“Rutherford’s Gold-Leaf  Experiment”
	Description
	This slide illustrates Ernest Rutherford’s experiment with alpha particles and gold foil and his interpretation of the results.
	Basic Concepts
	•When charged particles are directed at high speed toward a metal foil target, most pass through with little or no deflection, but some particles are deflected at large angles.
	•Solids are composed of atoms that are closely packed.  The atoms themselves are mostly empty space.
	•All atoms contain a relatively small, massive, positively charged nucleus.  The nucleus is surrounded by negatively charged electrons of low mass that occupy a relatively large volume.
	Teaching Suggestions
	Use this slide to describe and explain Rutherford’s experiment. Rutherford designed the apparatus shown in figure (A) to study the scattering of alpha particles by gold.  Students may have difficult with the concepts in this experiment because they lack the necessary physics background. To help students understand how it was determined that the nucleus is relatively massive, use questions 3 and 4 to explain the concept of inertia.
	Explain that the electrostatic force is directly proportional to the quantity of electric charge involved.  A greater charge exerts a greater force.  (Try comparing the electrostatic force to the foce of gravity, which is greater near a massive object like the sun, but smaller near an object of lesser mass, such as the moon.)  The force exerted on an alpha particle by a concentrated nucleus would be much greater that the force exerted on an alpha particle by a single proton.  Hence, larger deflections will result from a dense nucleus than from an atom with diffuse positive charges.
	Point out that Rutherford used physics to calculate how small the nucleus would have to be produce the large-angle deflections observed.   He calculated that the maximum possible size of the nucleus is about 1/10,000 the diameter of the atom.  Rutherford concluded that the atom is mostly space.
	Questions
	1.If gold atoms were solid spheres stacked together with no space between them, what would you expect would happen to particles shot at them?  Explain your reasoning.
	2.When Ernest Rutherford performed the experiment shown in diagram (A) he observed that most of the alpha particles passed straight through the gold foil.  He also noted that the gold foil did not appear to be affected.  How can these two observations be explained?
	3.Can you explain why Rutherford concluded that the mass of the f\gold nucleus must be much greater than the mass of an alpha particle?  (Hint:  Imagine one marble striking another marble at high speed.  Compare this with a marble striking a bowling ball.)
	4.Do you think that, in Rutherford’s experiment, the electrons in the gold atoms would deflect the alpha particles significantly?  Why or why not?  (Hint:  The mass of an electron is extremely small.)
	5.Rutherford experimented with many kinds of metal foil as the target.  The results were always similar.  Why was it important to do this?
	6.A friend tries to convince you that gold atoms are solid because gold feels solid.  Your friend also argues that, because the negatively charged electrons are attracted to the positively charged nucleus, the electrons should collapse into the nucleus.  How would you respond?
	7.As you know, like charges repel each other.  Yet, Rutherford determined that the nucleus contains all of an atom’s positive charges.  Invent a theory to explain how all the positive charges can be contained in such a small area without repelling each other.  Be creative!