(Symbol Hg. ) A metallic element, atomic number 80, atomic weight 200. 61; unique (for metals) in that it remains liquid under all but very extreme temperatures. Its density of 13. 596 g cm⁻³ and melting point of −38. 87°C (−37. 8°F) make it very useful as the medium for liquid barometers and thermometers. Mercury is very poisonous and can be absorbed through the skin. It can form organic derivatives that can enter the food chain, particularly via marine organisms. Atmospheric mercury is predominantly in the elemental form.