Periodic Table

Mendeleev came to believe in his periodic table to such a degree that he changed the atomic mass of known elements so that they fit where they "belonged" in his table. He did this with no experimental evidence, only his belief in his table. In one such case he changed the atomic mass of beryllium (Be) from 14, which placed it in Group 15 above nitrogen (N), to 9. This placed it in Group 2 above magnesium (Mg) with which it was more closely related chemically. Even more daring was the fact that Mendeleev predicted the properties of undiscovered elements. Based on gaps in the periodic table Mendeleev deduced that in these gaps belonged elements yet to be discovered. Based on other elements in the same group he predicted the existence of eka-aluminum, eka-boron, and eka-silicon, later to be named gallium (Ga), scandium (Sc), and germanium (Ge). Mendeleev predicted the atomic mass of each element along with compounds they each should form. Within 15 years of Mendeleev's predictions, these elements were discovered, and their properties were found to closely match his predictions. These fulfilled predictions went a long way toward convincing any remaining doubters of the infallibility of the periodic table.

Another change Mendeleev made based on chemical analogy and intuition was placing iodine (I) after tellurium (Te), even though the atomic mass of iodine was less than tellurium. This anomaly, along with the difficulty of where to place the inner transition metals, were problems that would soon be definitively solved. At the time of the periodic table's construction, little was known of atomic structure. With further scientific discoveries such as the existence of protons and the existence of electronic shells, these mysteries were explained and placed into their current places in the periodic table.

Refinement in the measurements of atomic mass, the ordering of the elements based on atomic number rather than atomic mass by Henry G. Moseley (1887-1915) in 1913, and the discovery of new elements have led to the continuing evolution of the periodic table. But since Mendeleev's time the periodic table has remained basically unchanged, providing testament to the power of his original insight.