Isotopes (or, as we called them, eggotopes)

Regular Hydrogen with its isotopes, deuterium and tritium

I've always thought isotopes were a little tricky to visualize. But the concept is critical to understanding radioactivity and the uranium enrichment process! Luckily I found this great idea from a junior high science teacher online. You use colored eggs to represent elements. Each isotope of an element is in the same color. Inside, you make a nucleus with beads, showing the number of protons and neutrons in each isotope. The model leaves out electrons altogether, as they aren't relevant here.

(One thing the children were SO interested in was half-life---specifically, how some elements have such a short half-life that they decay almost immediately. Protactinium, for example, would have been totally gone from the earth only hours after it first appeared. They loved that idea, for some reason.)

We really liked doing this, and we also used the eggs later on in the unit for reference. I had really small plastic eggs, so we only did some of the lighter elements, but it would have been fun to make an egg for U-235 and U-238 if we'd had one big enough to hold that many beads! I just looked up a list of common isotopes (some radioactive, some not) such as Carbon-14 and of course the three isotopes of Hydrogen. (That knowledge would be necessary for learning about nuclear fusion later!)

Here's a simple online explanation of isotopes.

Also, stringing beads is fun!

Sulfur-32 and Sulfur-35 (radioactive)