1. Thomson’s model of the atom ↑
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 of a fruitcake.
Thomson's model of the atom. Image credit: Fastfission/Public domain.
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.
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.
The predicted results of Rutherford's experiment. Image credit: Fastfission/Public domain.
The actual results of Rutherford's experiment. Image credit: Fastfission/Public domain.
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".
Rutherford's model of the atom. Image credit: Night Ink/Public domain.
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.
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 that different elements are composed of atoms of identical sizes. He did this by conducting experiments based on German-Swiss-American physicist Albert Einstein's explanation for Brownian motion.
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.
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.
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 physicists of the time to discuss 'Radiation and the Quanta'.
1911 Solvay Conference on Physics, 30th October - 3rd November 1911. Image credit: Benjamin Couprie/Public domain.
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 Sklodowska-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.