Mercury (element)

iso	NA	half-life	DM	DE M eV	DP
¹⁹⁴Hg	{syn.}	444 y	ε	0.040	¹⁹⁴Au
¹⁹⁶Hg	0.15%	Hg is stable with 116 neutrons
¹⁹⁸Hg	9.97%	Hg is stable with 118 neutrons
¹⁹⁹Hg	16.87%	Hg is stable with 119 neutrons
²⁰⁰Hg	23.1%	Hg is stable with 120 neutrons
²⁰¹Hg	13.18%	Hg is stable with 121 neutrons
²⁰²Hg	29.86%	Hg is stable with 122 neutrons
²⁰⁴Hg	6.87%	Hg is stable with 124 neutrons

SI units & STP are used except where noted.

Mercury, also called quicksilver, is a chemical element in the periodic table that has the symbol Hg (from the Latin hydrargyrum) and atomic number 80. A heavy, silvery, transition metal, mercury is one of only two elements that are liquid at room temperature (the other is bromine). Mercury is used in thermometers, barometers and other scientific apparatuses. Mercury is mostly obtained by reduction from the mineral cinnabar.

Notable characteristics

Mercury is a relatively poor conductor of heat but is a good conductor of electricity.

Mercury easily forms alloys with almost all common metals, including gold, aluminium, and silver, but not iron. Tellurium forms an alloy also, but it reacts slowly to form mercury telluride. The reaction of mercury with sulfur is more easily noticed. Any of these alloys is called an amalgam.

This metal also has uniform volumetric thermal expansion, is less reactive than zinc and cadmium and does not displace hydrogen from acids. Common oxidation states of this element are +1 and +2. Rare instances of +3 mercury compounds exist.

The commercial unit for handling mercury is the "flask," which weighs 76 lb.

Applications

Most mercury is used for the manufacture of industrial chemicals or for electrical and electronic applications. Mercury is used in some thermometers, especially ones which are used to measure high temperatures. Other uses:

The ease with which it forms amalgams with gold has resulted in its use in gold recovery from ores.
In addition to thermometers, mercury is used in barometers, diffusion pumps, coulometers, and many other laboratory instruments.
The triple point of mercury, -38.8344 °C, is a fixed point used as a temperature standard for the International Temperature Scale (ITS-90).
In some gaseous electron tubes, mercury arc rectifier
Gaseous mercury is used in mercury-vapor lamps and some "neon sign" type advertising signs and fluorescent lamps.
Mercury compounds are used as a preservative in vaccines and tattoo inks.

Miscellaneous uses; mercury switches, pesticides, dental amalgams/preparations, mercury cells for sodium hydroxide and chlorine production, anti-fouling paint, electrodes in some types of electrolysis, batteries (mercury cells), and catalysts.

History

Mercury was known to the ancient Chinese and Hindus and was found in Egyptian tombs that date from 1500s BC. By 500 BC it was used to make amalgams with other metals. The ancient Greeks used mercury in ointments and the Romans used it in cosmetics. Alchemists thought it to be the stuff from which all matter was formed and they also thought that when it hardened it turned into gold.

In the 18th and 19th centuries, mercury nitrate was used to remove fur from the animal skins from which felt hats were made. This caused many cases of brain damage among hatters, or milliners, leading, it is claimed, to the simile "as mad as a hatter", and thereby to the Mad Hatter of Alice in Wonderland fame.

It was named by alchemists after the Roman god Mercury. Its symbol Hg comes from hydrargyrum, a Latinised form of the Greek word hydrargyros, which was a compound word whose Greek roots meant 'water' and 'silver' — since it was liquid, like water, and yet had a silvery metallic sheen. Mercury is one of the few elements that has an alchemical symbol (left).

Occurrence

A rare element in the earth's crust, mercury is found either as a native metal (rare) or in cinnabar, corderoite, livingstonite, and other minerals with cinnabar (HgS) being the most common ore. Approximately 50% of the global supply comes from Spain and Italy, with much of the rest coming from Slovenia, Russia, and North America. The metal is extracted by heating cinnabar in a current of air and condensing the vapor.

Compounds

The most important salts are:

Mercury(I) chloride (AKA calomel and is sometimes still used in medicine)
Mercury(II) chloride (which is very corrosive, sublimates and is a violent poison)
Mercury fulminate, (a detonator widely used in explosives),
Mercury(II) sulfide (AKA vermilion which is a high-grade paint pigment),
Mercury(II) selenide a semi-metal,
Mercury(II) telluride a semi-metal, and
CdHgTe|Mercury Cadmium Telluride an infrared detector material.

Organic mercury compounds are also important. Laboratory tests have found that an electrical discharge causes the noble gases to combine with mercury vapor. These compounds are held together with van der Waals forces and result in HgNe, HgAr, HgKr, and HgXe. Methyl mercury is a dangerous compound that is widely found as a pollutant in water bodies and streams.

Isotopes

There are seven stable isotopes of mercury with Hg-202 being the most abundant (29.86%). The longest-lived radioisotopes are Hg-194 with a half-life of 444 years, and Hg-203 with a half-life of 46.612 days. Most of the remaining radioisotopes have half-lifes that are less than a day.

Precautions

Elemental, liquid mercury is slightly toxic, while its vapor, compounds and salts are highly toxic and have been implicated as causing brain and liver damage when ingested, inhaled or contacted. For this reason (along with exaggeration of the actual risk in the media), most thermometers now use pigmented alcohol instead of mercury, though some medical thermometers still use mercury for reasons of accuracy.

The main dangers associated with elemental mercury are that at STP, mercury tends to oxidize forming mercury (II) oxide, and that if dropped or disturbed, mercury will form microscopic drops, increasing its surface area dramatically.

Even though it is far less toxic than its compounds, elemental mercury still poses significant environmental pollution and remediation problems due to the fact that mercury forms organic compounds inside of living organisms. Methyl mercury works its way up the food chain, reaching high concentrations among populations of some species such as tuna. Mercury poisoning in humans will result from persistent consumption of tainted foodstuffs.

One of the most dangerous mercury compounds, dimethylmercury, is so toxic that even a few microliters spilled on the skin can cause death. One of the chief targets of the toxin is the enzyme pyruvate dehydrogenase (PDH). The enzyme is irreversibly inhibited by several mercury compounds, the lipoic acid component of the multienzyme complex binds mercury compounds tightly and thus inhibits PDH.

Mercury is a bioaccumulative toxin that is easily absorbed through the skin, respiratory and gastrointestinal tissues. Minamata disease is a form of mercury poisoning. Mercury attacks the central nervous system and endocrine system and adversely affects the mouth, gums, and teeth. High exposure over long periods of time will result in brain damage and ultimately death. It can pose a major health risk to the unborn fetus. Air saturated with mercury vapor at room temperature is at a concentration many times the toxic level, despite the high boiling point (the danger is increased at higher temperatures).

Mercury should therefore be handled with great care. Containers of mercury need to be covered securely to avoid spillage and evaporation. Heating of mercury or mercury compounds should always be done under a well-ventilated, filtered hood. Additionally, some oxides can decompose into elemental mercury, which immediately evaporates and may not be apparent.

References

Los Alamos National Laboratory – Mercury

External links

WebElements.com – Mercury
EnvironmentalChemistry.com – Mercury
Material Safety Data Sheet – Mercury
Hg 80 Mercury
Global Mercury Assessment report 2002 by the UNEP.
A summary of the previous report by the industry lobbying group GreenFacts.