Explain how Mendeleev changed some element orders based on chemical properties.

Mendeleev, a key figure in the development of the periodic table, recognized that the properties of elements were more important than their atomic weights. He rearranged certain elements to ensure that those with similar chemical properties were grouped together, even if this meant placing them out of strict atomic weight order. This approach allowed Mendeleev to predict the existence and properties of undiscovered elements, demonstrating the significance of chemical behavior in organizing the periodic table.