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. <strong>1982,</strong> 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.