1896, Discovery of Radioactivity

Photo Credit: www.nobelprize.org
Year: February 1896
Chemist: Henri Becquerel
Contribution: Discovery of Radioactivity

While studying the effect of uranium on photo paper, Henri Becquerel accidentally stumbled onto radioactivity when he found the decay had left an image on his photo plate. Radioactivity is an aspect of some elements that causes them to shoot off radiation as they change into something else. Becquerel did not pursue further study in radioactivity, but this discovery did impact the atomic theory timeline because radioactivity directly relates to an atom's structure.

Information Credits: Atomic Structure Timeline, Department of Chemistry at Duke University