What is it about?

The periodic system, which intertwines order and similarity among chemical elements, arose from knowledge about substances constituting the chemical space. Little is known, however, about how the expansion of the space contributed to the emergence of the system—formulated in the 1860s. Here, we show by analyzing the space between 1800 and 1869 that after an unstable period culminating around 1826, chemical space led the system to converge to a backbone structure clearly recognizable in the 1840s. Hence, the system was already encoded in the space for about two and half decades before its formulation. Chemical events in 1826 and in the 1840s were driven by the discovery of new forms of combination standing the test of time. Emphasis of the space upon organic chemicals after 1830 prompted the recognition of relationships among elements participating in the organic turn and obscured some of the relationships among transition metals. To account for the role of nineteenth century atomic weights upon the system, we introduced an algorithm to adjust the space according to different sets of weights, which allowed for estimating the resulting periodic systems of chemists using one or the other weights. By analyzing these systems, from Dalton up to Mendeleev, Gmelin’s atomic weights of 1843 produce systems remarkably similar to that of 1869, a similarity that was reinforced by the atomic weights on the years to come. Although our approach is computational rather than historical, we hope it can complement other tools of the history of chemistry.

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Why is it important?

The number and diversity of substances constituting the chemical space triggered, in two important steps, the convergence of the periodic system toward a stable backbone structure eventually unveiled in the 1860s. The first step occurred in 1826, and the second was between 1835 and 1845. Interestingly, the salient features of the periodic system of the 1860s can be detected as early as the 1840s, even when considering the effect of disagreement regarding the determination of atomic weights. The methods presented here become instrumental to study the further evolution of the periodic system and to ponder its current shape.


I hope this paper triggers further computational studies in the history of chemistry and, in particular, that it sheds light on the evolution of the periodic system. With this paper we are coming back to the chemical roots of the periodic system by studying chemical substances. After all, this was what the formulators of the system did in the 1860s with a small sample of data of the chemical space. Here we have a much larger sample and the advantage of computational and mathematical methods to explore that data.

Professor Guillermo Restrepo

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This page is a summary of: The expansion of chemical space in 1826 and in the 1840s prompted the convergence to the periodic system, Proceedings of the National Academy of Sciences, July 2022, Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2119083119.
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