There are altogether twelve quarks consisting of six pairs of particles and antiparticles. A particle and its antiparticle differ only in their charges. The names for the quark particles are Up, Down, Strange, Charm, Top and Bottom.

The problem concerning quark masses is that the mass of a pi meson with two quarks is only 273 elecron masses whereas a proton or a neutron with three quarks have masses of nearly two thousand electron masses.

The explanation is roughly that a nucleon has a spherical structure. Quarks are spherical shells of mass and a nucleon is three concentric quarkic spheres. A meson is two such concentric spherical shells. The quarks which make up a pi meson occupy a sphere of a radius that is about one half of the radius of a nucleon. They then occupy a volume which is about one eighth of the volume of the nucleon. If volume mass densities are the same then the meson would have one eighth the mass of a nucleon. Thus if the nucleon has a mass equal to two thousand electron masses then the meson would have a mass of 250=2000/8 electron masses. The full explanation, given in the webpages cited below, is only a bit more complicated.

• Estimates of the Mass Densities of Up and Down Quarks and Estimates of the the Outer Radii of the Small, Medium and Large Up and Down Quarks
• Spin, Mass and Size Characteristics of Hyperons Which Include a Strange Quark
• Spin, Mass and Size Characteristics of Hyperons Which Include a Charm Quark
• Spin, Mass and Size Characteristics of Hyperons Which Include a Bottom Quark
• Spin, Mass and Size Characteristics of Hyperons Which Include a Top Quark
• An Explanation for the Seemingly Relatively Low Mass of the Pi Mesons Compared to the Mass of a Proton
• Quark Masses