GOLD AND THE PLATINUM FAMILY

	SYMBOL	ATOMIC WEIGHT	DENSITY	HIGHEST OXIDE	HIGHEST CHLORIDE	MELTING POINT
Ruthenium	Ru	101.7	12.26	RuO₄	RuCl₄	Electric arc
Rhodium	Rh	103.	12.1	RhO₂	RhCl₂	Electric arc
Palladium	Pd	106.5	11.8	PdO₂	PdCl₄	1500°
Iridium	Ir	193.	22.42	IrO₂	IrCl₄	1950°
Osmium	Os	191.	22.47	OsO₄	OsCl₄	Electric arc
Platinum	Pt	194.8	21.50	PtO₂	PtCl₄	1779°
Gold	Au	197.2	19.30	Au₂O₃	AuCl₃	1064°

The family. Following iron, nickel, and cobalt in the eighth column of the periodic table are two groups of three elements each. The metals of the first of these groups—ruthenium, rhodium, and palladium—have atomic weights near 100 and densities near 12. The metals of the other group—iridium, osmium, and platinum—have atomic weights near 200 and densities near 21. These six rare elements have very similar physical properties and resemble each other chemically not only in the type of compounds which they form but also in the great variety of them. They occur closely associated in nature, usually as alloys of platinum in the form of irregular metallic grains in sand and gravel. Platinum is by far the most abundant of the six.

Although the periodic classification assigns gold to the silver-copper group, its physical as well as many of its chemical properties much more closely resemble those of the platinum metals, and it can he conveniently considered along with them. The four elements gold, platinum, osmium, and iridium are the heaviest substances known, being about twice as heavy as lead.