Think back to the Warm-Up at the beginning of this topic; when you rubbed the ruler or comb with the tissue, you were building up a charge on it. In turn, it was then attracted to anything else which has the opposite charge. This is because atoms have subatomic particles that are negatively, positively, and neutrally- charged. In this topic, you explored the discovery of an atom's negatively-charged particle - the electron. Through the experiments of scientists, like J.J. Thomson and Robert Millikan, the electron was examined as a subatomic particle with its own charge and mass. While early models of the atom, like the Plum Pudding Model, were later disproved, they set the foundation for what scientists study today.