Mercury – facts and properties
- Symbol
- Hg
- Atomic number
- 80
- Atomic mass
- 200.592 u
- Period
- 6
- Group
- 12
- Category
- metal
- Electron configuration
- [Xe] 4f¹⁴ 5d¹⁰ 6s²
- Electronegativity
- 2
- Atomic radius
- 176 pm
- Valence electrons
- 12
- State at standard conditions
- liquid
- Melting point
- -38.83 °C
- Boiling point
- 356.73 °C
- Density
- 13.534 g/cm³
- Discovered
- known since antiquity
Mercury is the only metal that is liquid at room temperature. This silvery, shiny heavy metal has atomic number 80. It melts as low as -38.83 °C. Its symbol Hg comes from the Latin hydrargyrum, meaning water silver. Mercury and its compounds are toxic, above all the vapour and the organic compound methylmercury. For that reason its use is being restricted worldwide. It was once found in thermometers, dental fillings and fluorescent lamps.
Position in the periodic table
Occurrence and extraction
Mercury is one of the scarcer elements. Tiny droplets of the metal are sometimes found in rock, but it almost always occurs as cinnabar, a bright red mineral made of mercury sulfide, HgS. Famous deposits were Almadén in Spain and Idrija in Slovenia, and both mines are closed today. It is now mined mainly in China, and recycling covers part of the demand.
To extract it, the cinnabar is roasted in air. The sulfur burns to sulfur dioxide, the mercury evaporates and is condensed again in coolers. Because of the toxicity this may be done only in closed plants with filters.
Mercury has been known since antiquity, so there is no year of discovery. Alchemists linked the metal to the planet Mercury, which is where its English name comes from. In 1774 Joseph Priestley obtained oxygen from mercury oxide, before the gas was understood as an element.
Properties and reactions
Mercury is in period 6 and group 12, below cadmium and to the right of gold. Its electron configuration, [Xe] 4f¹⁴ 5d¹⁰ 6s², has completely filled 5d and 6s subshells, and its electronegativity is 2. A density of 13.534 g/cm³ is more than thirteen times that of water, so even pieces of iron float on the liquid metal. The boiling point is 356.73 °C. The metal does not wet glass and draws itself into beads, and it expands very evenly when warmed.
Why is mercury liquid? Because of relativistic effects the two 6s electrons are held especially tightly by the nucleus and hardly take part in metallic bonding. The atoms therefore cling together only weakly, and the metal melts far below zero degrees.
Many metals dissolve in mercury to form alloys called amalgams, but iron does not.
Mercury and its compounds are toxic in different ways. Inhaled vapour passes through the lungs into the blood and damages mainly the nervous system and the kidneys, whereas swallowed liquid metal is hardly absorbed. Methylmercury is the most dangerous form: bacteria in water sediments make it from mercury compounds, and it builds up in the food chain, especially in predatory fish such as tuna and swordfish. It damages the brain and nerves and can cross the placenta.
In the Japanese city of Minamata a chemical factory discharged wastewater containing mercury into the bay for years. In 1956 the disease named after the city was first officially reported. Thousands of people who had eaten contaminated fish fell ill, and many children were born with severe damage. The Minamata Convention, in force since 2017, commemorates this and limits the use and release of mercury worldwide.
Uses in everyday life and technology
For a long time mercury was indispensable in measuring instruments. Clinical thermometers and barometers exploited its even expansion and the fact that it does not wet glass. Since 2009 mercury clinical thermometers may no longer be sold in the EU. Only the unit lives on: blood pressure is still given in millimetres of mercury, because Evangelista Torricelli built the first barometer with the metal in 1643.
In dentistry, fillings were made of amalgam, a mixture of mercury with an alloy of silver, tin and copper. They lasted well, but for the sake of the environment dental amalgam has been banned in the EU since 2025, with exceptions only where the dentist considers it medically necessary. Fluorescent tubes and energy-saving lamps contain tiny amounts of mercury vapour, which produces ultraviolet radiation that a coating turns into visible light. The amalgam process of chlor-alkali electrolysis, in which mercury served as an electrode, has largely given way to the membrane process.
Mercury has no place in school experiments, and devices containing mercury now count as hazardous waste.
Molecules containing this element
- Mercury(II) oxide HgO
- Mercury(II) chloride HgCl2
- Mercury(II) acetate Hg(CH3COO)2
Quick question
What is the atomic number of mercury (Hg)?
FAQ
Is mercury toxic?
The vapour and the organic compound methylmercury, which accumulates in predatory fish, are especially dangerous. They damage the nervous system. Swallowed liquid metal, by contrast, is hardly absorbed.
Why is mercury liquid at room temperature?
Relativistic effects hold its outer electrons tightly to the nucleus. The bonding between the atoms is therefore weak, and the metal melts far below zero degrees.
Where does the symbol Hg come from?
Hg stands for hydrargyrum, the Latinised Greek word for water silver. It describes the shiny, liquid metal.