Gold Foil
Takeaways / Key Points
Our Models of the Atom based on new evidence from Gold Foil Experiment
How Gold Foil (the target) Is Made
Alpha Particles (the projectile)
Experimental Set up
Experimental Set-up And Reuslts
EXPERIMENTAL SETUP
In this experiment, students were primed on several of the parts of the experimental set up prior to looking at the results.
First we looked a excerpts of the video about the making of gold leaf. This was to illustrate that the gold foil used in this experiment is extremely thin (only a few atoms thick).
Then we looked at the cloud chamber video. This video shows that certain materials (Americium 241) are capable of flinging off particles.
Then we discussed the idea that a scientist had in 1911. The was idea to take the material (Americium) that is flinging off the particles and shoot them at the extremely thin gold foil. He did this by putting the Americium in a lead box with a slit in the box that would allow him to make a beam of particles.
Since he could not see the particles, he used a piece of material that flashed when one of the particles from the Americium hit it.
BACKGROUND KNOWLEDGE
A key piece of reasoning that students need to consider is that the gold foil is made of gold atoms therefore the particles shooting off the Americium and "hitting" the foil are "hitting" gold atoms.
We know about electrons from the Green Beam Experiment (Crookes Tube) and we know they are extremely low mass and negatively charged.
We know that the particles coming from the Americium are significantly more massive than electrons but less massive than a gold atom.
The gold is a solid, meaning that the atoms must be strongly connected
OBSERVATIONS
About 99% of the particles coming from the Americium go through the gold foil without being deflected. There are no tears or holes in the foil.
About 1% of the particles coming from the Americium are deflected at large angles (bounce back etc...)
Thinking about the results
THINKING
If 99% of alpha particles are going straight through the foil (undeflected) then there must be a significant amount of empty space
This empty space can be in one of two places.
Either it is between individual atoms or
It is within the atom.
Model 1 - There is space between the Gold Atoms
Model 2 - There is space within the atom.
Additional Thinking
Since the gold foil is in the solid phase (not a liquid or a gas) then the gold atoms must be 1. Connected and 2. Fairly close together. If the gold atoms were very far apart, then they would not be in the solid phase (they would be a gas) so Model 1 does not seem to work. This means that the particles that have been shot at the gold foil must be going through the atoms. The fact that they go straight through the atoms 99% of the time suggests that the atoms are mostly empty space. Per Newton's first law, objects in motion will stay in motion in a stright line unless a force acts on them. Since the particles shot at the foil are moving in a straight line, no forces must be exerted on them, thus there is a large region with empty space.
The fact that a few of these particles are deflected suggests that they are experiencing forces. Per Newton's third law, objects that exert forces on each other exert equal forces in opposite directions. This means that the objects being shot are experiencing the same forces as the object that they are hitting. However, since the object being shot changes direction, it means that the object it hit is massive.