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Flashcards in The discovery of the nucleus Deck (11):

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.

10-15 m


Magnitude for atomic diameter.