Discover How We Came to Know the Cosmos

Chapter 9. Development of Atomic theory

18th December 2017 by Dr Helen Klus

9.1 Thomson’s model of the atom

British physicist Joseph John “J. J.” Thomson proposed the ‘plum pudding’ model of the atom in 1904,[1] seven years after he discovered the electron[2] (discussed in Chapter 6).

Thomson described the atom as being composed of electrons surrounded by a positive charge that neutralises the atom. The electrons are distributed like plums inside of a pudding, or raisins inside a fruitcake.

Diagram of Thomson’s model of the atom, where negative electrons are inside a positive nucleus.

Figure 9.1
Image credit

Thomson’s model of the atom.

9.2 Rutherford’s model of the atom

New Zealand physicist Ernest Rutherford disproved Thomson’s theory of the atom in 1911, when he showed that atoms are mostly composed of empty space. Rutherford discovered this by firing alpha rays - helium nuclei - at a thin sheet of gold foil.[3]

If Thomson’s theory were correct, then the alpha rays should pass straight through the gold atoms. Instead, Rutherford found that some of the nuclei were deflected at large angles. A few were even deflected back to where they had come from.

Diagram of Thomson’s model of the atom, showing that nuclei should travel straight through the atom.

Figure 9.2
Image credit

The predicted results of Rutherford’s experiment.

Diagram of Rutherford’s results, showing that some nuclei were deflected by a small nucleus in the centre of the atom.

Figure 9.3
Image credit

The actual results of Rutherford’s experiment.

Rutherford later described this as:

“...quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive center, carrying a charge”.[4]

Diagram of Rutherford’s model of the atom, where negative electrons are outside of a positive nucleus.

Figure 9.4
Image credit

Rutherford’s model of the atom.

Rutherford’s model of the atom is known as the planetary model because most of the mass of an atom is concentrated at the centre, and the electrons orbit the nucleus in a similar way to how planets orbit the Sun.

The main problem with Rutherford’s model was that he could not explain why negatively charged electrons remain in orbit, when they should instantly fall into the positively charged nucleus. This problem would be solved by Danish physicist Niels Bohr in 1913 (discussed in Chapter 10).[5]

9.3 Atoms and Brownian motion

By the end of 1911, French physicist Jean Perrin had finally proven that atoms exist, and verified British chemist John Dalton’s theory (discussed in Chapter 6) that different elements are composed of atoms of identical sizes.[6] He did this by conducting experiments based on German-Swiss-American physicist Albert Einstein’s explanation for Brownian motion.[7]

Brownian motion is named after British botanist Robert Brown who, in 1827, looked at pollen grains under a microscope and saw that they move around in the water in a similar way to how dust particles move in sunlight. He conducted more experiments and found that inorganic matter behaves the same way, but he didn’t know what caused this behaviour.[8]

In 1905, Einstein predicted that Brownian motion is caused by molecules of water converting heat into kinetic energy, causing them to move. The water molecules hit each other as well as the much larger pollen molecules, but because the water molecules cannot be seen, even under a microscope, it looks like the pollen is moving on its own.

Einstein showed that by studying the pollen grains you could calculate how many water molecules were colliding with them and their speed. Perrin worked out how to conduct these experiments and verified Einstein’s theory.

9.4 The 1911 Solvay Conference on Physics

1911 had been a groundbreaking year for atomic theory, and this cumulated with the first Solvay Conference on Physics, which was held in Belgium. Belgian chemist Ernest Solvay invited the greatest known physicists of the time to discuss ‘Radiation and the Quanta’. Those that attended included Jean Perrin, Max Planck, Heinrich Rubens, Arnold Sommerfeld, James Hopwood Jeans, Ernest Rutherford, Albert Einstein, Wilhelm Wien, and Marie Skłodowska Curie.[9]

Photograph of participants at the 1911 Solvay Conference on Physics.

Figure 9.5
Image credit

1911 Solvay Conference on Physics, 30th October - 3rd November 1911. Left-to right, Standing: Robert Goldschmidt, Max Planck, Heinrich Rubens, Arnold Sommerfeld, Frederick Lindemann, Maurice de Broglie, Martin Knudsen, Fritz Hasenöhrl, Georges Hostelet, Édouard Herzen, James Hopwood Jeans, Ernest Rutherford, Heike Kamerlingh Onnes, Albert Einstein, and Paul Langevin. Seated: Walther Nernst, Marcel Brillouin, Ernest Solvay, Hendrik Lorentz, Emil Warburg, Jean Perrin (reading), Wilhelm Wien (upright), Marie Skłodowska Curie, and Henri Poincare. Ernest Solvay was not present when the photo was taken and his portrait was pasted on before the picture was released.

9.5 References

  1. Thomson, J. J., The London Edinburgh and Dublin Philosophical Magazine and Journal of Science 1904, 7, 237–265.

  2. Thomson, J. J., The London Edinburgh and Dublin Philosophical Magazine and Journal of Science 1897, 44, 293–316.

  3. Rutherford, E., The London Edinburgh and Dublin Philosophical Magazine and Journal of Science 1911, 21, 669–688.

  4. Rutherford, E. in Background to Modern Science, (Eds.: Needham, J., Pagel,W.), Cambridge University Press, 1938.

  5. Bohr, N., The London Edinburgh and Dublin Philosophical Magazine and Journal of Science 1913, 26, 1–25.

  6. Nye, M. J., Before Big Science: The Pursuit of Modern Chemistry and Physics, 1800-1940, Harvard University Press, 1999.

  7. Einstein, A., Annalen der Physik 1905, 17, 549–560.

  8. Brown, R., The Philosophical Magazine or Annals of Chemistry Mathematics Astronomy Natural History and General Science 1828, 4, 161–173.

  9. Straumann, N., The European Physical Journal H 2011, 36, 379–399.

Back to top

How We Came to Know the Cosmos: Light & Matter

I Pre 20th Century theories

1. Atoms and Waves

2. Reflection, Refraction, and Diffraction

3. Newton's theory of Light

4. Measuring the Speed of Light

5. 19th Century Wave Theories

6. 19th Century Particle Theories

7. Spectral Lines

II Quantum Mechanics

8. Origin of Quantum Mechanics

9. Development of Atomic theory

10. Quantum Model of the Atom

11. Sommerfeld's Atom

12. Quantum Spin

13. Superconductors and Superfluids

14. Nuclear Physics

15. De Broglie's Matter Waves

16. Heisenberg's Uncertainty Principle

17. Schrödinger's Wave Equation

18. Quantum Entanglement

19. Schrödinger's Cat

20. Quantum Mechanics and Parallel Worlds

III Quantum field theories

21. The Field Concept in Physics

22. The Electromagnetic Force

23. The Strong Nuclear Force

24. The Weak Nuclear Force

25. Quantum Gravity

IV Theories of the mind

26. Mind-Body Dualism

27. Empiricism and Epistemology

28. Materialism and Conscious Matter

29. Material theories of the Mind

30. Material theories of the Mind vs. Descartes

31. The Mind and Quantum Mechanics

32. The Limitations of Science

V List of symbols

33. List of symbols

34. Image Copyright