The history of the atomic model is long—we could go back as far as the ancient Greeks, really—but for our purposes, we can start around 1900. It was about then that Sir Joseph John Thomson discovered the electron, which is the negatively charged part of an atom. He proposed that these electrons were captured in uniform spheres of positively charged matter. This was dubbed the “plum-pudding model,” since the electrons in the positive substance is a bit like plums in English pudding. New Zealand physicist Ernest Rutherford discovered that if you shoot positive particles at atoms (in the form of gold foil), they don’t all bounce off the way they should if there was a large mass of positive “pudding.” Instead, some bounce off, but most pass through, suggesting that electrons are spaced around a small mass of positive substance—a nucleus, if you will. He rejiggered the model in 1911 to have electrons orbiting a nucleus the way that planets orbit the sun, which was dubbed the “planetary model,” for obvious reasons. The planetary model has become the most famous symbol for the atom—even though it was refined only two years later by Danish physicist Niels Bohr.

 Steven Dutch of the University of Wisconsin Green Bay clearly sums up the next step in the atomic model: “By the 1920’s, physicists had discovered that matter also has wave-like properties and that it just doesn’t work at the atomic level to regard particles as tiny points with precise locations and energies. Matter is inherently ‘fuzzy.’ They gave up thinking of electrons as tiny planets altogether.” Electrons don’t really follow paths at all. Physicists discovered that they’re actually quantum particles that exist in many different places at once. They still occupy individual energy levels, but instead of a path, each electron’s many-places-at-once location could be thought of as a cloud. That’s why it’s known as the electron cloud model.