Physics 9702 · Paper 2

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.

The scattering result: most alpha particles pass nearly straight through (the atom is mostly empty space), a few deflect through large angles, and a very few bounce back (the positive charge and nearly all the mass are in a tiny, dense nucleus). A closer path gives a larger deflection.

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 very dense. Only something heavier than the alpha-particle can send it back, and only 1 in 20 000 hits it, so it is a very small target.
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.

The nuclear atom (not to scale): a tiny, dense nucleus of protons and neutrons at the centre, holding almost all the mass, with electrons far from it in a region that is mostly empty space.

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.

PreviousPotentiometer Calculations & Null Methods NextNucleon Number, Proton Number & Nuclide Notation