Why are protons and neutrons not elementary particles?

Protons and neutrons are not considered elementary particles because they possess an internal structure and are composed of smaller, indivisible entities known as quarks.512

While early 20th-century physics once categorized protons and neutrons as fundamental building blocks, high-energy experiments conducted in the latter half of the century revealed that they are actually complex, composite structures. Specifically, protons and neutrons are classified as baryons—a type of hadron—and each is formed by the combination of three quarks bound together by the strong force through the exchange of particles called gluons.1234

In contrast, an elementary particle is defined as an entity that contains no discernible structure and cannot be reduced or separated into smaller components. Quarks, along with leptons (such as the electron), are currently regarded as true elementary particles because they are pointlike and lack internal constituents.564