Unlike previous years, the periodic system between 1948 and 1980 is largely transient, as evidenced in its low preservation values in Fig. . This instability is mainly caused by the appearance of new similarities among known elements such as rare earths, in particular lanthanoids, as well as among the recently discovered actinoids. Noble gases chemistry also began to be explored and the chemistry of several known elements blossomed, such as B, F and Si. Regarding rare earths, although Ce and La compounds were known by 1814 and 1845, respectively; and early Sc and Y compounds were reported between 1870 and 1880 (Fig. , ), these elements are observed to form a family by 1912 (Fig. ). Nevertheless, its inner structure is modified around 1952, when the chemistry of lanthanoids was further developed as motivated by WW2 (SI). War efforts required pure uranium, which was often accompanied by rare-earth impurities . WW2 research brought about the development of the ion-exchange separation method, which besides providing pure U samples, offered chemists, for the first time, pure samples of lanthanoids, triggering the exploration of their chemistry (Fig. ) . Actinoid chemistry also began in this period by exploring different compounds of these new synthetic elements -mainly non-carbon combinations (SI Fig. ), which also motivated changes in some previous similarities. It is in this period, for instance, that U joined the actinoids after more than a century and a half of itinerating similarities with group-4 and -6 transition metals (Fig. ).