Dmitri Mendeleev was a chemist whose genius resulted in the creation of the Periodic Table of Elements. In 1869, he realized that when elements are arranged by atomic weight, their properties repeat periodically. He famously wrote each element on a card and played 'chemical solitaire' until the pattern emerged. Mendeleev’s greatest stroke of genius was leaving gaps in his table for elements that had not yet been discovered. He even predicted the properties (like density and melting point) of these 'missing' elements, such as Gallium and Germanium, with remarkable accuracy. When these elements were later discovered, his table was validated as the fundamental map of chemistry.