Explain the plum Pudding model.
J J Thompson’s original model of atom was ‘plum pudding’, where negative electrons were dotted in the atom like the currents in a ‘current bun’, and the positive charge was spread throughout the atom like a positive dough.
What was done in Rutherfords α scattering experiment?
- A source emitting a narrow beam of α particles, all with the same KE.
- Was directed at a thin layer of gold foil.
- Inside an evacuated container.
- A microscope was used to observe the pinpoints of light emitted when an α particle hit the fluorescent screen.
Why was the chamber evacuated?
To prevent α particles being absorbed by air molecules.
Why was the foil thin?
To prevent α particles being scattered more than once.
Why do the α particles need the same speed?
Slower α particles are deflected more than faster α particles on the same initial path.
What happened to the α particles approaching the nucleus?
- Most passed straight through with little to no deflection;
- 1 in 2000 were deflected.
- Few α particles were deflected more than >90 degrees; 1 in 10,000 were delfected.
What did the Rutherford scatterering experiment confirm?
- Most of the atom is empty space (as most α particles passed straight through)
- A central positively charged nucleus (as some α particles are deflected/repelled by the nucleus)
- The mass and charge are concentrated in the nucleus (if not then the α particles would not be deflected backwards).
For an α particle with charge, Qα, approaching a nucleaus of charge, QN:
- When the initial kinetic energy is equal to the potential energy of the nuclei's electric field.
- The alpha particle is at a distance of least approach d.
- EK = Qα*QN/4πε0d = +2e*+Ze/4πε0d
Why is the probabilty of α particles scattering once = 1 in 10,000n for n layers of atoms.
For each layer added, there are more atoms in the path of the beam, so the probablity of the alpha particle hitting the given nucleus ∝ 1/n.
typical value of n =104
Magnitude of nuclear diameter.
Magnitude for atomic diameter.