Explain how scientists' model of the atom changed from the ancient Greek idea of the atom to the nuclear model, describing the key developments in order.

Ancient Greek philosophers such as Democritus proposed that matter was made of indivisible particles called atoms, the smallest possible units of matter.

Later, J.J. Thomson discovered the electron in 1897, leading to the plum‑pudding model: the atom was a positively charged sphere with electrons embedded within it.

In 1909, Rutherford’s alpha‑particle scattering experiment showed that most alpha particles passed straight through a thin gold foil, indicating that atoms are largely empty space.

However, a small fraction of alpha particles were deflected through large angles or even rebounded, revealing a small, dense, positively charged nucleus at the centre of the atom.

Thus the nuclear model replaced the plum‑pudding model because the latter could not account for the large‑angle deflections; in the nuclear model electrons orbit the nucleus rather than being embedded in a positive cloud.