may 28, 1887 - Henry Moseley's discovery showed that atomic numbers were not arbitrarily assigned, but rather, they have a definite physical basis. Moseley postulated that each successive element has a nuclear charge exactly one unit greater than its predecessor. Moseley redefined the idea of atomic numbers to help sort the elements into an exact sequence of ascending atomic numbers that made the Periodic Table exact.
Added to timeline:
Date:
may 28, 1887
Now
~ 138 years ago