Protons, neutrons, and electrons are commonly referred to as elementary particles because they are considered fundamental particles, meaning they are not known to be composed of smaller, more fundamental constituents.
In the framework of the Standard Model of particle physics, which describes the known fundamental particles and their interactions, protons, neutrons, and electrons are classified as elementary particles. They are not made up of smaller particles or substructures that we are currently aware of.
Elementary particles are considered to be point-like, meaning they do not have a discernible size or internal structure. They are treated as mathematical entities with specific properties, such as mass, charge, and spin.
The term "elementary" does not imply that these particles cannot undergo interactions or participate in complex phenomena. Protons, neutrons, and electrons interact with other particles through various fundamental forces, such as the electromagnetic, weak nuclear, and strong nuclear forces. For example, protons and neutrons interact via the strong force to form atomic nuclei, and electrons interact with atomic nuclei through the electromagnetic force to create stable atoms.
It is worth noting that our understanding of the fundamental nature of particles continues to evolve with ongoing scientific research. Future discoveries or advancements in theoretical physics may lead to new insights about the ultimate structure of matter. However, at present, protons, neutrons, and electrons are considered elementary particles based on our current understanding of particle physics.