Why do cobalt and nickel demonstrate a limitation of arranging elements by atomic weight?

The arrangement of elements by atomic weight, as initially proposed by Dmitri Mendeleev, encountered a significant limitation with pairs like cobalt and nickel.

In these instances, the chemical properties of the elements required them to be placed in a specific order that contradicted their measured atomic weights. Specifically, cobalt has a greater atomic weight than nickel, yet it must occupy an earlier position in the periodic table to align with its chemical characteristics.12

This discrepancy demonstrated that atomic weight is not the fundamental factor determining an element's position. The issue was only resolved in the early 20th century with the discovery of atomic numbers, which represent the number of protons in an atomic nucleus. It is the atomic number, rather than the atomic weight, that dictates an element's unique identity and its correct, unambiguous placement within the periodic system.312