Gold (Element): A Dense, Soft, Noble Metal
Gold is a dense, soft, highly malleable transition metal prized for its chemical inertness, corrosion resistance, and infrared reflectivity. Its noble character makes it nearly unreactive, dissolving only in special reagents like aqua regia.
Gold (symbol Au, atomic number 79) is a dense, soft, bright-yellow transition metal. It crystallizes in a face-centered cubic lattice, melts at about 1064 °C, and has a density of 19.3 g/cm³ — roughly 2.5 times that of iron and close to tungsten. With a Mohs hardness around 2.5, pure gold is soft enough to dent with a fingernail, so jewelry and coinage usually use harder alloys. Gold is the most malleable and ductile of all metals. A single gram can be beaten into a gold leaf sheet about a square meter in area (down to roughly 100 nanometers thick, becoming translucent), and it can be drawn into wire approaching single-atom width before breaking. Its Young's modulus is about 79 GPa, lower than steel's ~200 GPa, so it is comparatively soft and not stiff. Chemically, gold is a noble metal — one of the least reactive elements, second only to platinum in inertness. It does not react with oxygen (it never tarnishes or rusts), and resists most acids, including hydrochloric and sulfuric acid alone. It does dissolve in aqua regia (a nitric/hydrochloric acid mixture) and in cyanide solutions used in mining. This inertness, combined with biocompatibility and high infrared reflectivity, drives gold's uses in electronics, dentistry, infrared optics, and spacecraft thermal control. See Aqua Regia: The Only Acid That Dissolves Gold and Chloroauric Acid: The Yellow Compound That Forms When Gold Dissolves in Aqua Regia.