Mercury is a chemical element with the symbol Hg and atomic number 80. It is commonly known as quicksilver and is the only metallic element that is liquid at standard temperature and pressure. A heavy, silvery-white transition metal, mercury is a poor conductor of heat compared to other metals but a fair conductor of electricity.
Characteristics
- Physical Properties: Mercury has a distinctive silvery appearance. It possesses a relatively high density (13.534 g/cm³ at 25 °C) and a low melting point (−38.83 °C) and boiling point (356.73 °C). Its high surface tension causes it to form spherical droplets. Mercury has a significant vapor pressure, and its vapor is highly toxic.
- Chemical Properties: Mercury is a relatively unreactive metal. It does not react with most acids but can be oxidized by concentrated oxidizing acids like nitric acid or hot concentrated sulfuric acid. It readily forms alloys with many metals, known as amalgams, with notable exceptions being iron and platinum.
- Isotopes: Mercury has seven stable isotopes, with Hg-202 being the most abundant. Several radioisotopes are also known, which are generally short-lived.
Occurrence
Mercury is a rare element in the Earth's crust. It occurs naturally, primarily as the mineral cinnabar (mercuric sulfide, HgS). Cinnabar is found in volcanic regions and hot springs, often associated with antimony, arsenic, and zinc deposits. The largest deposits are historically found in Spain (Almadén), Italy, China, and the United States (California).
Production
Historically, mercury was primarily extracted by heating cinnabar in air, which oxidizes the sulfide and releases mercury vapor. This vapor is then cooled and condensed into liquid mercury. ${\text{HgS (s)} + {\text{O}}{2}{\text{ (g)}} \xrightarrow{\Delta} {\text{Hg (g)}} + {\text{SO}}{2}{\text{ (g)}}$
Applications
Due to its unique properties, mercury has been utilized in various applications throughout history, though its use has significantly declined in recent decades due to its toxicity:
- Measurement Devices: Its consistent thermal expansion and high density made it ideal for thermometers, barometers, manometers, and sphygmomanometers.
- Electrical Applications: Used in silent electrical switches, tilt switches, and fluorescent lamps (where mercury vapor produces UV light).
- Dental Amalgam: An alloy of mercury with silver, tin, and copper was widely used for dental fillings.
- Chemical Industry: Used as a catalyst in certain chemical reactions, though this has largely been replaced by less toxic alternatives.
- Mining: Historically used in the amalgamation process to extract gold and silver from their ores, a practice now largely prohibited due to environmental concerns.
- Other Uses: Found in some old batteries, anti-fouling paints (now mostly banned), and some scientific instruments.
Toxicity and Environmental Impact
Mercury and its compounds are highly toxic and pose significant environmental and health risks.
- Neurotoxin: Elemental mercury vapor, inorganic mercury compounds, and organic mercury compounds (especially methylmercury) are potent neurotoxins. Exposure can lead to neurological damage, kidney damage, and developmental problems.
- Bioaccumulation and Biomagnification: Methylmercury, formed when inorganic mercury is methylated by bacteria in aquatic environments, is readily absorbed by living organisms. It then bioaccumulates in the food chain, leading to high concentrations in top predators, including fish, marine mammals, and humans who consume them.
- Environmental Release: Human activities such as coal combustion, artisanal gold mining, waste incineration, and industrial processes are major sources of mercury release into the environment.
Due to these severe health and environmental concerns, there has been a global effort, notably through the Minamata Convention on Mercury, to reduce and eliminate the production and use of mercury in many products and industrial processes.
Historical Context
The earliest recorded use of mercury dates back to ancient Egypt (around 1500 BCE) and China, where it was associated with alchemy and immortality. The Romans and Greeks used cinnabar as a pigment and for medicinal purposes. Its name "Mercury" is derived from the Roman god Mercury, known for his speed and fluidity, reflecting the element's liquid nature. Its chemical symbol "Hg" comes from its Latinized Greek name hydrargyrum, meaning "water-silver" or "liquid silver."