The group 12 elements

Although the Group 12 metals ( [link] ) are formally part of the d-block elements from their position in the Periodic Table, their electronic configuration in both their elemental form ( d ¹⁰ s ² ) and the vast majority of their compounds ( d ¹⁰ ) is that of the main group elements. The common oxidation state for all the Group 12 elements is +2, and the chemistry of zinc and cadmium compounds in particular is very similar to the analogous magnesium derivatives.

Discovery

Zinc

Artifacts with a high zinc content (as much as 90%) have been fond to be over 2500 years old, and possibly older. As such it is clear that several cultures had the knowledge of working with zinc alloys, in particular brass (a zinc/copper alloy). Zinc mines at Zawar, near Udaipur in India, have been active since the late 1 ^st millennium BC. However, the smelting of metallic zinc appears to have begun around the 12 ^th century AD.

The isolation of purified metallic zinc was reported concurrently by several people. The extraction of zinc from its oxide (ZnO) was reported as early as 1668, while John Lane is supposed to have smelted zinc in 1726. The first Patent for zinc smelting was granted to English metallurgist William Champion in 1738; however, the credit for discovering pure metallic zinc is often given to Andreas Marggraf ( [link] ) in 1746.

Cadmium

Cadmium was discovered in 1817 by Friedrich Stromeyer ( [link] ) as an impurity in calamine (zinc carbonate, ZnCO ₃ ). Stromeyer observed that impure samples of calamine changed color when heated but pure calamine did not. Eventually he was able to isolate cadmium metal by roasting and reduction of the sulfide.

Mercury

Mercury was known to the ancient Chinese and was found in Egyptian tombs that date from 1500 BC. In China and Tibet, mercury use was thought to prolong life, heal fractures, and maintain generally good health. The ancient Greeks used mercury in ointments; the ancient Egyptians and the Romans used it in cosmetics that sometimes deformed the face.

Abundance

The Group 12 elements mainly occur in sulfide ores, however, as with their Group 2 analogs, carbonate are known, but not as economically viable. The major zinc containing ore is zinc blende (also known as sphalerite), which is zinc sulfide (ZnS). Other important ores include, wurtzite (ZnS), smithsonite (zinc carbonate, ZnCO ₃ ), and hemimorphite (calamine, Zn ₂ SiO ₄ ). The basic form of zinc carbonate (hydrozincite, Zn ₅ (CO ₃ ) ₂ (OH) ₆ ) is also mined where economically viable. The main source of cadmium is as an impurity in zinc blende; however, there are several other ores known, e.g., cadmoselite (cadmium selenide, CdSe) and otavite (CdCO ₃ ). Mercury sulfide (cinnabar, HgS) is the major source of mercury, and in fact metallic liquid mercury droplets are often found in the ore. The terrestrial abundance of the Group 12 elements is given in [link] .