Elementary particle

A elementary particle is one that has no internal structure; that is, no experiment has observed constituent particles that combine to make it up. Nonetheless, and has proven true historically, particles considered "elementary" in one epoch, the atom for example, are later found to be composite.

In the Standard Model elementary particles fall into different groups: the group of "particles" themselves, which fall under the classifications of leptons and quarks, and the particles that mediate the interactions between them, the force carriers or field quanta, that fall under the categories of photons, weak bosons, and gluons.