Dobereiner's Law of Triads

The development of the periodic table begins with German chemist Johann Dobereiner (1780-1849) who  grouped elements based on similarities.  Calcium (atomic weight 40), strontium (atomic weight 88), and barium (atomic weight 137) possess similar chemical prepares.  Dobereiner noticed the atomic weight of strontium fell midway between the weights of calcium and barium:
     Ca   Sr   Ba      (40 + 137) ÷ 2 = 88
      40     88     137

Was this merely a coincidence or did some pattern to the arrangement of the elements exist? Dobereiner  noticed the same pattern for the alkali metal triad (Li/Na/K) and the halogen triad (Cl/Br/I).
   Li   Na  K         Cl   Br   I
      7     23     39           35    80   127

In 1829 Dobereiner proposed the Law of Triads: Middle element in the triad had atomic weight that was the average of the other two members. Soon other scientists found chemical relationships extended beyond triads. Fluorine was added to Cl/Br/I group; sulfur, oxygen, selenium and tellurium were grouped into a family; nitrogen, phosphorus, arsenic, antimony, and bismuth were classified as another group.

Source of this page:  http://mooni.fccj.org/~ethall/period/period.htm