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Every substance consists of ... . They consist of a compact nucleus and electrons sitting on their electron shells at a large ... from the nucleus. The nucleus is a collection of ... and neutrons, tightly bound to each other through interaction. Nuclear interaction is very strong, much more stronger than electromagnetic interaction, which is responsible for chemical ... . To study strong interactions, physicists accelerate ... particles, such as protons, and push them against each other. If the energy of the particles is small, then they simply bounce elastically from each other without any change. If the energy is large enough, then new unstable particles are born in the collision of protons. Numerous experiments on the collision of particles at high energies led physicists to the idea that both protons and neutrons are not elementary, but consist of other, more fundamental particles—quarks. The family of hadrons—that is, particles composed of quarks—is very large: protons, neutrons, pi—mesons, K—mesons, etc. A separate quark is not a hadron. All of them, except the proton, are unstable, ... to other particles. For example, a neutron decays into a proton, an electron, and an antineutrino; K—meson decays into two pi—mesons, which further decay into muon and neutrino.