The Nuclear Atom
The Nuclear Atom
- An atom has a tiny centre called the nucleus. One experiment showed this, and it gives us the picture of the atom we use for the whole chapter.
The gold-foil experiment
- Rutherford's team aimed a narrow beam of alpha-particles (fast, positive particles) at a very thin sheet of gold foil. A detector could be moved to any angle to see where the particles went.
- The experiment was done in a vacuum. Alpha-particles are stopped by a few centimetres of air, so any air left inside would have absorbed the beam before it reached the foil.
- Three things happened, and it is the mix of all three that matters:
- Almost all the alpha-particles passed straight through, with almost no change of direction.
- A few were pushed through large angles.
- A very few (about 1 in 20 000) bounced almost straight back.
What the results prove
- Each result is a clue about the inside of the atom. Read them one at a time.
- Most pass straight through, so the atom is mostly empty space. A solid atom would stop them.
- A few are pushed through large angles, so somewhere there is a concentrated positive charge that repels the positive alpha-particle. The closer the path, the stronger the push.
- A very few bounce back, so that positive region is tiny and contains nearly all the mass. A strong repulsion from a much more massive region can reverse an alpha-particle, and only about 1 in 20 000 passes close enough for this to happen, so that region is very small.
- Put together, the atom has a : a tiny centre holding nearly all the mass and all the positive charge, with electrons in the space around it.
Common mistake
Keep the three results separate, because each one proves a different thing. “Mostly empty” comes from the particles that pass through. “Small, dense and positive” comes from the few that come back. If you only say “most go straight through”, you have shown the atom is empty, but not that it has a nucleus.
Most pass through → empty space. A few deflect a lot → concentrated positive charge. A very few come back → that charge is tiny and holds nearly all the mass. That centre is the nucleus.
Your turn9702-style
In an alpha-scattering experiment, most alpha-particles pass straight through the foil and a very small number are deflected through angles greater than 90°. State what each observation shows about the atom. [4]
Show worked answer
Most pass straight through, so most of the atom is empty space. The rare large deflections show that positive charge and nearly all the mass are concentrated in a very small nucleus.