1.2.2 - Development of the Periodic Table

Early periodic tables were incomplete and elements were placed in inappropriate groups if what was to be followed?

The strict order of atomic weights.

Knowledge of what made it possible to explain why the order based on atomic weights wasn’t always correct?

isotopes

Mendeleev overcame some problems with the table by doing what?

Leaving gaps

Mendeleev also changed the order of some elements based on what?

atomic weights