*This activity was selected for the On the Cutting Edge Reviewed Teaching Collection This activity has received positive reviews in a peer review process involving five review categories.*

In this model, the removal of a penny or a cube corresponds to the decay of a radioactive nucleus.

The chance that a particular radioactive nucleus in a sample of identical nuclei will decay in each second is the same for each second that passes, just as the chance that a penny would come up tails was the same for each toss (1/2) or the chance that a cube would come up red was the same for each toss (1/6).

After the first toss, about 1/2 of the original pennies are left; after the second, about 1/4; then 1/8, 1/16, and so on.

These numbers can be written in terms of powers, or exponents, of 1/2: (1/2).

For uranium 238, the chance of decay is small: Its half-life is 4.5 billion years.

For radon 217, the chance of decay is large: Its half-life is one thousandth of a second.. Toss them and replace the nickels that land tail side up with pennies. Make a column with all the pennies that land tail side up, and replace all the nickels that land tail side up with more pennies.

It is a great way to introduce or reinforce the concepts involved in radioactive decay.

You will need enough coins (I use pennies) for each person in the class, some sort of graph paper printed on an overhead, an overhead projector and a pen for the overhead.

The chance that any penny will come up tails on any toss is always the same, 50 percent.

However, once a penny has come up tails, it is removed.

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