Those that decay are called radioactive (or parent) isotopes; those that are generated by decay are called radiogenic (or daughter) isotopes.The unit that we use to measure time is called half-life and it has to do with the time it takes for half of the radioactive isotopes to decay (see below).The element itself is defined by the atomic number (i.e., the number of protons).

Students often struggle with this concept; therefore, it should be stressed that it is impossible to know exactly when each of the radioactive elements in a rock will decay.If they can begin to comprehend that it is random and spontaneous, they end up feeling less nervous about the whole thing.Radioactive decay involves the spontaneous transformation of one element into another.Most students don't really know how isotopes are used to determine age.In particular, they have a hard time understanding that different systems are appropriate for different types of radiometric dating and why.They may ask, "What's the difference between an isotope and an atom?" Another way of explaining it is that when geologists talk about isotopes, they are talking about one element of differing masses.Showing this plot and asking them questions about the shape and changes in number of isotopes through time may help students to develop some intuition about half-life.Although most introductory students may not be prepared for the equation for exponential decay, discussion of half-life and radioactive decay prepares entry-level students for the introduction of more mathematical discussion of exponential growth and decay in upper level classes.Using demonstrations of half-life such as a coin toss for large classes or M&M demonstration for smaller classes can help students to better understand what is happening.Plotting the results of these demonstrations results in a curve of an exponential decay function.

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