Radiometric Dating Lab

In this lab, we will review the basic concepts related to radiometric dating and complete a practice activity simulating how radioactive decay occurs.

Background

Relative vs. Absolute Dating

Relative dating is the science of determining the relative order of past events (i.e., the age of an object in comparison to another), without necessarily determining their absolute age (i.e. estimated age). Relative dating gives an approximate age of something compared to some other event in history. Scientists can tell that the item is “older” or “younger” than certain events in time, but cannot give an exact age of the item. Relative dating is common when comparing layers of rocks in different regions, and figuring out which fossil is older by comparing the rock layers the fossils are in.

Absolute dating gives an actual date in history that the item was formed or died. The most common type of absolute dating for geologic material is radiometric dating. A rock containing a radioactive element can be dated by measuring how much of the element remains. Radiometric dating methods give absolute ages ranging from decades to billions of years.

Radiometric Dating 

Radiometric dating provides science with a powerful tool for reconstructing our planet’s history. The idea that radioactivity could be used as a measure of the age of geologic formations was first suggested in 1905 by a British physicist, Lord Rutherford. Not until the 1950s, however, was precise dating achieved and accepted by the scientific community. The methodologies and instruments for radiometric dating have been expanded and fine-tuned in the half-century since, and very accurate dating is now possible.

Atoms are composed of a nucleus orbited by negatively charged electrons. The nucleus is made up of protons, particles with a positive charge, and neutrons, particles with no charge. Every atom of a given element has the same number of protons in the nucleus. Each element may have one or more isotopes. Different isotopes of a given element have the same number of protons but a different number of neutrons. Remember the basic structure of an atom:

Figure 1. Atomic structure. www.wonderopolis.org

Radioactive elements are unstable atoms that give off particles. Emitting these particles transforms the unstable atoms into different, more stable elements. This is called radioactive decay, and it occurs at a constant rate specific to each isotope of each element. The original radioactive material is called the parent; the stable product is called the daughter. The rate of decay is described by the half-life of the isotope—the average time an atom of a radioactive element remains in the parent state. When the half-life has elapsed, half the parent element will have decayed into the daughter element.

Figure 2. Radioactive decay over time (www.schoolworkhelper.com)

Potassium-40, for example, decays into Argon-40 with a half-life of 1.25 billion years, so that after 1.25 billion years half of the Potassium-40 in a rock will have become Argon-40. This means that if a rock sample contained equal amounts of Potassium-40 and Argon-40, it would be 1.25 billion years old. If the sample contained three atoms of Potassium-40 for every one atom of Argon-40, it would be 625 million years old. And if it contained one atom of Potassium-40 for every three atoms of Argon-40 it would be 1.875 billion years old. 

Table 1. Half Lives of Selected Radioactive Elements.

Carbon-14 Dating

C14 dating is a method of age determination that depends upon the decay to nitrogen of radiocarbon (carbon-14). Carbon-14 dating is a way of determining the age of certain archeological artifacts of a biological origin up to about 50,000 years old.

Figure 3. Radioactive decay of C14 (www.sciencelearn.org.nz)

Because C14 has a half life of only 5730 years, it can’t be used to date very old materials. It has been used to date the following:

ShellsLeatherPeatLake muds and sedimentsSoilIce coresBone

PollenHairPotteryWall paintings and rock art worksAvian eggshellCorals and foraminifera

Textiles and fabricsPaper and parchmentFish remainsInsect remainsResins and gluesAntler and hornCharcoal, wood, and seeds

Example Problem : A sample of mussel shells is tested, and it is estimated that 1.5% of the original C14 is still present. When was that mussel shell likely produced (i.e. how old is it)?

Answer: We can answer this question by looking at this table. We now that the half life of C14 is 5730 years. So, after 5730 years 50% of the original is gone. After 11460 years (i.e. 2 half lives) 25% of the C14 remains, etc. So at which point is only ~1.5% of the C14 left? Approximately 34,380 years- that’s how old the mussel shell it.

Number of Half Lives	Years	% of Parent Material (C14)	% of Daughter Material (N14)
0	0	100%	0%
1	5730	50%	50%
2	11460	25%	75%
3	17190	12.5%	87.5%
4	22920	6.25%	93.75%
5	28650	3.125%	96.875%
6	34380	1.5625%	98.4375%
7	40110	0.78125%	99.21875%
8	45840	0.390625%	99.609375%
9	51570	0.1953125%	99.8046875%
10	57300	< 0.1%	>99.9%

Radiometric Dating

Materials

Student Supplied

– 100 pennies (you can also use any two-sided candy such as M&M or Skittles- make the side with the writing ‘heads’).

**Alternatively, you can use https://www.random.org/coins/ to digitally flip coins. Choose “100” for the number flipped, and choose your coin (scroll all the way down to U.S. to select pennies). After recording the number of tails, change the number of coins flipped, and repeat this process as described below).

Procedure:

1. Start with 100 pennies. Each represents an atom in the radioactive element Carbon-14.

2. Dump out all of the pennies and spread them out.

3. Remove all the coins that show tails to the side. These are atoms that have “decayed” and are

no longer radioactive.

4. Record the number of pennies with heads in Data Table 1. These represent atoms that are

still radioactive.

5. Put the pennies that had heads up back in the container, mix them, and spread them out on

the table and repeat the above process until all the pennies are gone or until you have

completed 15 trials.

6. Graph the data in Data Table 1, either using a graphing program such as Excel, or by hand using the graph paper provided.

7. Complete the review questions.

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