Dmitri Mendeleev’s arrangement of the elements in 1869 was not merely a catalog of known substances but a bold predictive framework that fundamentally changed how science understands the universe. His periodic table left intentional gaps, not as oversights, but as testable hypotheses regarding the existence and properties of undiscovered elements. By analyzing the recurring patterns in atomic weight, chemical valence, and physical behavior, Mendeleev demonstrated that the properties of elements are a periodic function of their atomic weights, allowing him to forecast the characteristics of elements that had not yet been isolated.
Number of Elements Predicted by Mendeleev
Most historical accounts and chemistry textbooks agree that Mendeleev successfully predicted the existence and properties of approximately **four to six elements** during his initial formulation of the periodic law. These predictions were remarkably accurate and served as powerful validations of his table’s structure. While he left spaces for more than a dozen gaps, some corresponded to non-existent elements or misidentified substances, but the specific, detailed predictions that came true cemented his legacy as a visionary scientist rather than just a meticulous organizer.
Specific Predictions and Their Success
Mendeleev’s predictive power was most evident in his descriptions of elements that filled the gaps in his table. He did not simply note missing entries; he calculated approximate atomic weights and described the expected chemical behavior, often suggesting where these new elements would fit within established families. His success transformed skepticism into widespread acceptance of the periodic system, proving that the table was a dynamic map of the elements rather than a static inventory.
Eka-aluminium (Gallium): Mendeleev predicted "eka-aluminium" would be a silvery metal with a density of about 5.9 g/cm³ and an oxide formula of Ea 2 O 3 . When Paul-Émile Lecoq de Boisbaudran discovered gallium in 1875, its density of 5.91 g/cm³ and the formula of its oxide Ga 2 O 3 matched Mendeleev’s predictions almost exactly.
Eka-boron (Scandium): He termed the missing element "ekaboron," forecasting a metallic element whose oxide would form a basic but weakly soluble salt. The discovery of scandium by Lars Fredrik Nilson in 1879 confirmed these properties, with scandium oxide (Sc 2 O 3 ) exhibiting the exact solubility and basicity Mendeleev described.
Eka-silicon (Germanium): Perhaps the most celebrated prediction was that of "ekasilicon." Mendeleev detailed its properties, including its density and the specific behavior of its tetrachloride. The 1886 discovery of germanium by Clemens Winkler revealed an element with a density of 5.5 g/cm³ and chemical reactions that aligned perfectly with the earlier conjecture, a triumph for the periodic law.
Additional Contextual Predictions
Beyond these three major successes, Mendeleev’s table implied the existence of other elements, though they were less definitively isolated. His arrangement suggested gaps that were later filled by elements such as indium, thallium, and cesium, which, while discovered around the same period, were consistent with the periodic trends he established. He even corrected atomic weights for some known elements, such as beryllium and uranium, when placing them in the table demonstrated that the accepted values were incorrect, further showcasing the table’s accuracy.
