MICHAEL GILBERT / SCIENCE PHOTO LIBRARY MICHAEL GILBERT / SCIENCE PHOTO LIBRARY
Diagram of the structure of the proton (or neutron). A proton consists of three smaller, fundamental particles called quarks, which are bound together by the strong nuclear force. Protons consist of 2 "up" quarks (blue) & 1 "down" quark (green). Neutrons consist of 2 "down" quarks & 1 "up" quark. Quarks are always found either in triplets (as in the proton & neutron) or in pairs of a quark & antiquark. Attempts to isolate individual quarks have been unsuccessful, & some theorists believe that the nature of the strong nuclear force makes the existence of free quarks impossible.
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