Development of the periodic table

• Periodic Table

  Arrangement of elements based on their properties
• Elements were arranged by atomic weight


  Early 1800s
• Ways to categorise elements in early periodic tables

  • Their physical and chemical properties
  • Their weight
• Early scientists had no idea of atoms, protons, neutrons or electrons, so there was no such thing as arranging elements by atomic number
• Protons and electrons were discovered, which led to arranging elements by atomic number


  20th century
• Early periodic tables were not complete and some elements were placed in the wrong order
• This was because elements were placed in the order of atomic weight and did not take into account their properties
• Mendeleev's contribution


  Left gaps in periodic table and predicted new elements based on patterns in properties
• Mendeleev arranged elements by atomic weight but adjusted the order if the properties did not fit
• Example of Mendeleev adjusting order


  • Placing lanthanides and actinides in separate groups despite similar atomic weights
• Mendeleev's predictions of new elements were confirmed when they were later discovered
• Discovery of isotopes confirmed Mendeleev was correct to arrange by atomic number, not just atomic weight