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What is Half-Life?

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Radioactivity

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Perhaps the most widely used evidence for the Theory of Evolution through Natural Selection is the fossil record. The fossil record may be incomplete and may never fully completed, there are still many clues to evolution and how it happens within the fossil record.

One way that helps scientists place fossils into the correct era on the Geologic Time Scale is by using radiometric dating. Also called absolute dating, scientists use the decay of radioactive elements within the fossils or the rocks around the fossils to determine the age of the organism that was preserved. This technique relies on the property of half-life.

What is Half-Life?

Half-life is defined as the time it takes for one half of a radioactive element to decay into a daughter isotope. As radioactive isotopes of elements decay, they lose their radio activity and become a brand new element known as a daughter isotope. By measuring the ratio of the amount of the original radioactive element to the daughter isotope, scientists can determine how many half-lives the element has undergone and from there can figure out the absolute age of the sample.

The half-lives of several radioactive isotopes are known and are used often to figure out the age of newly found fossils. Different isotopes have different half-lives and sometimes more than one present isotope can be used to get an even more specific age of a fossil. Below is a chart of commonly used radiometric isotopes, their half-lives, and the daughter isotopes they decay into.

Example of How to Use Half-Life

Let's say you have found a fossil you think to be a human skeleton. The best radioactive element to use to date human fossils is Carbon-14. There are several reasons why, but the main reasons is that Carbon-14 is a naturally occurring isotope in all forms of life and its half-life is about 5730 years, so we are able to use it to date more "recent" forms of life relative to the Geologic Time Scale.

You would need to have access to scientific instruments at this point that could measure the amount of radioactivity in the sample, so off to the lab we go! After you prepare your sample and put it into the machine, your read-out says you have approximately 75% Nitrogen-14 and 25% Carbon-14. Now it is time to put those math skills to good use.

At one half-life, you would have approximately 50% Carbon-14 and 50% Nitrogen-14. In other words, half (50%) of the Carbon-14 you started with has decayed into the daughter isotope Nitrogen-14. However, your read out from your radioactivity measuring instrument says you have only 25% Carbon-14 and 75% Nitrogen-14, so your fossil must have been through more than one half-life.

After two half-lives, another half of your leftover Carbon-14 would have decayed into Nitrogen-14. Half of 50% is 25%, so you would have 25% Carbon-14 and 75% Nitrogen-14. This is what your read out said, so your fossil has undergone two half-lives.

Now that you know how many half-lives have passed for your fossil, you need to multiply your number of half-lives by how many years are in one half-life. This gives you an age of 2 x 5730 = 11,460 years. Your fossil is of an organism (maybe human) that died 11,460 years ago.

Commonly Used Radioactive Isotopes

Parent Isotope Half-Life Daughter Isotope
Carbon-14 5730 yrs. Nitrogen-14
Potassium-40 1.26 billion yrs. Argon-40
Thorium-230 75,000 yrs. Radium-226
Uranium-235 700,000 million yrs. Lead-207
Uranium-238 4.5 billion yrs. Lead-206
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