The concept of the atom was first hypothesized by the ancient Greek philosopher Democritus, who described atoms as the fundamental, indivisible units of matter. In the early 1800s, John Dalton modernized this idea by proposing the atomic theory based on experimental data, which described atoms as solid spheres. Then in 1897, J.J. Thomson discovered the electron, showing that atoms are divisible and contain smaller charged particles. Later, Ernest Rutherford discovered the atomic nucleus, radically changing the model of the atom to one with a dense center surrounded by electrons.