Multiple choiceQuestion 1 · Recall
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By the end you can describe the alpha-particle scattering experiment and use its results to argue that the atom contains a small, positive, massive nucleus.
Fire a beam of alpha particles at a very thin gold foil and see where they go.
An alpha particle is a helium nucleus: two protons and two neutrons, so it is positively charged and fairly heavy. A detector (a screen that flashes when an alpha particle hits it) is moved around the foil to count how many particles arrive at each angle.
Before the experiment, the accepted picture was the "plum pudding" model: positive charge spread evenly through the atom with electrons dotted inside. If that were true, a fast, heavy alpha particle would barely be deflected by the foil.
Three observations, and they are not equally common:
The numbers matter as much as the directions. The third observation is rare, but it is the one that changed the model of the atom.
Imagine firing bullets at a target you cannot see. If almost every bullet passes straight through, the target must be mostly empty space.
If a bullet occasionally ricochets straight back, there must be something small, hard and heavy inside — something that can reverse the bullet's motion. A spread-out positive cloud could never do that to a fast, heavy alpha particle: the force on it would be far too weak.
So the atom is mostly empty, with positive charge and nearly all the mass concentrated in a tiny region: the nucleus. The deflection is electrical repulsion, because both the alpha particle and the nucleus are positive.
Where the analogy stops: bullets bounce off a hard surface, whereas the alpha particle is repelled before it ever touches the nucleus. Nothing physical is being hit.
The conclusion from alpha-particle scattering
The atom is mostly empty space, with a small, positively charged nucleus containing almost all of the atom's mass.