Image of Henri Becquerel                                                       Gif of Henri Becquerel, doing research on radioactivity

 Henri Becquerel (1852-1908), was a large part in the atomic theory, thanks to his research and discovery of radiation, and studying the effects of photographic film. By studying the effects of photographic film, he disovered radiation, which was a big part of the atomic theory, because it demonstrated that the atom was not divisble or changeable. 

This video here shows the backstory of Henri Becquerel, and how he discovered and experimented with radiation.  He was a scientist who was interested in the idea of fluorescence, because of this, in 1896, he started experimenting with a material called uranium, which glows, but was highly radioactive. In his experiment, he left non fluorescent uranium in complete darkness with photographic paper, but the paper did not respond to the properties of the uranium, and that was how radioactivity was discovered.

In conclusion, Henri Becquerel was a big contributor to the atomic theory, because of his discovery of radiation.