History of the Periodic Table

• Before scientists discovered sub-atomic particles (protons, neutrons and electrons), they tried to sort elements by their weight. Today, we rank elements by their atomic number (the number of protons in an atom's nucleus).
• Newlands' periodic table

  • John Newlands was the first chemist to devise a periodic table.
  • His periodic table was ordered by the weight of the element.
  • However, the table was incomplete, and some elements were placed in inappropriate groups.

  
• Mendeleev's periodic table

  • Dimitri Mendeleev realised that there may be undiscovered elements.
  • He added gaps to Newlands’ table to account for undiscovered elements.
  • He even predicted the properties and masses of these undiscovered elements!

  
• The modern periodic table

  • The discovery of protons and isotopes has shown that Mendeleev ordered elements exactly by atomic number (number of protons).
  • Therefore, the modern periodic table looks very similar to Mendeleev's (except the gaps are filled).