In the article on elementary particles, we read that, in 1804, John Dalton demonstrated the existence of atoms, which he imagined as hard spheres.
More than 90 years later, in 1897, the experiments of Joseph John Thomson, a British physicist, showed that atoms contain electrons, tiny particles with a negative electric charge. Thomson suggested that an atom is like a Christmas pudding with positive electric charge containing "raisins" with a negative charge, which he called electrons.
In 1911, Ernest Rutherford, a New Zealand scientist, could not explain the results of his experiments using Thomson's Christmas pudding model. He proposed a new atomic model where positive and negative charges are far apart. In Rutherford's atom, there is a central part with a positive charge called the nucleus, surrounded by electrons moving along orbits, just like planets in the solar system.
This type of atom is the one we are used to seeing in the logos of major scientific organizations, such as the International Atomic Energy Agency (IAEA) and the United States Atomic Energy Commission.
Unfortunately, these representations are misleading. According to Rutherford, the atom is more empty than full: the nucleus, which holds nearly all of the atom's mass, is about 10,000 times smaller than the entire atom. Electrons, which are incredibly tiny, are far from the nucleus. If the nucleus were scaled up to the size of a tennis ball, the electrons would be smaller than grains of sand and located more than 300 meters away, which is the length of three soccer fields.
Rutherford showed that the intuition of Leucippus and Democritus that matter is made up of extremely small building blocks surrounded by empty space (see the article on elementary particles) was correct.
A sense of immense emptiness is the first thing Millie notices when she arrives in the world of particles during her first adventure. To learn more, read the article on the empty space in the atom.
It is interesting to note that discoveries in particle physics are never based on direct observations, since atoms and their components cannot be seen. The discovery of the electron is an example of how the idea of the atom, which physicists call the atomic model, evolved over time to account for the results of successive experiments. An atomic model is discarded every time it fails to explain a new result. Rutherford’s atomic model, though still very popular, was actually replaced in 1913 by the model of the Danish physicist Niels Bohr. We will talk about this in the article dedicated to energy levels and quantization.
