Why did Mendeleev move some elements away from their atomic-weight order?

Dmitri Mendeleev moved certain elements out of strict atomic-weight order in his 1871 periodic table because he prioritized their chemical and physical properties over their atomic weights.12

Mendeleev observed that when elements were arranged strictly by increasing atomic weight, some elements did not align with others that shared similar chemical characteristics, such as valence. By placing these elements based on their properties instead, he was able to maintain the consistency of his periodic groups. For example, in pairs such as cobalt and nickel, or tellurium and iodine, the element with the higher atomic weight was placed in an earlier position to ensure it remained grouped with elements of similar behavior.132

Mendeleev’s intuition was later vindicated by the discovery of the atomic number, which is defined by the number of protons in an atom's nucleus. It is the atomic number, rather than the atomic weight, that fundamentally determines an element's position in the periodic system.12