Plum pudding model
In 1926 Erwin Schrödinger used the idea that electrons acted as a wave, as well as a particle, this is known as a wave-particle duality. This added a whole new layer to the atomic model and quantum physics. With a particle, you can know where it is in space if you observe (look) at it. But with a wave, it is all over the place, so you can't define where exactly it is. This is known as quantum uncertainty. With an electron, you can only know the probability of it being in a place, because it is a wave as well as a particle. (See diagram above)