jan 1, 1869 - Dmitri Mendeleev 1869- organised the elements by atomic mass but made it into a table form. Elements with similar properties were put into the same column. Left blanks where he believed missing elements were, and he was correct. Gave rise to the Periodic Law which was properties of elements are periodic functions of their atomic mass.
Added to timeline:
Date:
jan 1, 1869
Now
~ 157 years ago