Why did Dmitri Mendeleev pay close attention to valence when arranging the elements?

Dmitri Mendeleev paid close attention to valence—the number of single bonds an element can form—because it served as a crucial indicator of chemical properties and family resemblances among the elements.123

While previous attempts to classify elements relied primarily on atomic weight, Mendeleev recognized that valence patterns provided a systematic way to group elements that shared similar chemical behaviors. By correlating these bonding characteristics with atomic weights, he was able to organize the elements into a coherent structure that highlighted periodic trends. For example, he placed elements like oxygen and sulfur in the same family because they shared a typical valence of 2, while nitrogen and phosphorus were grouped together due to their shared valence of 3.4132

This focus on valence was essential for the development of the periodic table, allowing Mendeleev to identify gaps in his system and accurately predict the existence and properties of then-unknown elements.13