Elementary Particles

In our quest for the elementary building blocks of the universe, we delved inside the atomic nucleus and found that it is composed of protons and neutrons. Are the three particles, ${\displaystyle e}$, ${\displaystyle p}$ and ${\displaystyle n}$, the blocks we are looking for? The answer is no. Even before the structure of the atom was understood, Becquerel discovered the radioactivity (see Discovery of radioactivity) that afterwards puzzled physicists and forced them to look deeper - inside protons and neutrons.