James Chadwick

Chadwick
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Unfortunately, Rutherford's "planetary" model was still lacking in one crucial aspect. Hydrogen atoms contain one proton, while helium atoms contain two protons. Thus, the ratio of the mass of a helium atom to that of a hydrogen atom should be 2:1, but it is actually 4:1. Rutherford and his colleagues hypothesized there must be another subatomic particle in the nucleus. James Chadwick provided proof for the existence of this mysterious particle. His experiment involved firing alpha particles at a thin sheet of beryllium. This metal emitted high-energy radiation similar to gamma rays, and this radiation was shown to be electrically neutral. The subatomic particles associated with these rays were referred to as neutrons, and they easily explained the mass ratio mystery. Hydrogen atoms have one proton and no neutrons, while helium atoms contain two protons and two neutrons.
                                                                                                                                 Beryllium Experiment
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Atom Shot
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Hence, the basic model of the atom consists of three subatomic particles and a dense, central nucleus. This nucleus is positively charged, contains protons and neutrons, and is orbited by electrons.



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