24.1. Modeling the Atom http://www.ck12.org
FIGURE 24.2
The Geiger-Marsden gold foil experi-
ment.It was the backwards scattering that was most remarkable, and the rate of these was the subject of Geiger and
Marsden’s first paper. Rutherford described the results with astonishment. "It’s almost as incredible as if you fired
a fifteen-inch shell at a piece of tissue paper and it came back and hit you,” he wrote.
Implications for the Atom
This result can be considered with a simple parallel. Picture that you are outside of an area with black curtains
around it, not knowing what is inside. You can throw a bunch of baseballs through the curtains, and see where they
go. However, you cannot look inside.
FIGURE 24.3
A thought experiment similar to Geiger-Marsden, where someone throws baseballs through an area with an
unknown structure.What happens is that nearly all of the baseballs go straight through, without any apparent deflection. However, a
few deflect at different angles, and a tiny fraction even bounce straight backwards. What would you conclude?
Thomson’s Model of the Atom
At the time, the prevailing model of the atom was from J.J. Thomson, who had discovered the electron. Electrons
were known to have very little mass compared to the atom as a whole. He had proposed an atomic model,Figure
24.4, where tiny electrons were evenly distributed within a fluid of positive charge throughout the atom – dubbed
the “plum pudding” model for its resemblance to pudding with raisins suspended in it.
This model failed to explain Geiger and Marsden’s discovery. The soft “pudding” would not knock some particles