Physics 2000 Science Trek The Periodic Table

Evidence for a Nucleus

One of Thomson's most radical (and, as it turned out, best) ideas was to suggest that the electrons he'd discovered were part of the chemists' atoms. This contradicted Dalton's idea that atoms were "uncuttable," with no smaller parts. It did seem to make sense, though: Thomson knew that electric currents can be started by chemical reactions (as in a battery) and by light shining on metal, in the photoelectric effect, so electrons must have some connection with the world of atoms.

But if atoms have electrons in them, why are they electrically neutral?

They have to have a positively charged part, to cancel out the negative electrons.

Exactly. Thomson's original picture of the atom was called the "plum pudding model," though, in the spirit of the modern age, I prefer to think of it as the "chocolate chip cookie model." The electrons were the "chips," embedded in a positively charged hunk of "cookie dough."

In 1911, Ernest Rutherford announced experimental evidence showing that the cookie model was wrong.
Rutherford's experiments consisted of shooting alpha particles at thin sheets of metal. He then measured the angles at which they came sailing out.

So what happened?

Based on Thomson's model, Rutherford assumed the flying, positively charged alpha particles would be pushed a little to the side by the positive "cookie dough" in the metal atoms, and continue flying along at a slightly different angle. He was shocked by what he actually saw. Most of the alpha particles went right through the metal without changing course at all, but a few turned a full 180 degrees and went shooting back the way they'd come.

Rutherford compared the experience to shooting an artillery shell at a piece of tissue paper--and seeing the shell bounce back.

And that meant the cookie picture was wrong?

If the positive charge were spread throughout the whole atom, as in the cookie model, Rutherford calculated that there would be no possibility of the particles bouncing back that way. The only way his results made sense was if he assumed that all the positive charge, and almost all the atom's mass, was concentrated in a tiny lump at the center--what we now call the nucleus. He imagined the electrons orbiting around the nucleus like planets around the sun, with a (relatively) huge empty space between them.

Rutherford's model had a few problems, which helped inspire the development of quantum mechanics--I've told that story more fully elsewhere. The current picture of the atom has the electrons not orbiting but in "clouds" at different energy levels--see the Schrödinger Model or the Elements as Atoms section for more details.

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