Development of the periodic table

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  • Development of the Periodic Table
    • Early 1800s
      • John Dalton
        • Arranged elements in order of atomic mass
    • 1864
      • John Newlands
        • Noticed that every eighth element seemed similar
          • He assumed all the elements had been found
            • Did not take into account that scientists were still discovering new ones
        • 'Law of Octaves'
          • Only worked for the known elements up to Calcium
        • Some scientists ridiculed his ideas and refused to accept them
    • 1869
      • Dmitri Mendeleev
        • Placed them in order of atomic mass
          • He arranged them so that a periodic pattern in their properties could be seen
        • Left gaps for undiscovered elements
          • Used his table to predict the properties of elements that were yet to be discovered
        • Not all elements fit in with his pattern
          • e.g. Argon (noble gas) and Potassium (very reactive metal)
            • Argon atoms have a larger relative atomic mass than potassium
              • Following Mendeleevs rule, argon was in the same group as reactive metals such as sodium and lithium
              • Potassium would be in a group with the noble gases
            • Simply changed the order so that elements with similar properties could be in the same group
      • Around 50 elements had been identified
    • Early 1900s
      • Solved the issue of elements breaking the periodic pattern
        • In order of number of protons (atomic number)
        • Existence of isotopes

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