To demonstrate that the rates of decay of unstable nuclei can be measured, that the exact time that a certain nucleus will decay cannot be predicted, and that it takes a very large number of nuclei to find the rate of decay.
This is the second lesson in a three-lesson series about isotopes, radioactive decay, and the nucleus.
Have them go directly to the Nuclear Structure Systematics Home Page.
Once to that page, students should then go to the Isotope Discovery History, a graph of the number of known isotopes versus the date, and to the Chart of Aristotle and Plato (found at the bottom of the page), which the site planners cleverly call "the first chart" of isotopes.
If half of the uranium-238 has turned into lead-206 the rock will be 4500 million years old.