The development of the Periodic Table on Planet Earth. By 1860 about 60 elements were discovered, with some basic data on them- not much more than their masses. Using this data alone, some patterns were discovered.
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) รท 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.
2. What was it that Dobereiner noticed about the masses of elements that he thought was interesting? Please use your own words to describe this.
3. Did Dobereiner arrange the triads into columns, or rows? ___________ Why do you suppose he did that?
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