Why did Dmitri Mendeleev move some elements out of atomic-weight order?

Dmitri Mendeleev did not move elements out of atomic-weight order simply to ignore that metric; rather, he prioritized the consistency of chemical and physical properties when organizing the periodic table.12

While Mendeleev used atomic weight as a foundational guide, he also placed significant emphasis on an element’s valence and its overall chemical behavior. When the strict sequence of atomic weights conflicted with the observed periodic patterns of these properties, Mendeleev prioritized the patterns.1

His confidence in this approach was so strong that he even proposed that the accepted atomic weights for several elements were incorrect. By leaving gaps in his table for elements yet to be discovered—such as scandium, gallium, and germanium—he ensured that elements with similar characteristics remained grouped together. It was not until the 20th century that scientists understood that an element's position is actually determined by its atomic number, or the number of protons in its nucleus, which resolved the discrepancies Mendeleev encountered.12