More Chemistry

Abstract

The trends within rows and columns of the periodic table are quite well known and are not repeated here. Instead, I concentrate on a number of other chemical trends, some of which challenge the form of reductionism that attempts to provide explanations based on electronic configurations alone. In the case of one particular trend described here, the knight’s move, the chemical behavior defies any theoretical understanding whatsoever, at least at the present time. As is well known to students of inorganic chemistry, a small number of elements display what is termed diagonal behavior where, in apparent violation of group trends, two elements from adjacent groups show greater similarity than is observed between these elements and the members of their own respective groups. Of these three classic examples of diagonal behavior, let us concentrate on the first one to the left in the periodic table, that between lithium and magnesium. The similarities between these two elements are as follows:1. Whereas the alkali metals form peroxides and superoxides, lithium behaves like a typical alkaline earth in forming only a normal oxide with formula Li2O. 2.Unlike the other alkali metals, lithium forms a nitride, Li3N, as do the alkaline earths. 3.Although the salts of most alkali metals are soluble, the carbonate, sulfate, and fluorides of lithium are insoluble, as in the case of the alkaline earth elements. 4.Lithium and magnesium both form organometallic compounds that act as useful reagents in organic chemistry. Lithium typically forms such compounds as Li(CH3)3, while magnesium forms such compounds as CH3MgBr, a typical Grignard reagent that is used in nucleophilic addition reactions. Organolithium and organomagnesium compounds are very strong bases that react with water to form alkanes. 5.Lithium salts display considerable covalent character, unlike their alkali metal homologues but in common with many alkaline earth salts. 6.Whereas the carbonates of the alkali metals do not decompose on heating, that of lithium behaves like the carbonates of the alkaline earths in forming the oxide and carbon dioxide gas. 7.Lithium is a considerably harder metal than other alkali metals and similar in hardness to the alkaline earths.