Group 12
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Group 12 is the section of the periodic table containing the elements zinc (Zn), cadmium (Cd), mercury (Hg), and copernicium (Cn). The periodic table is a systematic arrangement of elements ordered by atomic number and electron configuration. These elements share a common electronic structure: the d subshell is completely filled (d10), and two electrons occupy the outermost s subshell.
Although Group 12 is classified among the transition metals in the conventional layout of the periodic table, the strict IUPAC definition of a transition metal requires that the element have an unfilled d subshell, whether in its neutral state or in a common ionic state. Zinc, cadmium, and mercury do not meet this criterion and are therefore sometimes classified as post-transition metals rather than true transition metals.
General properties
Zinc, cadmium, and mercury are metals with melting points lower than those of true transition metals. Mercury is distinctive: it is the only metal that is liquid at room temperature. The most common oxidation state is +2, although mercury also forms the +1 state in dimeric form (Hg2^2+). They generally form complexes with ligands such as ammonia and halides, but these complexes are usually colourless, since the filled d subshell prevents electron transitions that would produce colour.
Uses and occurrence
Zinc is obtained from the mineral sphalerite (zinc sulfide) and is used for coating steel (galvanization) to prevent corrosion, and in alloys such as brass. Cadmium is obtained primarily as a byproduct in zinc refining and has been used in nickel-cadmium batteries and pigments. Mercury is obtained from the mineral cinnabar (mercury sulfide) and was historically used in thermometers and barometers.
Toxicity and regulation
Cadmium and mercury are toxic elements that accumulate in living organisms (bioaccumulation), particularly organic forms of mercury such as methylmercury. The Minamata Convention on mercury has established international controls on the production, trade, and use of mercury. Zinc, by contrast, is an essential nutrient for the human body, and many enzymes require zinc ions.
Copernicium
Copernicium (Cn, atomic number 112) is a synthetic element with extremely limited stability, produced through nuclear reactions. Its half-life is very short, which constrains chemical research. Theoretical predictions based on relativistic effects suggest it may show properties more similar to gases or have lower chemical affinity than mercury, but experimental data remain limited.