The physicist known for discovering the electron is J.J. Thomson. In 1897, Thomson conducted experiments with cathode rays and found that they could be deflected by electric fields, leading to the identification of the electron as a negatively charged particle. His groundbreaking work fundamentally changed the understanding of atomic structure and contributed to the development of modern physics. Thomson's discovery not only advanced scientific knowledge but also paved the way for future research in quantum mechanics and the field of particle physics.