development of the periodic table

• early periodic tables classified elements by what? why was this a problem?

  • atomic weight
  • they were incomplete & elements were placed in inappropriate groups
• Mendeleev overcame some problems w the table by doing what?

  • leaving gaps for elements he thought were undiscovered
  • changed the order based on atomic weights
• what did Mendeleev predict about the undiscovered elements?

  properties
• knowledge of what made it possible to explain why the order of atomic weights wasn't correct?

  isotopes