<unk> <unk> <unk> <unk> <unk>  Passage: Although the word atom originally denoted a particle that cannot be cut into smaller particles, in modern scientific usage the atom is composed of various subatomic particles. The constituent particles of an atom are the electron, the proton and the neutron; all three are fermions.<unk> The constituent particles of an atom are the electron, the proton and the neutron; all three are fermions. However, the hydrogen-1 atom has no neutrons and the hydron ion has no electrons.<unk> However, the hydrogen-1 atom has no neutrons and the hydron ion has no electrons.  Question: What was initially known to be the smallest particle?<unk>Question: What was initially known to be the smallest particle?
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