He left gaps in his table, and predicted elements no one had found yet

In 1869, Dmitri Mendeleev arranged the known elements by atomic weight and recurring properties, and where the pattern broke he left blank spaces rather than force a bad fit. He used those gaps to predict the exact properties of undiscovered elements like gallium and germanium; when chemists found them years later, the match was almost exact.

— Dmitri Mendeleev, On the Relationship of the Properties of the Elements to their Atomic Weights — Journal of the Russian Chemical Society, 1869