Complex sentence: However, in the 20th century, the "atoms" of the chemists were found to be composed of even smaller entities: electrons, neutrons, and protons, and further experiments showed that protons and neutrons are made of quarks.
Split into simpler sentences: However, in the 20th century, the "atoms" of the chemists were found to be composed of even smaller entities: electrons, neutrons, and protons. Further experiments showed that protons and neutrons are made of quarks.