Atomic particles are the smallest components of matter and can be divided into two categories: fermions and bosons. Fermions include electrons, protons, neutrons, and mesons, while bosons include pions, positrons, muons, neutrinos, and baryons. An ELECTRON is a negatively charged particle that orbits the nucleus of an atom. A proton is a positively charged particle found in the nucleus of an atom. Neutrons are neutral particles found in the nucleus of an atom. Mesons are subatomic particles composed of one quark and one antiquark. Pions are short-lived particles composed of a quark and an antiquark. Positrons are positively charged particles that are the antiparticles of electrons. Baryons are particles composed of three quarks. Muons are subatomic particles similar to electrons but with a much greater mass. Ions are atoms or molecules with a net electric charge due to the loss or gain of one or more electrons. Neutrinos are electrically neutral particles that interact only via the weak nuclear force and gravity.