Final answer:
J. J. Thomson discovered the electron, challenging the completeness of Dalton's Atomic Theory, and Robert Millikan measured the electron's charge and calculated its mass through his oil drop experiment.
Step-by-step explanation:
The contributions of J. J. Thomson and Robert Millikan to atomic theory were pivotal in understanding the substructure of the atom. J. J. Thomson, through his experiments with cathode rays, discovered the electron and suggested that atoms consisted of smaller particles, which he initially called 'corpuscles'. This finding challenged Dalton's Atomic Theory by introducing subatomic particles, changing the way we view the fundamental structure of matter. Robert Millikan furthered this research through his famous oil drop experiment, which allowed him to measure the charge of an electron with high precision. Combining his findings with Thomson's mass-to-charge ratio, Millikan calculated the electron's mass, proving that it was only a tiny fraction of the atom's mass, with most of the atom's mass being in its nucleus.