Despite the advantages of our approach for the quantification of chemical similarity between elements, as compared to previous ones , the method, and thus the approach to the evolution of the periodic system, is still based on chemical formulas. A more appropriate level of abstraction should consider molecular structure which, if included, would allow for gauging local bonding resemblances of atoms within molecules. Although this may be considered for future studies on the periodic system, it also faces some challenges, in particular it lacks the generality that the use of molecular formulas provide. Similarity among elements based on molecular structure requires information on these structures for most of the compounds and also a standard for encoding them. Popular formats include SMILES and InChIs, which are however not a good fit for the aim of this work, as they assume fixed composition chemical rules, such as atom valences, whose flexibility is of primal importance for studies of this kind. Moreover, molecular structures are based on a graph-theoretical setting where bonds correspond to edges of the graph representing binary relations between atoms. Graph-theoretical models leave aside substances whose molecular representations are out of the atomic binary relationships, as is the case of boranes, metallocenes and molecules holding mechanical bonds . Although mathematical settings have been suggested to solve these issues by using hypergraphs , these approaches need still to be further discussed. The rise of material sciences has also brought to the fore the necessity of computational standards for annotating alloys and glasses, for instance . At any rate, future approaches to similarity should explicitly use the most amount of available data. We envision further similarity studies, including our approach of elements in molecular formulas, plus different further levels of chemical description of diverse complexities, ranging from elements in molecular structures, elements in chemical reactions, up to elements in reaction networks and in networks of classes of chemical reactions. Clearly, these approaches to similarity must go hand in hand with discussions on the concept of chemical substance and with common work among chemists, mathematicians, computer scientists, among other specialists. Further aspects of the similarity among chemical elements, as provided by the chemical space, could be explored, for instance by analysing the role of substances that are exclusive to particular elements. This approach is currently addressed by our research group when analysing the status of this property for the current system .