"He Who Shrank" by Henry Hasse was published in Amazing Stories in 1936. It's an old idea.

Edit to add: The solar-system-like atom model is the Bohr model of the atom, dating to 1913. I don't know who came up with the idea of extending it to the point of making atoms into actual solar systems, and vice-versa.

Fred

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"For shame, gentlemen, pack your evidence a little better against another time."
-- John Dryden, "The Vindication of The Duke of Guise" 1684