidation states of I or II; experiments have failed to unequivocally demonstrate any higher oxidation states.