The Place of Zinc, Cadmium, and Mercury in the Periodic Table

Following an earlier article on the positions of lanthanium and lutetium in the periodic table (J. Chem. Educ. 1982, 59, 634-636), the author notes that introductory textbooks, inorganic textbooks, and advanced monographs on coordination and organometallic chemistry are increasingly treating zinc, cadmium, and mercury as transition or d-block elements, rather than as main-block elements. The author reviews the historical evolution of the concepts of transition elements and d-block elements, evaluates the chemical and spectrosopic evidence for each placement, and concludes that these elements are unambiguously main-block elements and that there is a fundamental bifurcation of group 2 at magnesium into a Ca–Ra branch and a Zn–Hg branch. The author also reviews various ways of representing this bifurcation using spatial position in the periodic table and various labeling schemes.