1.2.2 Development of the Periodic Table

• Before the discovery of protons, neutrons and electrons, how did scientists attempt to classify elements?

  By arranging them in order of their atomic weights.
• What were the early periodic tables?

  Incomplete.
• Why was the order based on atomic weights not correct?

  Isotopes.
• How did Mendeleev overcome some problems of the periodic table?

  Mendeleev organized the elements based on their atomic mass and left gaps for undiscovered elements.