Development of the Periodic Table

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Development of the Periodic Table
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John Newlands (1865)

• Arranged the known elements in order of
  increasing atomic mass
• Noticed similar chemical and physical properties
  after every 8 elements
• Called the Law of Octaves
• Did not leave room for undiscovered elements

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Dmitri Mendeleev (1869)

• Corrected many irregularities in Newlands
  Octaves
• Still arranged by atomic mass
• Periodic table close to our own
• Lead to massive increase in discovery of new
  elements
• Still a few irregularities

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• ekaboron (Eb), ekaaluminium (Ea), ekamanganese
  (Em), and ekasilicon (Es), proved to be good
  predictors of the properties of scandium,
  gallium, technetium and germanium respectively

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Henry Moseley (1915)

• Showed atomic number not arbitrary
• Arranged periodic table in order of atomic number
• Proved that there were still a few gaps in the
  periodic table, later filled with Tc, Pm, Hf and
  Rh
• Killed by sniper in Battle of Gallipoli in WWI at
  age 27, 1915

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• Isaac Asimov once wrote
• "In view of what he Moseley might still have
  accomplished ... his death might well have been
  the most costly single death of the war to
  mankind generally."
• Because of Moseley's death in the War, the
  British government began a policy of no longer
  allowing their scientists to enlist for combat.

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You Should Know

• Periodic Law The elements are arranged in order
  of INCREASING ATOMIC NUMBER, so elements with
  similar properties occur at regular intervals
  (they exhibit periodicity)
• Periods elements in horizontal rows
• Groups (families) elements with similar
  properties arranged in vertical columns
• Main Group elements Groups 1, 2, and 13 - 18