Dmitri Mendeleev was a Russian chemist and inventor. He formulated the Periodic Law and created his version of the periodic table of elements.
He observed that when the elements (helium, hydrogen, etc.) were listed in an orderly sequence based on their atomic weight, certain patterns emerged among groups of elements. Using these observations, he was able to correct some of the properties of already-known elements.
But then he noticed that there were gaps in the order where there should be elements -- elements that had not been discovered yet. Based upon his periodic table, he calculated what the atomic weights and chemical properties of these unknown elements should be.
Many years later, the elements Scandium, Gallium, and Germanium were discovered and found to match exactly the properties predicted by Dmitri Mendeleev.
