Why did Mendeleev leave some gaps in his periodic table of elements?

Dmitri Mendeleev, the Russian chemist credited with creating the first widely-accepted periodic table of elements, left gaps in his table for several reasons:

Predictive Power: Mendeleev used the gaps to predict the existence and properties of elements that had not yet been discovered. He arranged the elements based on their atomic masses and chemical properties, and he noticed that certain elements seemed to be missing from his table. By leaving gaps, Mendeleev suggested that there were elements yet to be discovered that would fit into those spaces.

Pattern Recognition: Mendeleev recognized that there were patterns in the properties of elements that repeated periodically. By leaving gaps in his table, he believed that he was preserving these patterns and making it easier for future scientists to predict the properties of undiscovered elements.

Maintaining Similarities: Mendeleev arranged the elements in his periodic table based on their chemical properties. By leaving gaps, he ensured that elements with similar properties were grouped together, even if some of those elements had not yet been discovered.

Maintaining the Periodic Law: The periodic law states that the properties of elements are periodic functions of their atomic numbers. By leaving gaps, Mendeleev ensured that the periodicity of the elements was maintained in his table.

Ultimately, Mendeleev's decision to leave gaps in his periodic table was a demonstration of his insight into the underlying patterns and principles governing the properties of chemical elements. Many of the elements that he predicted to fill these gaps were later discovered and found to fit remarkably well into his proposed system.