Teaching Modern Physics - QuarkNet - Fermilab

Modern Physics
Quantum Mechanics Catches on—and Gives a New Atomic Model
Electrons were discovered in 1898 by J. J. Thomson. It had been known before that that the atom
was composed of positive and negative charges, but that most atoms were indeed neutral overall.
J. J. Thomson found the source of the negative charge—the electron. He constructed a model of
the atom called the “Plum Pudding” model of the atom in which the majority of the “stuff” that
composed the atom was positive, and the electrons were stuck inside like chocolate chips in a
chocolate chip cookie (or like plums in a plum pudding). Draw a picture of this model of the
atom.
Check out the regular (incandescent) light bulb with the specs on. Describe what you see.
Now check out the three lamps with the specs on. Describe what you see.
What’s the difference between the lamps and the bulb?
Why do you think this difference occurs?
What was Balmer’s formula, what did it describe, and how did he come up with it? (We’ll put it
in Rydbergh’s notation to make it less complicated.)
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