Which experiments prove atomic theory?
Sub-atomic theories:
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atoms have: nuclei; electrons; protons; and neutrons.
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That the number of electrons atoms have determines their relationship with other atoms.
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That the atom is the smallest elemental unit of matter – that we can't continue to divide atoms into anything smaller and have them retain the characteristics of the parent element.
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That everything is made of atoms.
These sub-theories might spur more thoughts of individual experiments that prove individual sub-atomic theories (my guess is more was able to be proven after more experiments followed).
Best Answer
I would say that one experiment that demonstrates the atomic nature of things is the observation of Brownian motion. But it is not the experiment itself that convinces that things are made of atoms, rather its theoretical explanation given by Einstein in one of his 1905 papers (actually Einsteins work for his PhD was on the subject of atomic theory and there are several publications in the period 1903-1905). Of course there is also the observation of Rayleigh who calculated Avogadro’s number by the distance from which he could make out the figure of Mount Everest, assuming that light is scattered by atoms and that is why far away objects look fuzzy (1,2). Also scattering experiments demonstrated the atomic nature of things.
(1) Rayleigh, On the transmission of light through an atmosphere containing small particles in suspension, in Scientific Papers by Lord Rayleigh Vol. 4, pp. 247–405, New York: Dover, 1899/1964.
(2) P. Pesic, Eur. J. Phys. 26, 183 (2005).
(3) Patterson, G. Jean Perrin and the triumph of the atomic doctrine (2007) Endeavour, 31 (2), pp. 50-53.