Mendeleevs Natural System of Elements

FIGURE 5 Mendeleev s Natural System of the Elements (1871). Reproduced from Mendelejeff (1871). 

As a result, the fourth period gave rise to two series (the fourth and the fifth) of 10 and 7 elements respectively. The fourth series was termed an even series, while the fifth series was called an odd series. Mendeleev 

FIGURE 6 Mendeleev s table with eight periods. 

The values in square brackets are those that were effectively used by Mendeleev in 1870. Notice that the atomic weight of erbium was tripled, while the atomic weight of thorium was doubled, just as in the case of uranium  

After having changed the atomic weights of indium, uranium, and cerium, Mendeleev decided to check the correctness of the new values by determining the heat capacity of these elements (Trifonov, 1970). 

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