jan 3, 1869 - Dimitri Mendellev used John Newlands' grouping and organized the elements into what is now known as the periodic table. He used the atomic mass as the primary characteristic to decide where each element belonged in his table. The elements were arranged in rows and columns. He even left spaces for elements to be discovered because of the pattern he saw once he started organizing those elements known at that that time.
Added to timeline:
Date:
jan 3, 1869
Now
~ 156 years ago