A gold atom has 79 electrons around its nucleus. There are 2 in the K shell, 8 in the L shell, 18 in the M shell, 32 in the N shell, 18 in the O shell and 1 in the P shell.
Gold is in group 11, like copper and silver. It can form ions with a charge of +1 or +3, but it forms ions very reluctantly.
Listing metals in order, from those that form ions most readily to those that form them least readily, gives the electrochemical series of metals. Gold comes at the end of the series: of all metals, it is the least inclined to form ions.
So gold does not corrode in air or in water, and it does not dissolve in hydrochloric acid, nitric acid or hot concentrated sulfuric acid. Gold ornaments made thousands of years ago are still being found with their shine intact.
A mixture of one part concentrated nitric acid and three parts concentrated hydrochloric acid, by volume, is called aqua regia. Aqua regia is a very strong oxidizing agent, and it dissolves even gold and platinum.
\mathrm{Au} + \mathrm{HNO_3} + 4\mathrm{HCl} \longrightarrow \mathrm{H[AuCl_4]} + \mathrm{NO} + 2\mathrm{H_2O}
Gold does not dissolve in concentrated nitric acid alone or in concentrated hydrochloric acid alone.
Of all metals, gold is the most malleable and the most ductile: it can be beaten out into thin sheets and drawn out into fine wire.
When 1 g of gold is made into gold leaf about one ten-thousandth of a millimetre thick, it spreads over about 0.5 m², a square about 70 cm on each side. Drawn into a fine wire, it can be stretched to a length of about 3 km.
In traditional Japanese crafts, gold leaf has long been used to decorate Buddhist altars, folding screens and buildings such as Kinkaku-ji, the Golden Pavilion.
Pure gold is soft, so ornaments are often made from alloys of gold with silver, copper and other metals. The proportion of gold in an alloy is given in carats, with pure gold counted as 24 carats; 18-carat gold contains 18 parts in 24, that is, 75 % gold.
Gold does not corrode and conducts electricity well, so it is also used for the contacts of electronic parts and for plating.