Therefore, although increasing in number, new isomers of organic compounds did not contribute further to increasing diversity of the chemical space, which left the periodic system essentially untouched by the growing presence of organic chemistry. Nevertheless, this latter compound-diversity unbalance of the 1830s and 1840s did pose a semiotic challenge for the nomenclature and theoretical settings of the early 19 th century, which ultimately triggered the formulation of the molecular structural theory . As we will see, this theory motivated further changes in the periodic system. Shown in Fig. are the evolving families of similar elements -analogous to, and often coinciding with groups in the periodic table. There, it is observed how before 1826 some recurrent similarities appeared, which are today parts of the alkali metals, pnictogens (excluding N), chalcogens, halogens and some subfamilies of transition metals such as {Ti, Zr}, {Mo, W} and {Pd, Pt}. It can be seen that, despite the transient character of the similarities among chemical elements before 1826, some of them often remained for the years to come, and constitute the corner stone of the current periodic system. A representative periodic table of the periodic system of this period is shown in Fig. . Between 1826 and 1860 chemists expanded the chemical space more and more into the direction of organic chemistry . In this period, compounds containing organogenic elements such as H, C, N and O, increased in number and diversity (Fig. ) to the extent that H transitioned from the alkali elements to the family of organogenic elements (Fig. ). Alkali metals were, in turn, consolidated by including Rb, Cs and Tl, thanks to their +1-valence participation in compounds reported at that time. Valence +1 for Tl is today recognised as an evidence of the inert pair effect, explained in terms of electronic relativistic effects . Mg, in turn, joined alkaline earths, along with Pb, and also the family Fe, Co, Ni. Nb and Ta formed a family with sporadic inclusions of V (current group 5 in the periodic table); this has remained largely untouched over the years, however V is currently a singular element. By singularity we mean the difficulty in assigning one element to one or few families, which may occur because the element has no similarity to any element, but also because of multiple ties in similarity. This shows how multiple classifications may be possible for some elements as it has been recently recognised . Overall, around 65% of the periodic system of this period has made its way until the present (Fig. ), and the salient structure of the periodic system by the time of its formulation is shown in Fig. .