How did Mendeleev's arrangement of the periodic table allow for the prediction of undiscovered elements?

Mendeleev arranged the elements by increasing atomic weight and noticed patterns in their properties. He left gaps in his table for elements that had not yet been discovered, predicting their existence and properties based on the trends observed in the surrounding elements.

This answer is strong because it directly addresses how Mendeleev's method of arranging elements led to the prediction of undiscovered elements, demonstrating an understanding of the historical development of the periodic table. The question tests the student's ability to recall and explain a key aspect of Mendeleev's contributions.