Mercury is a heavy element with the symbol Hg and an atomic number of 80. It is unique among the common metals for being liquid at standard temperature and pressure, possessing a silvery-white appearance. Chemically, it is a soft, malleable metal with high density. Historically, the element has found applications in various devices, including specialized thermometers, barometers, and switches due to its high conductivity and predictable thermal expansion. However, the elemental form and its various compounds require careful handling due to the toxicity associated with its vapor and environmental persistence.