How did Dmitri Mendeleev identify errors in accepted atomic weights?

Dmitri Mendeleev identified errors in accepted atomic weights by organizing the chemical elements into a systematic table based on their atomic weight and their chemical properties, such as valence.12

By arranging elements in this manner, Mendeleev observed recurring patterns of properties. When an element did not fit the expected pattern based on its assigned atomic weight, he concluded that the accepted weight was incorrect. In his 1871 version of the periodic table, he correctly predicted that the atomic weights of 17 elements were wrong.12

Mendeleev’s confidence in his periodic system was further bolstered by his ability to predict the existence and properties of then-unknown elements—such as scandium, gallium, and germanium—by leaving gaps in his table where those elements logically belonged. While his work was foundational, it was later discovered in the early 20th century that an element's position is determined by its atomic number rather than its atomic weight.123