By | May 25, 2015

Objective: To demonstrate the concepts of exponential radioactive decay and half life in a hands – on and graphical manner.

Standards: Energy in the Earth System (internal sources of energy – radioactive decay) and Origin and Evolution of the Earth System (geologic dating methods).

Materials: One hundred pennies and a container to hold the pennies.


(1) Toss all 100 pennies onto a table surface.

(2) Remove all pennies that landed tails side up.

(3) Place the removed pennies on the left side of the table top and arrange them in a straight, vertical column.

(4) Collect the remaining pennies and toss them again.

(5) Again remove the tails side up pennies and place them in another vertical column directly besides the first column.

(6) Repeat this process until all pennies are removed. If no pennies land tails side up during a toss, leave that column empty and continue tossing the pennies.

Science Behind It: In this demo, the removal of a penny is analogous to the decay of a radioactive nucleus. Each time a penny is tossed, it has the same 50% chance of being removed. Therefore, after the initial toss, about one half of the pennies are removed. After the second toss, about one – fourth of the total pennies remain, followed by one – eighth, one – sixteenth, and so on. This pattern of repeated decrease by a fixed fraction is known as exponential decay. The time it takes for one half of the pennies to be removed (essentially one toss given the 50% chance of removal per toss) is deemed the half – life. Different substances have different half lives, and this principle could be demonstrated by tossing dice or colored blocks. Dice have longer half lives than pennies because each side of a die has a 1/6 chance of being removed compared to the ½ chance of the penny. Arranging the pennies in columns allows the students to view this concept graphically.

Fractures in earth’s crust