How did the discovery of isotopes contribute to the understanding of atomic weight order in the periodic table?

The discovery of isotopes showed that elements can have atoms with the same number of protons but different numbers of neutrons, leading to different atomic weights. This explained why some elements did not fit neatly into the order of atomic weights based solely on their atomic number.

This answer is strong because it directly addresses the role of isotopes in atomic weight discrepancies, demonstrating understanding of atomic structure and its implications for the periodic table. The question tests the student's ability to connect isotopic knowledge to historical developments in chemistry.