For example, Mendeleev predicted the existence of an element that he called "eka-silicon." He predicted that this element would have an atomic weight of about 72 and would be similar to silicon in its chemical properties. In 1886, the element germanium was discovered, and it was found to have an atomic weight of 72.5 and chemical properties that were similar to silicon. This confirmed Mendeleev's prediction.
Mendeleev was able to make similar predictions for other elements, and many of these predictions were later confirmed by the discovery of new elements. His periodic table was a major breakthrough in chemistry, and it is still used today as a way to organize and understand the elements.