What changes did Mendeleev make to ordering of the elements to make his ideas work?

Mendeleev arranged the elements in his Periodic Table primarily by increasing Atomic Mass rather than atomic number, which allowed him to group elements with similar chemical properties. He also left gaps for undiscovered elements, predicting their properties based on trends in the table. This approach enabled him to correct some atomic masses of known elements, leading to a more coherent arrangement that reflected periodicity. Mendeleev's willingness to deviate from strict mass order in favor of chemical properties was a key innovation in his formulation of the periodic law.