How did Mendeleev use gaps in his table to predict scandium, gallium, and germanium?

Dmitri Mendeleev developed the periodic table by arranging elements according to their atomic weight and chemical properties, specifically noting the valence of each element.12

When organizing these elements, Mendeleev observed a recurring pattern of properties within groups. He discovered that if he strictly followed the order of atomic weights, certain elements did not align with the properties of their group members. To maintain the consistency of his periodic law, he intentionally left gaps in his table where he believed undiscovered elements would eventually fit.23

By analyzing the properties of the elements surrounding these gaps, Mendeleev was able to predict the existence and characteristics of scandium, gallium, and germanium before they were actually discovered. The subsequent discovery of these elements—gallium in 1875, scandium in 1879, and germanium in 1886—provided powerful evidence for the validity of his system and led to its widespread scientific acceptance.13