Development of the Periodic Table

• Before the discovery of protons, neutrons, and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights
• The early periodic tables were incomplete, and some elements were placed in inappropriate groups if h=the strict order of atomic weights was followed
• Mendeleev overcame some of the problems by leaving gaps for elements he thought had not yet been discovered, and in some placed changed the order based on atomic weights
• Elements with properties predicted by Mendeleev were discovered and filled the gaps. Knowledge of isotopes made it possible to explain why the oder based on atomic weights was not always correct