A Ductile and Noble Metal

Gold is the most ductile of all metals. It can be rolled to foils with a thickness of 0.1 gm (0.1 micron 0.0000004 in) and drawn to wire with a diameter of 10 [mi (0.00004 in). 

Gold is bright yellow but finely divided it is black like other metal powders. Suspended as a colloid in water the color ranges from ruby red to purple. 

Between silver in period 5 (atomic number 47) and gold in period 6 (atomic number 79) 32 electrons have been added, 14 of them in the 4f subshell. The number of positive protons in the nucleus has, of course, also increased by 32. Electrons in an f subshell are, however, not very effective in screening outer-shell electrons from the nucleus. As a result the outer-shell electrons are held tightly by the nucleus. If we go from an atom with 47 electrons (Ag) to one with 79 (Au) an increased atomic radius would be expected. Surprisingly, this is not the case. The covalent radii are 1.5 A for silver and 1.44 A for gold. As a consequence, the outer electrons are strongly attracted by the nucleus and the tendency to form ions is extremely small. 

Gold is important to the total world economy. In the whole world about 500000 employees work in gold production. Of the gold on the market, about 75% comes from mining and 25% from recycling processes, including quantities sold by government monetary authorities. 

The two biggest demand sectors are jewelry and electronics. In 1997, 2360 tonnes (86%) were used for jewelry while 184 toimes (6.7%) were used for electronics. Dental materials used 2.4% of the total gold supply, 2750 tonnes, that year [5.6]. 

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