The scientist who discovered the electron was Sir Joseph John "J. J." Thomson, a British physicist. He made this discovery in 1897 through his experiments with cathode rays, where he demonstrated that cathode rays were composed of previously unknown negatively charged particles, which he initially called "corpuscles" and later became known as electrons. Thomson was the first to suggest that these particles were fundamental building blocks of atoms, much smaller than atoms themselves. This discovery was announced on April 30, 1897, and was a significant milestone in atomic physics as it showed atoms were not indivisible but had internal structure. For this and related work, Thomson was awarded the Nobel Prize in Physics in 1906.