Compounds bring back chemistry to the system of chemical elements

Abstract

The periodic system of chemical elements was historically devised by assessing order and similarity relationships among the elements from their compounds, that is, using the accumulated results of chemical practice and knowledge. However, the current approach to the system is based on an ontology of isolated atoms where similarities, especially, are addressed through resemblances of electronic configurations. Here we show how the historical approach can be combined with computational tools for data analysis to build up the system based on the compounds reported by chemists. The approach produces well-known similarities of chemical elements when applied to binary compounds. The results come from the analysis of 4,700 binary compounds of 94 chemical elements, whose resemblances are quantified based on the elements they form compounds with and the proportions of those combinations. It is found that similarities do not always correspond to columns of the conventional periodic table and that besides robust similarities such as those of alkali metals, halogens and lanthanoids, there are other mixed similarities involving transition metals and actinoids, some of which were already known for a long time. These similarities are described. Finally, the advantages and disadvantages of the electronic and the compound approach to the system are discussed. It is concluded that the current data availability and computational facilities make possible to think of a periodic system closer to the chemical milieu of compounds, bringing chemistry back to the system.