Experiments suggested that atoms were actually made up of smaller particles, which motivated the development of new models involving protons, neutrons, and electrons. One was Thomson’s Plum Pudding Model, which described the atom as a positively charged “pudding” filled with negatively charged electrons. Rutherford later proposed the idea of a positively charged nucleus, but couldn’t explain why electrons didn’t just fall into it. A Danish physicist named Niels Bohr proposed the idea that electrons travel around the nucleus in specific orbits and advanced the atomic theory to a point very close to where it is today.