Controversy on Position Reversal of Four Element Pairs in the Periodic Table
In the periodic table of chemical elements, there was a contro-versy on the position of four element pairs—tellurium-iodine, cobalt-nickel, argon-potassium, and thorium-protactinium—where higher atomic weight species were placed just before the lower, i.e., the positions of these pairs of elements were reversed. The proper justiﬁcation of this position reversal appeared after the publication of Moseley’s work in 1913 when periodicity was redeﬁned as the function of atomic number, not the atomic weight. This controversy surrounding the housing of these four pairs of chemical elements according to their atomic weights, which spanned for more than forty years, and its ultimate solution are brieﬂy delineated in this article.
Volume 27 | Issue 6