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Why did Mendeleev move some elements away from the order suggested by their atomic weights?

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Periodic table of the elements with color-coded categories and labeled element groups, showing atomic numbers, symbols, and basic information for each chemical element. — periodic table Modern version of the periodic table of the elements. To learn an element's name, atomic number, electron configuration, atomic weight, and more, select one from the table.

Dmitri Mendeleev moved certain elements away from the strict order of their atomic weights because he prioritized their chemical and physical properties—such as their valence—when organizing the periodic table.12

Mendeleev observed that arranging elements solely by increasing atomic weight sometimes placed them in groups with elements that did not share similar characteristics. To maintain the consistency of his periodic system, he intentionally repositioned elements to ensure they aligned with others of similar behavior. For example, in pairs such as tellurium and iodine, or cobalt and nickel, the element with the higher atomic weight was placed earlier in the table to match its chemical properties.13

It was not until the early 20th century that the scientific community understood that an element's position is actually determined by its atomic number—the number of protons in its nucleus—rather than its atomic weight. This discovery resolved the inconsistencies Mendeleev encountered, as isotopes can have different atomic weights while sharing the same atomic number and chemical properties.13