Atoms are composed of a nucleus, which contains protons and neutrons, surrounded by electrons. Protons and neutrons, in turn, are made up of quarks held together by the strong nuclear force.
Quarks are elementary particles that come in six different types or flavors: up, down, charm, strange, top, and bottom. Protons are made up of two up quarks and one down quark, while neutrons consist of two down quarks