The third period spans the years 1860-1900 and is characterised by 80% of preservation of its periodic system until 1970 and 70% until the present. In this period, the organic chemistry side of the chemical space was strongly developed , as evidenced in the increase of organogenic element compounds and, above all, in their diversity (Fig. ). There was a surge of different molecular formulas made of H, C, N and O, as well as some variations of them including Cl and S (). This organic chemistry emphasis turned H most similar to organogenic elements, especially Cl through one-to-one substitutions of one element for the other in organic compounds. H's organogenic similarities have remained so ever since (Fig. ) as a consequence of the still growing content of organic chemicals in the chemical space .