Protons, electrons, and neutrons are considered the fundamental particles that make up atoms. However, within the realm of particle physics, there are subatomic particles that are smaller than protons, electrons, and neutrons.
One such example is quarks, which are elementary particles and are the fundamental building blocks of protons and neutrons. Quarks come in different types or flavors: up, down, charm, strange, top, and bottom. Protons are composed of two up quarks and one down quark, while neutrons consist of two down quarks and one up quark.
Furthermore, there are other elementary particles like leptons, which include electrons, muons, and neutrinos. Electrons are negatively charged leptons that orbit around the nucleus of an atom. Muons are similar to electrons but are heavier, and neutrinos are neutral, very light particles that hardly interact with matter.
Particle physics also explores smaller particles beyond those mentioned above. For instance, there are smaller elementary particles like gluons, which mediate the strong force that binds quarks together, and photons, which are elementary particles of light and electromagnetic interactions.
It's important to note that these particles are considered point-like, meaning they do not possess a size or structure in the classical sense. They are treated as mathematical entities with specific properties and behaviors within the framework of quantum mechanics and the Standard Model of particle physics.