The atomic structure [1]: the discovery of fundamental particles

Until the second half of the 18th century, the most widely accepted idea of atom was Dalton's one. His theory was based on 5 fundamental points:

1. Everything is composed by invisible and indivisible particles, called atoms. This idea was present also in Greece over 2500 years earlier, because of Democrito.
2. Atoms cannot be created and cannot be destructed;
3. In a certain element, all the atoms are equal, sharing the same mass and chemical properties;
4. Different elements are made by different atoms, with different mass and chemical properties;
5. Different atoms can combine each other to form more complex particles

It revealed to be false at the beginning of 19th century, when it was discovered that atoms weren't fundamental particles, but were composed by three other particles: electrons, protons and neutrons.

Electrons were the first ones to be discovered, because of Goldstein's researches  on cathode rays. Those one were emitted on a tube filled with rare gas and inside a strong electric field by the negative electrode (cathode), and appeared to cause fluorescence to certain elements. The fact that those particles got deflected by magnetic fields made Goldstein realize that they were negative-charged. They also resulted to form with every possible gas used., and independently by the material the cathode was made by.

In 1897 Thomson succeeded in calculating the charge/mass ratio. Later Millikan succeeded in calculating also the real value of its charge. Those discoveries led Thompson to hypothesize a model in which the atom was made by a positive charged nucleus, with electrons distributed on its surface, to justify the global neutral charge.

This atomic model resulted to be wrong in 1911, because of Geiger-Mardsen experiment. A source of alpha particles was disposed in the front of a thin sheet of gold, with a fluorescent screen, to analize their direction.  The result was that 99% of particles passed through the sheet without any deviation, a little amount were deflected, and only few were reflected.

Rutherford hypothesized that atoms were mainly empty, while most of the mass was concentrated in the nucleus, which is positively charged. Electrons had to orbit around the nucleus, in a way similar to planets one.They were on stationary state, in equilibrium between coulombian and gravitational force.  Rutherford calculated also that  the protons that made the nucleus were only half of the mass of the atom, and that electrons contributed in a very limited way. Another neutral massive particle was required, the neutron. It was discovered only in 1932 by Chadwick, because of neutral high energy radiation emitted by nucleus.

Rutherford's atomic model was however in contradiction with classical laws of electromagnetism. A charged particle moving will indeed slowly loose its energy, emitting electromagnetic waves. This is why an electron orbiting around a nucleus would loose its energy in a split second, not allowing the existence of any stable atom.