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The Element Mercury |
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Basic Properties
An atomic number of 80An atomic weight of 200.59, with common isotopes of 198 to 202.
Lies in the sixth row (period) of the Periodic Table between Gold and Thallium and in the IIB column where it shares reaction and atomic characteristics with Cadmium and Zinc.
Its general atomic configuration is 5d106s2 meaning there are two electrons available for bonding and ionization in its outermost electron shell.
Melts at -39oC (234K), boils at 357oC (630K).
It is dense, 13.6 times that of water.
It takes 240 kilocalories to ionize it (remove one electron).
Its "triple point," the temperature where solid, liquid and gaseous Mercury coexist equally (at a very low pressure of .22 mm of Hg--1 normal atmosphere of pressure = 760 mm of Hg) is a mere -38.8344oCelsius and acts as a fixed point to measure from on the International Temperature Scale.
History
Mercury was known in ancient times and was an important chemical for medieval alchemists, suspected of being an element of which all matter is made from, and when solid it was thought to become gold. It is also known as "quicksilver" because it is liquid and silver-white in appearance.Its Latin name hydrargyrum means "liquid silver" and is the origin of Mercury's chemical symbol Hg.
Mercury oxide (found in both red and yellow forms) led to the discovery of Oxygen by Joseph Priestley in 1774 who heated the oxide and found a gas was emitted that allowed burning to be more vigorous.
States of Existence
Though occasionally found as small liquid globules in rock, it is usually mined from cinnabar, a mineral made from Mercury and Sulphur. It is heated in small furnaces (retorts), oxygen combining with cinnabar to decompose the metal liquid Hg out of the ore and emitting SO2, Sulphur Dioxide, a poisonous gas. Typically it must be purified in iron flasks, filtered through chamois skin or slowly passed through a diluted solution of nitric acid.Though it doesn't combine easily into compounds with other elements, it will alloy--form a solution with two or more metals. These alloys are called amalgams.
Compounds
It is the only common metal liquid at ordinary room temperatures.Mercury is not an active element, combining with other elements only with some difficulty. It is therefore a noble metal, like gold, silver and the platinum group which are all highly resistant to chemical reactions and oxidation. It will combine only at high temperatures with Oxygen to form Mercury Oxide. If found in vapor form and given an electrical discharge it will form weak compounds with Neon, Argon, Krypton and Xenon.
Mercury forms two kinds of compounds
- Mercurous -- uses both electrons in the bonding process.
- Mercuric -- uses just one electron to bond with another element.
Mercuric compounds include halides (salts) like Mercuric Chloride (HgCl2) or Mercuric Flouride. The former does not conduct electricity and melts easily at 280 degrees Celsius, bonding covalently rather than ionically**. Normally a white crystalline salt soluble in water it is also called corrosive sublimate and is extremely poisonous. It has been used as a germicide. It can destroy kidneys in humans. If combined with the protein albumen, Mercuric ions form an insoluble white solid that acts like what heat does to egg whites!
Mercurous compounds use Mercurous ions Hg
Mercurous nitrate is one of the few soluble mercurous salts; it will dissolve into other liquids.
When testing for the presence of Mercurous ions, tin ions are added which creates the formation of metallic mercury or mercurous chloride.
Benign Uses
As mentioned above, many of Mercury's salts do have some medicinal values.As a liquid that expands uniformly with increasing temperature, it is
- often found in meteorological equipments, notably thermometers and barometers.
- it's even used in pendulum clocks, its varying volume changing with temperatures acts as a compensator for the expansion of metals in the pendulum.
- It can be found in many home temperature regulators--as it gets warmer, it expands, shifts and the thermostat goes on or off.
Mixed with gold and silver and other materials it is used as a dental filling.
Some boilers used mercury instead of water to attain greater heat efficiencies.
It is the chief component in mercury vapor lamps lighting our highways (and a chief cause of light pollution at night).
Used in Mercury batteries.
In paints, it is used for its anti-fouling properties and as mercury sulfide it is a paint pigment (color--vermillion).
It is used in the production of caustic soda and chlorine.
Dangers!
Mercury, treated with nitric acid and alcohol yields mercuric fulminate [Hg(ONC)2],
an extremely unstable and explosive compound. This compound is used in
percussion caps as a detonator, starting up the explosion of TNT or
gunpowders.
Mercury vapor and salts of Mercury are poisonous--Never INHALE or SWALLOW Mercury! In fact, don't ever leave Mercury exposed in an open container at room temperature--it will slowly evaporate placing a toxic gas into the atmosphere. Mercury can be absorbed through the respiratory and gastrointestinal systems and even through unbroken skin, further, it is a cumulative poison. It doesn't easily flush out and levels can accumulate over time until it becomes toxic and deadly.
A principle mercury pollutant is methyl mercury, easily soluble in water. It can cause permanent brain and kidney damage and harm fetal development.
Recently, scientist Ellen Silbergeld of the University of Maryland discovered that high mercury levels make animals--and people--more susceptible to malaria. Mercury may reduce the immune system's ability to ward off infectious disease. She discovered this while trying to gauge the impact of mercury contamination on Brazilians who were using Mercury to separate gold from sediment during gold-mining.
Mercury has been found in fish and shellfish, making the fish poisonous to eat. The mercury is stored in their muscles which, when eaten by humans, gets released to cause liver, kidney and skin damage.
The highest concentration of mercury in freshwater sediments in the world is found in a section of Berry's Creek in NJ, near the Meadowlands Sports Complex (home of football's Giants and Jets). Several defunct industrial plants, including one mercury plant, are parts of some SuperFund environmental cleanup sites.
Miscellaneous
Half the world's supply of Mercury, in its cinnabar form, comes from Spain and Italy.
Research for this section used:
- "University Chemistry," Bruce Mahan, Addison-Wesley Publishing, 1969
- "College Chemistry," G. Brooks King and William E. Caldwell, American Book Company, 1967
- Article by Linell Smith, Baltimore Sun, 1997
- Article in NJ "Record," August 17, 1997
- "The New York Public Library Science Desk Reference" by Patricia Barnes-Svarney, MacMillan, 1995
**Ionic bonds involve one atom taking one or more electrons from another atom, causing one to become positively charged, the other negatively, the two becoming linked as the opposite charges attract each other. A covalent bond is when the electrons are shared equally by the two atoms. The net charge distribution in each atom, normally a net of zero, is distorted so there is a kind of negative pole and positive pole on opposite sides of each atom. The two act like magnets attracted to each other but without actually exchanging electrons--a looser type of bond!