The Rutherford Model

In 1909 Ernest Rutherford conducted what is now a famous experiment where he bombarded gold foil with alpha particles (Helium nuclei). A source which undergoes alpha decay is placed in a lead box with a small hole in it.   Any of the alpha particles which hit the inside of the box are simply stopped by the box.  Only those which pass through the opening are allowed to escape, and they follow a straight line to the gold foil.  The animation below shows the experiment in action.

Observations

• Most of the alpha particles pass straight through the gold foil.
• Some of the alpha particles get deflected by very small amounts.
• A very few get deflected greatly.
• Even fewer get bounced of the foil and back to the left.

Conclusions

• The atom is 99.99% empty space.
• The nucleus contains a positive charge and most of the mass of the atom.  
• The nucleus is approximately 100,000 times smaller than the atom.

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The deflection of the alpha particle through large angles and even bouncing off the gold foil is best described by the animation below.  Keep in mind that the gold nuclei have a charge of +79 and the alpha particle has a charge of +2.  (remember your electrostatics)  These two positive charges repel each other.  The closer they get, the greater the force.   The greater the force, the greater the amount of deflection.

©1999 Science Joy Wagon