Development of the periodic table

• • Before the discovery of protons, neutrons and electrons, scientists categorised elements by atomic weight
• • Early periodic tables were incomplete and some elements were placed in the wrong group - order of atomic weight
• • Mendeleev created the modern periodic table
  • He arranged elements in order of atomic weight but switched the order with the influence of properties
  • Mendeleev left gaps for elements that had not been discovered
  • The discovery of isotopes confirmed why the order of elements based on atomic weight was not correct