What is the difference between the three stages in the development of elementary particle physics?

At the first stage, figuring out what matter is made of, it was determined that it consists of atoms. Then it turned out that the atom has a complex structure. The found particles that make up the atom, electron, proton and neutron were considered indivisible – the building blocks of the universe. At the second stage, it became clear that immutable particles do not exist, the main property of elementary particles is interconversion.
At the third stage, the theory of quarks was created. The number of quarks is six. Quarks have not been found experimentally. In the first two stages, experiment played the main role. At the third stage, a model of the structure of elementary particles was proposed.

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