A quark is a type of elementary particle that acts as one of the fundamental building blocks of matter, making up protons and neutrons inside atomic nuclei. Quarks always appear bound together (never alone) and interact through the strong nuclear force, which “glues” them into composite particles called hadrons.

Basic idea

  • Quarks are considered point‑like, with no known internal structure, so they are treated as fundamental in the Standard Model of particle physics.
  • They combine in groups (like twos and threes) to form particles such as protons and neutrons, which then form the nuclei of atoms.

Flavors and charges

Physicists classify quarks into six types, called “flavors”: up, down, charm, strange, top, and bottom.

  • Up, charm, and top quarks carry electric charge +2/3+2/3+2/3 (in units of the proton charge).
  • Down, strange, and bottom quarks carry electric charge −1/3-1/3−1/3.

In ordinary matter:

  • Protons are made of two up quarks and one down quark.
  • Neutrons are made of one up quark and two down quarks.

Forces and “color” charge

  • Quarks feel all four known fundamental interactions: strong, electromagnetic, weak, and gravitational (though gravity is negligible at particle scales).
  • They carry a special kind of charge called “color charge,” which is what makes them interact via the strong force; this is unrelated to visual color but uses the same word as a metaphor.

Because of a phenomenon known as color confinement, individual quarks are never observed in isolation; they are always locked inside larger particles like protons, neutrons, mesons, or short‑lived exotic states such as tetraquarks and pentaquarks.

Why quarks matter

  • All familiar, visible matter is built from a small set of ingredients: mainly up quarks, down quarks, and electrons.
  • The mass of protons and neutrons is dominated not by the tiny masses of the quarks themselves, but by the strong-force energy binding them together, so quarks and the fields around them effectively account for most of the mass of everyday matter.

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