What discovery lead J.J. Thomson to decide that atoms were not indivisible but are actually composed of smaller parts?

J.J. Thomson's discovery of the electron in 1897 was pivotal in demonstrating that atoms are not indivisible. Through his experiments with cathode rays, he identified these negatively charged particles, suggesting that atoms contain smaller components. This finding challenged the long-held notion of the atom as a fundamental, indivisible unit and laid the groundwork for the modern understanding of atomic structure.