* J.J. Thomson (1897): He discovered the electron and proposed the "plum pudding model" where electrons were embedded in a positively charged sphere. This model did not explicitly state electrons moved in empty space, but it did suggest they weren't fixed in a specific location.
* Ernest Rutherford (1911): Conducted the famous gold foil experiment, which proved that atoms have a small, dense, positively charged nucleus at their center. This discovery overturned the plum pudding model and led to the idea of a mostly empty atom with a nucleus surrounded by electrons.
* Niels Bohr (1913): Proposed the Bohr model, which suggested that electrons orbit the nucleus in specific energy levels. While the Bohr model was later superseded, it was the first to introduce the concept of electrons moving in quantized orbits around the nucleus.
* Quantum Mechanics (early 20th century): This groundbreaking theory fundamentally changed our understanding of atoms. Quantum mechanics showed that electrons do not move in orbits like planets, but rather exist as probability clouds called orbitals. This means that while we cannot pinpoint the exact location of an electron at any given time, we can calculate the probability of finding it in a specific region around the nucleus.
Therefore, while Rutherford's gold foil experiment provided the most direct evidence for the mostly empty structure of an atom, it was the development of quantum mechanics that ultimately solidified the idea that electrons move in empty space within the atom, albeit in a probabilistic way.
