Why did Mendeleev leave gaps in his periodic table?

Mendeleev left gaps in his periodic table primarily to indicate undiscovered elements and to group elements with similar properties. At the time he developed the periodic table, certain elements had not yet been discovered, but Mendeleev predicted their existence based on the trends he observed in the properties and atomic masses of the elements around the gaps. By intentionally leaving spaces, he demonstrated an understanding that the periodic table was not yet complete and that new elements would fit into the established order, maintaining the table's organization based on properties. This foresight was significant, as later discoveries confirmed his predictions, validating the accuracy and usefulness of his table.