The Model of the Atom
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By doing experiments, scientists 
determined that atoms had a positive, 
dense nucleus and a cloud of charged electrons (e). 
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Many years later, scientists found that the 
nucleus was composed of protons (p) and neutrons (n).
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Now it turns out that protons and neutrons aren't fundamental -- they are 
comprised of more fundamental particles called quarks. 
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Physicists believe that 
electrons are fundamental particles.