Rutherford's model of the atom for GCSE Physics

Ernest Rutherford's model of the atom, published in 1911, showed that an atom is mostly empty space with a tiny, dense, positively charged nucleus at the centre. Electrons move around this nucleus, far from it. This nuclear model replaced J.J. Thomson's plum pudding model and is the foundation of the modern atomic picture taught at GCSE.

This guide explains how the gold foil experiment worked, why the results forced a rethink of atomic structure, how Rutherford's model compares to Bohr's later refinement, and the exam-ready facts AQA mark schemes want to see in 4 and 6 mark questions.

Before 1911, the accepted picture was J.J. Thomson's plum pudding model from 1904. Thomson had discovered the electron in 1897 and pictured the atom as a sphere of positive charge with negative electrons embedded in it like raisins in a pudding.

The model explained why atoms were neutral overall (the positive sphere balanced the negative electrons) and why electrons could be knocked out. What it could not predict was what would happen if you fired something at an atom and watched how it bounced off. That is the gap Rutherford's experiment filled.

In 1909, Hans Geiger and Ernest Marsden, working under Rutherford at the University of Manchester, fired alpha particles at a very thin sheet of gold foil. Alpha particles are positively charged and have about 4 times the mass of a hydrogen atom. They surrounded the foil with a fluorescent screen that flashed when an alpha particle hit it.

If the plum pudding model were correct, every alpha particle should have passed straight through with only a tiny deflection. The positive charge would be too spread out to deflect anything by much. What they actually saw was very different.

Rutherford published his nuclear model in 1911. He concluded that almost all the mass and all the positive charge of an atom is concentrated in a tiny region at the centre, which he called the nucleus. The rest of the atom is empty space, with electrons moving around the nucleus at relatively large distances.

This was a huge shift from the plum pudding model. The atom went from being a fuzzy ball of mixed charge to being a mostly empty space with a tiny, dense core. The diameter of the nucleus is around 100,000 times smaller than the diameter of the atom itself.

Rutherford's model had one big problem. Classical physics predicted that electrons orbiting a positive nucleus should spiral inwards and crash into it, which clearly does not happen. In 1913, Niels Bohr proposed that electrons orbit only at fixed distances from the nucleus, called energy levels or shells.

This solved the stability problem and explained why atoms only absorb and emit light at specific wavelengths. Later, James Chadwick discovered the neutron in 1932, completing the picture of the nucleus as protons plus neutrons. The model you draw at GCSE (nucleus with shells around it) is really the Bohr-Chadwick refinement of Rutherford's original idea.

AQA examiner reports flag the same handful of mistakes every year. The most common one is describing the gold foil experiment results without explaining what they showed about atomic structure. Examiners want both: The observation and the conclusion.

Another classic error is mixing up Rutherford and Bohr. Rutherford gave us the nucleus. Bohr gave us the fixed electron shells around it. If the question asks who proposed that electrons orbit at fixed energy levels, the answer is Bohr, not Rutherford.

Question: Describe how the results of the alpha particle scattering experiment led to a change in the model of the atom (4 marks).

Mark scheme answer: Most alpha particles passed straight through the gold foil, showing that the atom is mostly empty space (1). Some were deflected by small angles, showing that the centre of the atom has a positive charge (1). A very small number bounced almost straight back, showing that the positive charge and mass are concentrated in a very small region called the nucleus (1). This evidence replaced the plum pudding model with the nuclear model (1).