Before 1897, the atom was considered the smallest unit of matter and was indivisible. In 1897, however, the English scientist J.J. Thomson changed the ideas of matter drastically. With his cathode ray tube experiment, Thomson discovered the electron, particle more than 1,000 times smaller than the lightest known particle at the time, the hydrogen atom. Thomson proved that the atom was not indivisible and began study of matter at the sub-atomic level.
To learn more about Thomson's experiment, click on this link- J.J. Thomson's Cathode Ray Experiment