1. Rutherford & Discovery of the Nucleus
Topic 15.3

2. Outcomes
You will explain, qualitatively, the significance of the results of Rutherford’s scattering experiment, in terms of scientists’ understanding of the relative size and mass of the nucleus and the atom
You will explain, qualitatively, how emission of EMR by an accelerating charged particle invalidates the classical model of the atom

3. Review: Thomson & Millikan
Thomson and Millikan’s work, charge-to-mass ratio and elementary charge, respectively, led to a new model of the atom… the “plum pudding” model
Electrons were embedded in a positively charge fluid which accounted for most of the atoms mass and volume

4. Rutherford’s Scattering Experiment
Rutherford shot alpha particles at a piece of gold foil
He hypothesized that the alpha particles would travel through the foil and on to the screen (similarly to shooting bullets at a Kleenex)
However, some particles were deflected backwards along their incident path

5. Rutherford’s Conclusion
I shall call it… A NUCLEUS!!!
Meaning “kernel” or “inner part”
That the mass was not evenly distributed throughout the atom
The positive particles were densely packed – would have to be to create enough electrostatic force to cause the alpha particles to deflect backwards
He called this “densely packed” area THE NUCLEUS!

6. Rutherford’s Atomic Model
The “positive fluid” was contained in the nucleus, while the electrons were external to the nucleus
It was also determined that the nucleus was very small relative to the atom, so much of the gold foil was empty space

7. How small is an atom?

8. The Problem with Rutherford’s Model
According to the Law of Charges…what is wrong with Rutherford’s model?
Scientist then predicted that the electrons must have enough kinetic energy to counterbalance the electrostatic forces between the protons and electrons
Think of Earth and the Sun and gravitational forces
This was called the planetary model of the atom

9. The Problem with the Planetary Model
Circular motion of electron = centripetal acceleration
An accelerating charge = EMR
If EMR is being emitted, then the electron is losing energy
If the electron is losing kinetic energy it is losing speed
If the electron is losing speed is should eventually crash into the nucleus
According to Maxwell’s Theory… the planetary model could not work
Empirical evidence supported the Rutherford planetary model, and yet atoms do not collapse.

10. TO BE CONTINUED!

11. Homework
p. 770 #1, 2, & 4