feb 12, 1964 - Murray Gell-Man and George Zweig - Quarks (1964)
Gell-Man and Zweig presented the idea of quarks, which were either up, down, or strange and had charges of 2/3, -1/3, and -1/3, respectively. These subatomic particles have never been directly observed but theoretical predictions regarding them have been experimented and confirmed.
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Atomic Theory Timeline