Mendeleev noticed a recurring pattern in chemical properties when elements were arranged by increasing atomic weight.
- He observed that the valence (combining power) tended to follow a repeating sequence across the rows of his table: 1, 2, 3, 4, then 3, 2, 1, and then repeating for subsequent elements. This pattern helped him predict properties of undiscovered elements and place them in appropriate groups.
- The pattern extended to recurring chemical behaviors, such as the types of compounds elements tended to form and their affinities in reactions, which reinforced the idea of periodic repetition in properties.
- Mendeleev used this periodicity to justify leaving gaps for elements that hadn’t been discovered yet, predicting their properties and atomic weights with remarkable accuracy. This was a key factor in his development of the periodic law.
For additional context and historical framing:
- His table was originally organized by atomic weight, with the understanding that the order would reveal periodic trends in properties. Later work showed that arranging by atomic number also yields the same overall structure and clarifies certain anomalies.