Rutherford and the Alpha Scattering Experiment

Question 1

Define

Alpha Particle

Answer 1

a helium nuclei - a massive positively charged particle

Question 2

How far could an Alpha Particle travel?

Answer 2

Alpha particles could not travel more than a few centimetres in the air and could be stopped by a sheet of paper.

Question 3

What happened in the experiment?

Answer 3

The experiment saw a beam of alpha particles directed through a very thin gold sheet ( a couple of atoms thick) in a darkened room.

Question 4

What were the two reasons the experiment was carried out in a vacuum?

Answer 4

• prevent ionisation of the alpha particles with the air
• ensure that the deflection of the alpha particles was due to the collision with the gold atoms and not air.

Question 5

What is:

Thomson’s Plum Model

Answer 5

A model of an atom that would have meant that the + and - charges of the atom were spread out evenly.

Question 6

What does the Plum Pudding model mean for the alpha particle experiment?

Answer 6

The electrostatic repulsion of the positive alpha particle and the atom would be too weak to scatter the alpha particles at large angles because the net charge at any point would be small.

Question 7

List

The three observations of the alpha scattering ecperiment.

Answer 7

1. The majority of alpha particles passed straight through
2. Some were scattered at large angles
3. A very small number (1 in 8000) were deflected by more than 90 degrees - they bounced back

Question 8

What does the first observation mean?

(The majority of alpha particles passed straight through)

Answer 8

• There must be a majority of empty space in an atom becuase due to the sheet being several hundred atoms thick, the positive charge must be significantly smaller than the atom as there was a very small chance of an alpha particle passing close to a positive charge.
• The (-) electrons must orbit the (+) protons far away as the negative charge did not cancel out the positive charge.

Question 9

What does the second observation mean?

(Some were scattered at large angles)

Answer 9

All the protons are clumped together in the nucleus as the particle must have encountered an electrostatic repulsion and therfore must have encountered a concentrated positive charge .

Question 10

What does the third observation mean?

(A very small number (1 in 8000) were deflected by more than 90 degrees)

Answer 10

Must have encountered something with a much larger mass and a concentrated positive charge

Question 11

What was Rutherford’s model of the atom?

Answer 11

• Positive charge was concentrated in a dense cluster nucleus at the center of the atom
• Nucleus surrounded by electrons that were orbiting the nucleus