By composition
The atomic nucleus of helium-4 is composed of how many protons and how many neutrons?
Subatomic particle
The elementary particles of the Standard Model include: Six "flavors" of quarks: up, down, bottom, top, strange, and charm; Six types of leptons: electron, electron neutrino, muon, muon neutrino, tau, tau neutrino; Twelve gauge bosons (force carriers): the photon of electromagnetism, the three W and Z bosons of the weak force, and the eight gluons of the strong force; The Higgs boson. Various extensions of the Standard Model predict the existence of an elementary graviton particle and many other elementary particles. Composite subatomic particles (such as protons or atomic nuclei) are bound states of two or more elementary particles. For example, a proton is made of two up quarks and one down quark, while the atomic nucleus of helium-4 is composed of two protons and two neutrons. Composite particles include all hadrons: these include baryons (such as protons and neutrons) and mesons (such as pions and kaons).