Half-life and carbon-14 dating

Because radioactive isotopes have predictable decay rates they can be used to determine the age of objects within a certain range.  The idea is shown in the following diagram

http://science.howstuffworks.com/carbon-141.htm

carbon-14 is created continuously in the atmosphere when cosmic rays hits nitrogen-14. Over time the carbon-14 becomes incorporated in the entire food chain along with the stable carbon-12 isotope. When an organic organism (something based on carbon) dies, the carbon-14 continues to undergo radioactive decay but the carbon-12 doesn’t. So with time the ratio of carbon-14 to carbon-12 becomes smaller and smaller. By measuring the ratio of carbon-14 to carbon-12 in a dead organic material compared to the ratio in a living material, it is possible to calculate the age.

Today we did the M&M exercise which introduced you to the concept of half-life. If we start with 100%, one half-life later only 50% will be left. Two half-lives later we will have 50% of 50% = 25%. In three half-lives we will be down to 50% of 25% = 12.5% and so on. By seven half-lives less than 1% of the original amount remains. For carbon-14 seven half-lives equals about 40,000 years and this is about the useful limit for carbon-14 dating. 

 Homework: write up the M&M as a lab
1. Title page (be creative or use the web)
2. Introduction
3. Procedure
4. Results (table and graph)
5. Analysis – answer all questions

The writing must be neat and legible. If your handwriting is hard to read please use a computer. The axes of the graph must be drawn with a ruler. There must be a title for the graph and the axes must be neatly labelled.

Due tomorrow.

 Mr.T.

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