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Absolute Dating

LEARNING TARGET: 

I can use radiometric dating to calculate the absolute age of rocks and fossils.

 

FOCUS QUESTIONS: 

What is radiometric dating?

How are radioactive elemets used to determine the age of rocks and fossils?

 

KEY CONCEPT:

The absolute age of rocks and fossils can be measured with a unit called half-life.

Radioactive decay is a random process.

Radioactive decay can be related to probability.

The breakdown of a radioactive element into a decay element occurs at a constant rate.  By measuring the ratio of radioactive element to the decay element, the age of the fossil can be estimated.

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EXAMPLE 1:

If you started with a sample of 600 radioactive nuclei, about how many would remain un-decayed after three half-lives?

Solution:

600 divided by 2 = 300 nuclei (after 1 half-life)

300 divided by 2 = 150 nuclei (after 2 half-lives)

150 divided by 2 = 75 nuclei (after 3 half-lives)

The correct answer is 75 nuclei.

 

EXAMPLE 2:

Carbon-14 has a half-life of about 5,730 years.  How many half-lives will it take a sample of 100.0 grams to decay to 12.50 grams?

Solution:

100 grams divided by 2 = 50 grams (after 1 half -life)

50 grams divided by 2 = 25 grams (after 2 half-lives)

25 grams divided by 2 = 12.5 grams (after 3 half-lives)

 

EXAMPLE 3:

Carbon-14 has a half life of about 5730 years.  An anthropologist finds some human bones containing only 3% un-decayed carbon-14 nuclei.  About how old are the hominid bones?

Solution:  (Use the radiometric dating table of carbon-14 to the right  of this page)

Locate 3% undecayed nuclei on the table.

The corresponding age of a set of bones containing 3% undecayed carbon-14 nuclei is about 28,000 years old

 

VOCABULARY:

Radiometric Dating:  the method of obtaining a rock’s age by measuring the relative abundance of radioactive nuclei.

Half-life:  the amount of time required for half of an element’s radioactive nuclei to decay.

Radioactive Decay:  the spontaneous emission of radiation from the unstable nucleus of an atom.

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