In contrast, the similarity relation has not found such a stable state given its fundamental dependence on the corpus of compounds populating the chemical space, particularly on the stoichiometric combinations encoded in empirical and molecular formulae. Furthermore, the exponential expansion of the chemical space has been accompanied with the appearance of new chemical elements and of new combination patterns among elements, revealing in some cases new valencies; both of which have been key factors affecting the similarities among chemical elements . Aiming at studying how similarities among elements are affected by changes in the chemical space, here we developed a method to quantify similarity among chemical elements, further improving upon that reported in , which is based on the replaceability of elements in compounds, as originally used by the formulators of the periodic system.