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How did the model of the atom change as new experimental evidence was gathered?

The historical development of the atomic model from Dalton to the nuclear model, including the plum pudding model, the alpha particle scattering experiment, the discovery of the nucleus, Bohr's shells, and the discovery of the neutron.

A focused answer to OCR Gateway GCSE Chemistry A topic C1.2 on how the atomic model developed, covering Dalton, the plum pudding model, the alpha particle scattering experiment and the nuclear model, Bohr's energy levels, and the discovery of protons and neutrons.

Generated by Claude Opus 4.88 min answer

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  1. What this topic is asking
  2. Dalton's model
  3. The plum pudding model
  4. The alpha particle scattering experiment
  5. The nuclear model and Bohr's shells
  6. Protons and neutrons

What this topic is asking

OCR wants you to describe how the model of the atom changed over time as scientists gathered new experimental evidence, and to explain how each piece of evidence led to a new model. This is a classic example of how science develops: a model is accepted until new data forces it to be revised. You need the key scientists, their models, and the experiments behind them.

Dalton's model

Dalton's model treated the atom as a single solid particle with no internal structure. It explained the laws of chemical combination, but it could not account for charge, which later experiments revealed.

The plum pudding model

In 1897 J. J. Thomson discovered the electron, a tiny negatively charged particle, showing that atoms are not indivisible after all. To include these electrons, Thomson proposed the plum pudding model: the atom is a ball of positive charge with negative electrons embedded in it, like pieces of fruit in a pudding. The overall charge is neutral because the negative electrons balance the positive charge of the "pudding".

The alpha particle scattering experiment

The plum pudding model was tested by an experiment carried out by Geiger and Marsden under Ernest Rutherford around 1909. Positively charged alpha particles were fired at a very thin sheet of gold foil, and the directions they travelled in were recorded. The plum pudding model predicted that the alpha particles should pass almost straight through, only slightly deflected.

The nuclear model and Bohr's shells

From these results Rutherford proposed the nuclear model: a tiny, dense, positively charged nucleus at the centre, holding most of the mass, with the electrons moving around the outside and most of the atom being empty space. This replaced the plum pudding model because it explained the scattering results.

There was a problem: the laws of physics suggested the orbiting electrons should lose energy and spiral into the nucleus. Niels Bohr solved this by proposing that electrons orbit the nucleus at fixed distances in specific energy levels (shells). The Bohr model matched experimental observations and is close to the model still used at GCSE.

Protons and neutrons

Later experiments showed the nucleus contains positively charged particles, the protons (associated with Rutherford). About 20 years after the nucleus was discovered, James Chadwick (1932) provided evidence for the neutron, a neutral particle in the nucleus. This completed the picture of a nucleus of protons and neutrons surrounded by electrons in shells.

Exam-style practice questions

Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

OCR 20196 marksDescribe how the model of the atom changed from the plum pudding model to the nuclear model, and explain how the results of the alpha particle scattering experiment provided evidence for the nuclear model.
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A Higher tier six-mark Level of Response question. Reward describing the plum pudding model (the atom is a ball of positive charge with negative electrons embedded in it, like fruit in a pudding) and then the nuclear model (a tiny, dense, positively charged nucleus at the centre with electrons around the outside, mostly empty space). Then explain the experiment: positive alpha particles were fired at a thin gold foil. Most passed straight through (showing the atom is mostly empty space), some were deflected through small angles (showing the centre is positively charged and repels them), and a very few bounced almost straight back (showing the centre is tiny but very dense and holds most of the mass and charge). These results could not be explained by the plum pudding model, so it was replaced by the nuclear model. Markers reward the two models, the three observations, and the conclusions drawn from each observation.

OCR 20213 marksState the contribution of Niels Bohr to the model of the atom, and name the two scientists associated with the discovery of the proton and the neutron.
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A C1.2 structured recall question. Reward: Niels Bohr adapted the nuclear model by proposing that electrons orbit the nucleus at specific distances in fixed energy levels (shells), which explained why electrons did not spiral into the nucleus and matched experimental observations. The proton is associated with Ernest Rutherford (who showed the nucleus contained positively charged particles), and the neutron is associated with James Chadwick (who discovered the neutral particle in the nucleus about 20 years after the nucleus itself was identified). Markers credit the idea of electrons in fixed shells or energy levels for Bohr, Rutherford for the proton, and Chadwick for the neutron.

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