Protons and neutrons are fundamental particles with no internal structure.

After the neutron was discovered in 1932, the atomic nucleus became easier to describe. Nuclei could be built from protons and neutrons, collectively called nucleons.

For a time, it was reasonable to treat those nucleons as elementary particles with no smaller components.

High-energy scattering changed that picture.

The basic idea is similar to using an echo to learn about an object you cannot see directly. Physicists fired energetic electrons at protons and measured how the electrons bounced away.

If a proton were a smooth point-like particle, the scattering should follow one pattern. Instead, at high energies the electrons behaved as though they were striking much smaller charged objects inside the proton.

These experiments became known as deep-inelastic scattering and supplied strong evidence for internal constituents. The smaller objects were identified with quarks.

Modern quantum chromodynamics describes quarks as bound together by gluons, the carriers of the strong interaction.

A proton is not simply three tiny hard balls sitting motionless inside a shell. Its interior is a highly dynamic quantum system containing gluon fields and temporary quark-antiquark pairs. Much of the proton’s mass comes from the energy of those strong-interaction fields and motions.

Protons and neutrons remain extremely useful building blocks for nuclear physics. They are composite rather than fundamental.