1. Rutherford Experiment and Atomic Structure

2. New Models of the Atom The Dalton Model The Thomson Model p p p p p p

3. Rutherford Experiment
Lead Block
Radioactive Source
Alpha Particles
Very thin sheet of Gold
Screen for detecting
Particles

4. Experimental Notes
Alpha Particles are very small, “heavy” and positively charged radiation particles
They are heavier than protons, but lighter than most atoms
The gold foil is really, really thin, just a few atoms thick
When the alpha particles hit the screen, they would give a little flash of light…one could sit back and count the flashes from different parts of the screen
Lead Block
Radioactive Source
Very thin sheet of Gold
Screen for detecting
Particles
Alpha Particles

5. Thomson Model: Expected Results
Gold Atom p p p p p p p p
Protons
Scattered
Through
Nucleus
Particles
“sprayed”
out
Alpha Particles 

6. Rutherford Experiment
Gold Atom
A few get
scattered a bit
Alpha Particles 
Most Protons Go
Straight
Through
Nucleus
A very few get
“bounced”
completely

7. Interpretations Gold Atom A few hit the nucleus on the side
Alpha Particles 
nucleus
Most protons
missed the
nucleus
A very few
“bounced”
completely
off the nucleus

8. Interpretations
Given that the vast majority went straight through the atom, the nucleus must be very, very small.
The fact that some of the alpha particles bounced off completely meant that all the protons must be concentrated at one point.
So that “one point”, the nucleus, must be very small, heavy, and so very very dense.
The rest of the volume of the atom must be filled with the left over material, the electrons

9. The atom starts to look a little weird….
The great majority, 99.95%, of the mass is in the nucleus
The nucleus is really tiny, about % of the volume of the atom
The great majority, % of the volume of the atom is electron space
But that part has almost no mass

10. An Analogy… for an atom of carbon
Imagine that a nucleus was the size of a lead marble…
Place it in middle of Giant stadium
The rest of the volume of the atom would be filled with six fleas, flying around inside the stadium

11. Measurements of the atom
The nucleus is about nanometers wide {
About 0.2 nanometers wide