- Mercury (Hg) is a unique transition metal with atomic number 80, belonging to Group 12 of the periodic table, along with zinc and cadmium.
- Its electron configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s², giving it a filled d-subshell and two outer s-electrons that account for its metallic character. Each atom of mercury contains eighty protons, eighty electrons, and typically 120 to 124 neutrons depending on the isotope.
- Naturally occurring mercury is a mixture of seven stable isotopes, with mercury-202 (²⁰²Hg) being the most abundant.
- Mercury has been known since ancient times. It was used by civilizations such as the Chinese, Hindus, and Greeks in medicine, cosmetics, and rituals. The symbol Hg derives from the Latin word hydrargyrum, meaning “liquid silver,” due to its silvery appearance and liquid state at room temperature. In alchemy, mercury was considered one of the fundamental elements and was associated with transformation and immortality.
- Physically, mercury is remarkable for being the only metallic element that is liquid at standard temperature and pressure. It has a silvery-white appearance, a density of 13.53 g/cm³, a melting point of –38.83 °C (–37.89 °F), and a boiling point of 356.7 °C (674.1 °F). It expands and contracts uniformly with temperature changes, which historically made it ideal for use in thermometers and barometers. Mercury also forms amalgams with many metals, such as gold and silver, a property once exploited in mining and dentistry.
- Chemically, mercury is relatively unreactive, showing resistance to oxidation in air and water. It does not react with most acids, though it dissolves in concentrated nitric acid and aqua regia. Mercury commonly exhibits oxidation states of +1 (mercurous) and +2 (mercuric). Examples of its compounds include mercuric chloride (HgCl₂), a highly toxic substance, and mercurous chloride (Hg₂Cl₂), also known as calomel, which was once used in medicine. Organomercury compounds, such as methylmercury (CH₃Hg⁺), are particularly hazardous due to their high toxicity and bioaccumulation in food chains.
- Mercury has had many historical applications, though its use has declined due to toxicity concerns. It was widely employed in scientific instruments such as thermometers, barometers, and manometers, and in electrical switches and relays. Mercury vapor lamps were used in lighting, and mercury compounds served in fungicides, cosmetics, and pharmaceuticals. Today, mercury’s primary industrial use is in the production of chlorine and caustic soda through the mercury cell process, although safer alternatives are replacing it. Small amounts are still used in scientific research and artisanal gold mining, where mercury forms amalgams with gold for extraction.
- Biologically, mercury has no essential role and is highly toxic to humans and animals. It can damage the nervous, digestive, and immune systems, with organic mercury compounds being the most dangerous. Chronic exposure leads to mercury poisoning, historically known as “mad hatter’s disease,” from its use in felt hat production. Methylmercury accumulates in fish and shellfish, posing risks to human health through dietary exposure.
- Environmentally, mercury is a rare element in Earth’s crust, occurring at about 0.08 parts per million. It is primarily found in the mineral cinnabar (HgS), from which it is extracted by roasting. Major producers include China, Mexico, and Kyrgyzstan. Due to its mobility and persistence, mercury pollution is a global concern, leading to international agreements such as the Minamata Convention on Mercury, which aims to reduce emissions and protect ecosystems.
