QUARKS
These particles make up protons, neutrons and a veritable zoo of lesser-known particles. They have never been observed in isolation.

Electric charge: +2/3
Mass: 2 MeV
Constituent of ordinary matter; two up quarks, plus a down, make up a proton.

Electric charge: -1/3
Mass: 5 MeV
Constituent of ordinary matter; two down quarks, plus an up, compose a neutron.

Electric charge: +2/3
Mass: 1.25 GeV
Unstable heavier cousin of the up; constituent of the J/theta particle, which helped physicists develop the Standard Model.

Electric charge: -1/3
Mass: 95 MeV
Unstable heaver cousin of the down; constituent of the much studied kaon particle.

Electric charge: +2/3
Mass: 171 GeV
Heaviest known particle, comparable in mass to an atom of osmium. Very short-lived.

Electric charge: -1/3
Mass: 4.2 GeV
Unstable and still heavier copy of the down; constituent of the much studied B-meson particle.

LEPTONS
These particles are immune to the strong force and are observed as isolated individuals. Each neutrino shown here is actually a mixture of neutrino species, each of which has a definite mass of no more than a few eV.

Electric charge: 0
Immune to both electromagnetism and the strong force, it barely interacts at all but is essential to radioactivity.

Electric charge: -1
Mass: 0.511 MeV
The lightest charged particle, familiar as the carrier of electric currents and the particles orbiting atomic nuclei.

Electric charge: 0
Appears in weak reactions involving the muon.

Electric charge: -1
Mass: 106 MeV
A heavier version of the electron, with a lifetime of 2.2 microseconds; discovered as a component of cosmic-ray showers.

Electric charge: 0
Appears in weak reactions involving the tau lepton.

Electric charge: -1
Mass: 1.78 GeV
Another unstable and still heavier version of the electron, with a lifetime of 0.3 picosecond.