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1.5 History of the Atom

GCSE Chemistry · AQA · Chapter 1: Atomic Structure

A free GCSE Chemistry lesson for AQA (8462) from OBA School: how new evidence changed our model of the atom, from Dalton’s solid spheres to the nuclear model. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe the models of the atom suggested by Dalton, Thomson, Rutherford and Bohr
  • Explain how the alpha particle scattering experiment led to the nuclear model
  • Describe the differences between the plum pudding model and the nuclear model
  • Describe how protons and neutrons were discovered, and why scientific models change

Key words

  • Model: A simplified description or picture used to explain observations and make predictions.
  • Plum pudding model: Thomson’s model: a ball of positive charge with negative electrons embedded in it.
  • Alpha particle: A small, positively charged particle given out by some radioactive substances.
  • Nuclear model: A model with a tiny, positive nucleus that contains most of the mass, and electrons around it.
  • Energy level (shell): A fixed distance from the nucleus at which electrons orbit (Bohr).
  • Evidence: Observations and measurements used to test an idea or model.

Solid spheres and plum puddings

Over 2000 years ago, Greek thinkers suggested that everything was made of tiny particles that could not be cut. In the early 1800s, John Dalton used results from experiments to suggest that atoms are tiny solid spheres that cannot be divided, and that each element has its own type of atom.

In 1897, J. J. Thomson discovered the electron, a tiny negatively charged particle. He found that rays given off in a glass tube at a very high voltage were attracted towards a positive plate, so they must be made of negative particles. The particles were the same whatever gas or metal he used, and each had a mass about 2000 times smaller than a hydrogen atom, so they had to come from inside atoms. This showed that atoms can be divided. Because atoms have no overall charge, Thomson suggested the plum pudding model: a ball of positive charge with negative electrons scattered through it.

The alpha particle scattering experiment

In 1909, Hans Geiger and Ernest Marsden, working with Ernest Rutherford, fired positive alpha particles at a very thin sheet of gold foil in a vacuum. They used a detector to count where the alpha particles went. The plum pudding model predicted that the alpha particles would all pass through with, at most, tiny deflections, because the positive charge was spread out.

  • Most alpha particles went straight through.; Conclusion Most of the atom is empty space.
  • Some were deflected (changed direction).; Conclusion The centre of the atom has a positive charge that repels the alpha particles.
  • A very few bounced back.; Conclusion The positive charge and most of the mass are concentrated in a tiny nucleus.

Worked example: how rare were the bounce-backs?

In one run, 40 000 alpha particles were counted and 5 bounced back.
fraction = 5 ÷ 40 000 = 1 in 8 000
percentage = (5 ÷ 40 000) × 100 = 0.0125%

Answer: 1 in 8000 (0.0125%), so the nucleus must be tiny

Rutherford used these results in 1911 to suggest the nuclear model: a tiny, positive nucleus with electrons moving around it, and mostly empty space.

Bohr, protons and neutrons

In 1913, Niels Bohr adapted the nuclear model. He suggested that electrons orbit the nucleus at specific distances, in shells (energy levels). Bohr’s evidence came from light. When atoms are heated they give out light, but only of certain exact energies (colours). Bohr explained this: an electron can only have certain fixed energies, so when it drops from a higher shell to a lower one it gives out one fixed amount of energy. His theoretical calculations agreed with experimental observations, so the model was accepted.

Later experiments showed that the positive charge of any nucleus could be split into a whole number of smaller particles, each with the same amount of positive charge. These were named protons. About 20 years after the nucleus was proposed, in 1932, James Chadwick provided the evidence that the nucleus also contains neutrons, which have no charge. This explained why atoms are heavier than their protons alone could account for.

Why scientific models change

A model is kept as long as it explains the evidence. When new experiments give results that a model cannot explain, scientists change the model or replace it. The plum pudding model was not “silly”: it explained the evidence known in 1904. The scattering results could not be explained by it, so it was replaced.

  • 1803; Scientist(s) Dalton; Idea or discovery atoms are solid spheres that cannot be divided
  • 1897–1904; Scientist(s) Thomson; Idea or discovery discovered electrons; plum pudding model
  • 1909–1911; Scientist(s) Geiger, Marsden, Rutherford; Idea or discovery alpha scattering; nuclear model
  • 1913; Scientist(s) Bohr; Idea or discovery electrons in shells at specific distances
  • 1919–1920; Scientist(s) Rutherford and others; Idea or discovery protons in the nucleus
  • 1932; Scientist(s) Chadwick; Idea or discovery neutrons in the nucleus

Exam tip: In 6-mark questions about the history of the atom, give each model in order, and for each change say what new evidence caused it.

Common mistakes

  • Saying the electrons in the plum pudding model orbit the centre. They are embedded in a ball of positive charge.
  • Saying alpha particles bounced off electrons. They were repelled by the positive nucleus.
  • Mixing up observations and conclusions: “most went straight through” is the observation; “mostly empty space” is the conclusion.
  • Saying Rutherford discovered the neutron. Chadwick did, in 1932.
  • Thinking the plum pudding model had a nucleus. It did not: the nucleus came from the scattering experiment.

Revision questions

Try these, then check your answers below each one.

Question 1. Which describes the plum pudding model? (Choose from: A ball of positive charge with electrons embedded in it, A tiny nucleus with electrons orbiting it, A solid sphere that cannot be divided, A nucleus with electrons in fixed shells.)

Answer: A ball of positive charge with electrons embedded in it

Working: Thomson’s plum pudding model was a ball of positive charge with negative electrons scattered through it.

Question 2. In the alpha scattering experiment, most alpha particles passed straight through the gold foil. What did this show? (Choose from: Most of the atom is empty space, The nucleus is positive, The nucleus contains neutrons, Electrons are negative.)

Answer: Most of the atom is empty space

Working: If most alpha particles pass straight through, most of the atom must be empty space.

Question 3. True or false? The plum pudding model predicted that some alpha particles would bounce back from the gold foil.

Answer: False

Working: False. With the charge spread out, the model predicted that all the alpha particles would pass through. The bounce-backs were a surprise.

Related lessons

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