Evidence for a Nucleus
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
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.