Inquiry-Based Analogy Experiment to Rutherford's Discovery of the Nucleus

As I mentioned in a previous post on the electron, I believe atomic structure is taught WAY too early in a typical chemistry curriculum.  Having looked through a few chemistry textbooks laying around my house, I continuously found that atomic structure (and atomic theory as a whole) is introduced as early as Chapter 2 and as "late" as Chapter 5.  Considering the hundreds of years worth of scientific achievement and failure that took place before the discovery of subatomic particles, why do so many teachers present the inner workings of the atom so early?  Having knowledge of such information is simply not needed in order to explain some of the most important early concepts in chemistry such as:
1.) conservation of mass
2.) the role of energy in phase transitions
3.) particle motion
3.) gas laws
4.) density
5.) the existence of atoms
6.) atomic attraction
7.) Law of Definite Composition and Multiple Proportions
8.) Avogadro's Hypothesis and the mole
9.) % composition and empirical formulas

Though I'm sure there are other important concepts in chemistry that were known before the discovery of subatomic particles, I believe the above list suggests that a great deal of chemistry can be learned before having knowledge of atomic structure.  PLUS....this is how it was historically done!  Before the 1890's, early chemists had no knowledge of subatomic particles.  So in our classrooms, why do so many teachers simply bypass the knowledge accumulated prior to Thomson and present the whole idea of atomic theory so early?  It's simply not needed and it makes things more complicated.

I typically don't introduce the unit of atomic structure until 2nd semester.  By the time we reach this unit, my students' only true knowledge of atomic theory lies on the shoulders of Dalton.  Obviously I know they are aware of words like electron, proton, and neutron, but I don't allow them to use those words in their explanations prior to this unit because they have yet to understand the evidence for such ideas.  Early chemists couldn't use those words either because the words didn't exist yet!  

In order to keep it historically accurate, we approach the discovery of the electron first which I've written a post about here.  Though I deviate from the history accuracy a little bit by including Milkian's oil drop experiment, I only bring it up because we are currently talking about the electron anyways.  After we analyze the evidence for the existence of electrons, students realize that Dalton's model of the atom needs to be replaced and in comes the plum pudding model.  

Up next comes Rutherford.  But before I even mention anything about Rutherford's gold foil experiment, I get the students to perform a simply analogy experiment.  They don't know what it's an analogy for yet, but they will once we start talking about the details of Rutherford's experiment.  The goal of the analogy experiment is this:

Indirectly calculate the diameter of an unknown object by recording the number of times it is hit with objects of a known diameter

A link to the lab activity can be found here

The setup is incredibly simple and the I would imagine nearly every science teacher has access to similar materials that would get the job done.  The general procedure is simple as well.
1.) Students setup a well-defined path with meter sticks parallel to each other.
2.) A whiteboard (or some sort of large opaque material) is positioned at the end of the path on top of the meter sticks so that there is a cm or two of space below.  
3.) The group is given 100 pennies and they are to measure the diameter and therefore determine the radius of the penny.  They also measure the width of their path.
4.) One student is assigned the role of "shooter" or "bombarder" and is given 100 pennies and is also blindfolded.  I've also gone to the extreme and made the "bombarder" wear headphones as well.
5.) The other group member is given an object and is told to place that object on the other side of the whiteboard.  The "bombarder" does not know where the object is and knows nothing about its size or mass.
6.) Once the non-blindfolded group member has measured the diameter of the "unknown" object, that group member places the object wherever he wants behind the whiteboard.
7.) The "bombarder" now shoots the pennies one by one, typically in groups of 10, and the other group member counts how many times the object is hit.  Because the "bombarder" cannot see, it is important that the other group member makes sure the path is cleared of pennies in order to ensure that each penny has a fair chance at hitting the object.
8.) Once all 100 pennies have been shot, the total number of hits is recorded and the analysis ensues.

The entire setup looks like this:

Using the information recorded and a simple equation, students are able to arrive at an experimentally-determined diameter of the unknown object.  I have had great success with this and I often see groups get within 1 or 2 cm of the actual diameter.  In order to ensure success, it's really important that the "bombarder" is truly shooting at random.  If the "bombarder" knows where the object is, it's almost impossible to convince a teenager to NOT try to purposely hit the object.

So why the heck did we just do this?

Well, without going into much detail, not only did Rutherford's experiment lead to the discovery of the nucleus, but it also led to the indirect determination of the size (diameter) of the nucleus relative to the rest of the atom.  In my experience, students have no issue with the conclusion of alpha particle scattering being the result of hitting the nucleus.  However, using that scattering information to determine the size of the nucleus is a much more conceptual idea and much tougher to grasp.  By doing this analogy activity, students see that it is actually possible to determine the diameter of something without actually being able to see it.  I think that's pretty cool!

After the activity is over and they've calculated and compared their experimental diameter to the actual diameter, class is pretty much over.  When they go home, I tell them to watch this video where I describe some of the fundamental points about what we just did in relation to Rutherford's experiment--which we talk about the next day.

If you haven't already figured it out by now, here are the most fundamental points to the analogy activity:

• penny is analogous to alpha particle
• unknown object is analogous to nucleus
• path width is analogous to diameter of atom
• penny bouncing back a various angles is analogous to extreme alpha particle deflection when hitting the nucleus
• vast majority of pennies going straight through suggests unknown object is small relative to path width which is analogous to vast majority of alpha particles go straight through gold fold with little to no deflection due to the nucleus being so small relative to the size of the atom
• empty space directly behind whiteboard is analogous to the majority of the atom being empty space
• penny bouncing back suggests unknown object is much more massive than penny which is analogous to the idea of the nucleus containing nearly all the mass of an atom (and much more massive than an alpha particle--at least gold's nucleus)

I believe having this experience PRIOR to discussing Rutherford's experiment lays a strong foundation for our students to more easily connect the rather conceptual findings from Rutherford's experiment with what we did the previous day.  

By no means do I think this is the BEST way to do things but I do know that it's better than simply approaching the inner workings of Rutherford's experiment head on and assuming everyone will just "get it".  When teaching such abstract concepts in chemistry, the more connections we can give our students to make with prior experience, the more easily they will be able to assimilate such experiences with the appropriate concept.