History of periodic table

• Steps of discovery of periodic table

  
  

  1. Before the discovery of the subatomic particles, scientists arranged the elements in order of their atomic weight and not their atomic number
• 2. The earlier tables were incomplete as some elements were forced into a position to fill gaps which appeared during the sorting process

  • Other elements were placed in the wrong group as they were sorted strictly on their atomic weight and had their chemical properties ignored
• 3. Mendeleev overcame some of the problems in the periodic table by leaving gaps for elements he had not yet discovered and in some places changed the order based on atomic weights
• • He also switched the order of the elements to maintain consistency down the columns
  • He quickly realised that elements with the same properties should be placed in the same column
• • He used the properties and trends of other elements in the group with the gap to predict the properties of these undiscovered elements
• 4. Elements with properties predicted by Mendeleev were discovered and filled the gaps. Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct