The first half of the 20th century corresponds to the fourth period in the evolution of the periodic system, where 80% of the families are still present today (Fig. ). In this period, organic chemistry had a smooth growth without sudden changes in the number of compounds and their diversity (Fig. ), however it was still the driving force of expansion of the chemical space . Major changes in this period were on the side of some metals and other non-organogenic elements (Fig. ). For instance compounds containing Ga, Ge and Re and some lanthanoids increased in number and diversity, which led them to form new families on the periodic system. In particular, organic-analogous compounds started to be produced for Ge and Si after 1925, following the periodic trend encoded in the belonging of C, Si and Ge to the 14 th group of the periodic system . This was carried out by applying known C-reactions to Si and Ge, for instance analogous of the Grignard reaction , which led to the formation of the family {Si, Ge, Sn} (Fig. ). In the early years of the period, S, Po and Ra increased the diversity of its compounds; while Hf, Eu, Gd and Yb increased their compounds (Fig. ). F chemistry, in turn, gained momentum, as evidenced in the rapid increase of diverse F compounds, ranging from small representatives such as freon and UF 6 , to large compounds such as Teflon . Two blue vertical lines around 1918 and 1945 in Fig. indicate a drop in the diversity and number of new compounds, which demonstrate the toll of World Wars (WWs) upon chemistry. Further details on the effect of WWs on the chemical space are found in and its consequences for the periodic system are discussed below.