Mass Spectrometer Model

"What are we doing to help kids achieve?"

My students just finished an activity about isotopes (see Isotopes, Nuts, Bolts and Eggs). Each year there is always one or more student who ask about the mass of isotopes. How do scientists solve for that mass experimentally?

One way scientists do this is by using a mass spectrometer. Mass spectrometers first form atoms into ions and then pass them through magnetic fields. Atoms of different masses respond differently to the magnetic field. Flinn Scientific has a nice model of this. It is simply a piece of plexiglass with a magnet underneath. The "atoms" are metal spheres rolled down a ramp. The magnet effects the spheres differently based on their masses. I have used this model but with a twist suggested by Irwin Talesnick. Irwin does a similar experiment but with one added feature. The plexiglass is coated with paper and the students do not know the magnet is present. They must predict what will happen. The first "atom" is a large billard ball and it goes straight.The next "atom" is as large as the billiard ball but is metallic, which represents the "ion" idea. Subsequent "isotopes" are deflected more or less based on their mass. This is similar to an actual mass spectrometer. Students usually have an "aha" moment when the second "atom" is deflected and then they quickly want to look underneath the plexiglass.

I have been doing this for several years. It always seems to get a nice reaction from students. They want to explore the idea more and it generates questions. It also costs much less than an actual spectrometer.....

Do you have a great quick demonstration? Would love to see it...don't be afraid to share...