Anomalies of Mendeleev's Periodic Table

Mendeleev's table, built on atomic weight, showed two kinds of internal contradiction that pointed to a deeper ordering principle waiting to be found.

Misplaced similar/dissimilar elements. Some elements with genuinely similar chemical behaviour ended up in different groups, while some elements placed in the same group did not actually behave alike.

Atomic-weight order reversed. In a few pairs, Mendeleev had to place the heavier element before the lighter one to keep elements with matching properties in the same group -- directly against his own stated law that properties are periodic functions of atomic weight. The two textbook examples are cobalt (atomic weight 59) placed before nickel (58.7), and tellurium (127.6) placed in Group VI ahead of iodine (127.0, placed in Group VII), because chemically cobalt clearly belongs with the Group VIII triad and iodine's halogen-like properties clearly belong with Group VII, whatever …