may 1, 1829 - Johann Dobereiner's triads were an early attempt to sort the elements into some logical order. He also noted that some quantifiable properties of elements (e.g. atomic weight and density) in a triad followed a trend whereby the value of the middle element in the triad would be exactly or nearly predicted by taking the arithmetic mean of values for that property of the other two elements.
Added to timeline:
Date:
may 1, 1829
Now
~ 196 years ago