The discovery of the nucleus

Question 1

Explain the plum Pudding model.

Answer 1

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.

Question 2

What was done in Rutherfords α scattering experiment?

Answer 2

• 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.

Question 3

Why was the chamber evacuated?

Answer 3

To prevent α particles being absorbed by air molecules.

Question 4

Why was the foil thin?

Answer 4

To prevent α particles being scattered more than once.

Question 5

Why do the α particles need the same speed?

Answer 5

Slower α particles are deflected more than faster α particles on the same initial path.

Question 6

What happened to the α particles approaching the nucleus?

Answer 6

• 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.

Question 7

What did the Rutherford scatterering experiment confirm?

Answer 7

• 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).

Question 8

For an α particle with charge, Q_α, approaching a nucleaus of charge, Q_N:

Answer 8

• 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.
• E_K = Q_α*Q_N/4πε₀d = +2e*+Ze/4πε₀d

Question 9

Why is the probabilty of α particles scattering once = 1 in 10,000n for n layers of atoms.

Answer 9

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

Question 10

Magnitude of nuclear diameter.

Answer 10

10^-15 m

Question 11

Magnitude for atomic diameter.

Answer 11

10^-10