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We know that radioactive substances disintegrate at a known rate, however. It is the length of time required for the disintegration of one-half of a given number of nuclei of a radioactive element. Suppose we have 100 nuclei of a radioactive isotope.

After one half-life, half of the nuclei will have disintegrated, leaving 50 nuclei." Have students write their answers to these questions in their science journals.

You may group them in any size, but working in pairs is optimal for this exercise.

Weigh out 80 candies for each group into cups before students arrive, as described in the Planning Ahead section above.

Tell students: "We measure our rate of speed in a car in miles per hour.

This method of measuring a rate won't work for radioactive decay.

"Today we will simulate radioactive decay to understand what we mean by half-life.

Games with manipulative or computer simulations should help them in getting the idea of how a constant proportional rate of decay is consistent with declining measures that only gradually approach zero.

The exercise they will go through of predicting and successively counting the number of remaining "mark-side up" candies should help them understand that 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 lesson can be done in two, 45-minute class periods.

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.

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