I started by setting the context. The students had studied atoms and molecules the previous year so I started by drawing a molecule of water (two hydrogen atoms and one oxygen atom) and reminding them of the evidence for atoms and molecules. Then we zoomed in to one hydrogen atom and discussed the Rutherford experiment showing that atoms are very fluffy; most of their volume is nearly empty while nearly all their mass is concentrated in a tiny volume in the center (nucleus). Then we zoomed in further by a factor of 10,000 to the nucleus. For a hydrogen atom, the nucleus is a single positively charged particle called a proton. I held up a ping-pong ball as a proton and said that if protons really were that size, the atom would have to be the size of South Davis.
To reinforce the sense of scale, I showed the movie Powers of Ten. This classic ten-minute movie should be seen by anyone wanting to understand the universe. I also took the time to answer questions about it.
The basic rules of nuclear physics are actually understandable by anyone. Last year we investigated the effects of electrical charge, and concluded that like charges repel while opposite charges attract. Atoms beyond hydrogen in the periodic table have more protons. But why do the protons stick together if they repel each other? There must be some form of glue. I demonstrated two magnets which repelled each other. They were "donut" magnets threaded onto a rod so they didn't flop around and the repulsion was clear. But when I turned the rod vertically and one magnet fell with enough speed onto the other one, they touched briefly. That was enough for the velcro on their surfaces to attach and keep them together. The velcro is a short-range force, like the strong nuclear force which keeps a nucleus together.
But protons alone can't generate sufficient strong nuclear force to keep nuclei together. Another type of particle, with similar mass but no charge and called a neutron, provides the glue. Nuclei need roughly equal amounts of protons and neutrons to be stable. I modeled this with a bunch of ping-pong balls I had wrapped with velcro. The "protons" had velcro hooks and the "neutrons" had velcro loops, so that you needed roughly equal numbers of each to build up a large nucleus. (The different types were also different colors to make the idea plainly visible.) Adding a neutron to a nucleus adds mass, but doesn't otherwise change the properties of the atom. For example, a proton plus a neutron is still hydrogen, but we call it a different isotope of hydrogen. Similarly, carbon-12 (usually written with a superscript 12 on the left) and carbon-14 are different isotopes of carbon which differ by two neutrons.
With that in mind, we can start building up more complicated elements from hydrogen. Element number 2 (two protons) is helium, and we need two neutrons to provide the glue so the most common isotope of helium is helium-4. The protons have to be smashed together at very high speed if they are to ever get close enough for the "velcro" of the strong nuclear force to make them stick, so we need very high temperatures to make this fusion process happen. (High temperature means the individual microscopic particles are wiggling or bounding around at high speed.) We find it difficult to make these high temperatures on Earth, but the core of the Sun is 15 million degrees (Celsius; tens of millions of degrees if you think in Fahrenheit) and this happens quite routinely. In fact, most stars turn hydrogen into helium in their cores.
Fusing helium into even heavier elements is harder, but most stars will do that as well by the ends of their lives. It turns out that crashing two heliums together results in an unstable isotope of element 4 (beryllium), which quickly decays back into two helium-4 nuclei. But if you manage to crash a third helium into the two heliums before the two-helium complex has a chance to decay, you make carbon-12 (the most common isotope of element 6, carbon; again, equal amounts of protons and neutrons). Then, if you crash another helium into that, you get element number 8: oxygen. Another helium into that produces element 10, argon. These helium capture reactions are common in massive stars (substantially more massive than the Sun), and they create more of the even-numbered elements than the odd-numbered elements (nitrogen, fluorine, etc). They go all the way up to iron (element 26). I modeled all this with the velcro-covered ping-pong balls.
Have you noticed what we've done here? We've explained the origin of the elements using basic, well-understood physical processes. That's pretty cool! Here's a graph of the observed abundances:
You can see that hydrogen is the most abundant, followed by helium, then the even-numbered elements carbon, oxygen....through iron. But why are there elements beyond iron if stars only make up to iron? Well, stars make up to iron when they are in equilibrium. But when they explode (a supernova), so much energy is released that even more complicated nuclei can be made. I won't explain the details here, but the abundances of all the elements beyond iron are well understood as consequences of supernovae. That we can understand all the features of the above plot is, to me, one of the most amazing things in all of science.
The supernova explosions are also what throw the newly-manufactured elements back into space, where they can mix into gas clouds that eventually collapse to form new stars. That means that the atoms in your body were once inside another star. (Not from the Sun, because new atoms made in the Sun won't escape until the end of its life.)
Supernovae make some unstable elements, like uranium. The most common type of decay for a heavy element is to violently eject a "bullet" made of two protons and two neutrons, in other words a helium nucleus (again I modeled this with the ping-pong balls). This is why there is helium on Earth; our gravity is too weak to hold on to helium gas, but helium produced by radioactive decays is trapped in rocks underground. When we drill for natural gas, we can capture some of this helium and eventually use it to fill balloons. When it escapes from the balloon, it eventually escapes into space.
