Scientific models of the atom

1. Atoms are far too small to be seen directly, so scientists describe them using a scientific model built from experimental evidence.
2. A good model explains all of the observations made so far and allows scientists to make predictions about new experiments.
3. When an experiment produces a result that the current model cannot explain, the model has to be adapted or replaced by a better one.
4. A new model is only accepted once other scientists have repeated the experiment, checked the results and agreed with the conclusions, a process called peer review.
5. Each earlier model of the atom was a sensible explanation of the evidence available at the time, so it was replaced because of new evidence rather than because it was careless.
6. The atomic model changed several times between the early 1800s and the 1930s, and each change was driven by a new discovery.

Dalton's solid sphere model

1. In the early 1800s John Dalton described atoms as tiny solid spheres that could not be divided into anything smaller.
2. In Dalton's model each element was made of its own type of atom, and atoms of different elements had different masses.
3. This model explained how elements combine in fixed proportions, but it gave atoms no internal structure at all.

Thomson's plum pudding model

1. In 1897 J.J. Thomson discovered the electron, a particle with a negative charge and a mass far smaller than that of any atom.
2. Because electrons could be removed from atoms, the atom could no longer be an indivisible solid sphere, so Dalton's model had to be replaced.
3. Atoms are electrically neutral overall, so Thomson reasoned that there must also be positive charge in the atom to cancel the negative charge of the electrons.
4. In his plum pudding model, the atom was a ball of evenly spread positive charge with negative electrons scattered through it, like dried fruit in a Christmas pudding.
5. In this model the mass and the positive charge were spread evenly through the whole volume of the atom, with no dense centre.

The alpha particle scattering experiment

1. In 1909 Hans Geiger and Ernest Marsden, working under Ernest Rutherford, fired a narrow beam of alpha particles at a very thin sheet of gold foil inside a vacuum chamber.
2. Gold was chosen because it can be hammered into a foil only a tiny fraction of a millimetre thick, so the alpha particles had very few atoms to pass through.
3. The chamber had its air removed so that the alpha particles would not be stopped or scattered by air molecules before reaching the foil.
4. A small zinc sulfide screen gave a tiny flash of light whenever an alpha particle struck it, and the screen was moved round the foil to count the flashes at each angle.
5. If the plum pudding model were correct, the thinly spread positive charge would only nudge each fast alpha particle slightly, so every particle should have passed through with a very small deflection.
6. Most alpha particles did pass straight through the foil undeflected, which showed that most of the atom is empty space.
7. A small number were deflected through large angles, which showed that the positive charge is concentrated in a very small region that repels the positive alpha particles.
8. A very few, roughly 1 in every 8000, bounced back towards the source, which showed that this small central region also contains most of the mass of the atom.
9. In 1911 Rutherford explained these results with the nuclear model, in which a tiny, dense, positively charged nucleus sits at the centre with electrons moving around it.
10. The plum pudding model could not explain the large deflections, so it was replaced by the nuclear model once other scientists had confirmed the results.

Bohr's energy levels

1. Rutherford's nuclear model did not explain where the electrons are or why they stay in the atom.
2. Heated gases give out light of only certain specific frequencies, and the nuclear model could not explain this pattern either.
3. In 1913 Niels Bohr adapted the nuclear model by proposing that electrons orbit the nucleus at fixed distances, called energy levels or shells.
4. An electron in a particular energy level has a fixed amount of energy, so it can only move to another level by absorbing or giving out an exact amount of energy.
5. This explained the specific frequencies of light given out by each element, and Bohr's calculations agreed with the experimental measurements.
6. Because the adapted model matched the evidence, the Bohr model was accepted and it is the picture of the atom still used at GCSE.

Protons and neutrons

1. Later experiments by Rutherford showed that the positive charge of a nucleus is carried by smaller particles, each with the same positive charge, which were named protons.
2. The mass of most nuclei turned out to be greater than the mass of their protons alone, which suggested that the nucleus contained another particle.
3. In 1932 James Chadwick provided experimental evidence for this particle, the neutron, which has no charge.
4. Chadwick's discovery completed the model of a nucleus made of protons and neutrons, surrounded by electrons in energy levels.