Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum (from "hydr-" water and "argyros" silver). A heavy, silvery d-block element, mercury is the only metal that is liquid at standard conditions for temperature and pressure; the only other element that is liquid under these conditions is bromine, though metals such as caesium, francium, gallium, and rubidium melt just above room temperature. With a freezing point of −38.83 °C and boiling point of 356.73 °C, mercury has one of the narrowest ranges of its liquid state of any metal.[2][3][4]
Mercury occurs in deposits throughout the world mostly as cinnabar (mercuric sulfide). The red pigment vermilion is mostly obtained by reduction from cinnabar. Cinnabar is highly toxic by ingestion or inhalation of the dust. Mercury poisoning can also result from exposure to water-soluble forms of mercury (such as mercuric chloride or methylmercury), inhalation of mercury vapor, or eating seafood contaminated with mercury.
Mercury is used in thermometers, barometers, manometers, sphygmomanometers, float valves, mercury switches, and other devices though concerns about the element's toxicity have led to mercury thermometers and sphygmomanometers being largely phased out in clinical environments in favor of alcohol-filled, galinstan-filled, digital, or thermistor-based instruments. It remains in use in scientific research applications and in amalgam material for dental restoration. It is used in lighting: electricity passed through mercury vapor in a phosphor tube produces short-wave ultraviolet light which then causes the phosphor to fluoresce, making visible light.
Mercury is the innermost of the eight planets in the Solar System. It is also the smallest, and its orbit has the highest eccentricity of the eight.[a] It orbits the Sun once in about 88 Earth days, completing three rotations about its axis for every two orbits. Mercury has the smallest axial tilt of the Solar System planets. The perihelion of Mercury's orbit precesses around the Sun at an excess of 43 arcseconds per century beyond what is predicted by Newtonian mechanics, a phenomenon that was explained in the 20th century by Albert Einstein's General Theory of Relativity.[12]
Mercury, being an inferior planet, appears as a morning star and an evening star, but is much more difficult to see than the other inferior planet, Venus. At its brightest, Mercury is technically a very bright object when viewed from Earth, but it is not easily seen in practice because of its proximity in the sky to the Sun.
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