You're likely thinking of J.J. Thomson, a physicist, not a chemist. He was a crucial figure in the development of atomic theory and is most famous for his discovery of the electron in 1897.

Here's a breakdown of his key contributions:

* Discovery of the electron: Thomson's experiments with cathode rays led him to conclude that they were made up of negatively charged particles, much smaller than atoms. This was the first discovery of a subatomic particle and revolutionized our understanding of the atom.

* Plum Pudding Model: Thomson proposed a model of the atom where negatively charged electrons were embedded in a positively charged sphere, much like plums in a pudding. Although later proven incorrect, it was a significant step towards understanding atomic structure.

While not a chemist himself, Thomson's work had a profound impact on the development of chemistry:

* Understanding chemical bonding: The discovery of the electron helped to explain how atoms bond together to form molecules.

* Explaining chemical reactions: Thomson's work provided insights into the nature of chemical reactions and how atoms interact with each other.

* Foundation for modern chemistry: His work laid the groundwork for the development of quantum mechanics and our modern understanding of atomic structure, which is crucial for modern chemistry.

So, while J.J. Thomson wasn't a chemist, his discoveries were fundamental to the progress of the field.