In 1817, Johann Wolfgang Dobereiner thought that he had made a great observation about elements. He thought that there was a relationship between the atomic weights of elements and their properties. His theory was that, in groups of three (triads), the middle element had the mean atomic mass of the two elements to either side of it. This works with many elements, like Lithium, Sodium and Potassium. (Li + K) / 2 = Na (7 + 39) / 2 = 23 (jan 1, 1817 – 23h 59min, dec 31, 1817 y)
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Date:
jan 1, 1817
23h 59min, dec 31, 1817 y
~ 12 months