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Radioactive decay causes a reduction in the number of unstable nuclei in a sample.

In turn, this reduces the count rate measured by a detector such as a Geiger-Muller tube. Another way to define the half-life of a radioactive isotope is the time taken for count rate from a sample to decrease by a half. The table shows how the count rate of an isotope might change over time. Notice how the count rate falls to half its previous value every four days. This is the half-life of the isotope. Note that this is equal to: Use the table to determine the net decline, expressed as a ratio, after 6 half-lives.


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Half-life Radioactive decay is a random process. Understand the relative ages of layers from the law of superposition and what fossils are, but need not yet know exactly how the absolute ages of rock layers or fossils are determined. Have knowledge of the geologic time scale and the names of the eons throughout history. Instructional Suggestions This lesson is intended to be completed in 1. Begin class with a brief introduction to the concept of radioactivity. If you do not have access to these, a class discussion, read aloud from an appropriate selection of text , or other video review would be appropriate.

Be sure to define the following words with the class: Discuss the possible uses of radioactive material in science. BBC Bitesize goes on to explain radioactive dating in the following section , though it is only text and images, without an interactive video. Introduce the MEA lesson concept: Your students' help has been requested by the Natural History Foundation in order to help them select the proper elemental isotopes to use in their radioactive dating process.

The students must decide which elemental isotope is best suited for determining the absolute age of one of three ancient discoveries. Inform students that they will be completing a full essay written response with their decisions and reasoning at the end of this project. Arrange students in groups of to prepare for the project it may be helpful to have predetermined groups ready.

Allow students in each group to read the client letter. This image will help the students in their understanding of the client letter as well as developing their response essay. Ask the students what they think of the information, and how would they determine which elemental isotope is the best for each item? Use their responses to transition into the next section.

What is Radioactive Dating? Define and explain "radioactive dating" and "half-life. A half-life is a set amount of time after which half of the radioactive element will have disappeared because it turned into a new element!

GCSE Physics - Radioactivity - Half-Life and Carbon Dating 1 of 2

Radioactive dating is when scientists use the amount of radioactive materials in a rock or fossil and the half-life duration to calculate the absolute age of the rocks or fossils. They can then complete calculations to see how many half-lives have passed, which will tell them the age of the rock!

With your class, complete the following examples on the board: If a rock sample has g of radioactive material with a half-life of years, how much of the radioactive material will remain after years? What about after years? A scientist finds a gram rock composed of grams of Lead and 50 grams of Uranium They know that the rock used to be entirely made of Uranium The half-life of U is million years.

How old is the rock? After 1 half-life, how much Uranium will I have left? What about after a second half life? Take how much Uranium we have left g and divide that by 2. Is that how much the scientist found in the rock? Yes And how many half-lives had to pass to get us to 50g? So, if each half-life is million years, and we did 2 of them, how old is our rock? If available, use manipulatives whenever they will be of assistance.

Use the readiness questions to be sure each group understands the assignment. They must think of a rationale to explain why they would select a particular elemental isotope for each of the three items being dated. You may discuss the readiness questions with the entire class, or go over them with each group separately. A teacher of advanced classes may be able to point the group to the questions and allow them to discuss only amongst each other, without direct instruction from the teacher.

Once the students understand what is requested of them, allow them to work in their groups. They should have a written explanation of the steps used to make this selection. The response letter must be written in paragraph form as a professional response to the client. This response letter is a full 5 paragraph essay: Students must have 1 written paragraph for each of the items to be dated. This paragraph must provide the rationale as to why the particular element was selected. Collect all student work and wrap up: Ask students which choice they made and why they chose it.

Open discussion with the entire class. What item do they think would be the oldest?

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Which one would be hardest to date? Block Day 2 min. Begin class with a review of Radioactive Dating and Half-Life. The BrainPop "Carbon Dating" video is a great review tool. If you do not have access to this, a class discussion, read aloud from an appropriate selection of text , or other video review would be appropriate. Complete the BrainPop post-lesson quiz as a class.

If you do not have access to this, a few review demonstrations using the blocks and some half-life calculations would be appropriate. Inform your students that you have heard back from the Natural History Foundation. They have more work for you! Arrange your students in the groups from the first section of this MEA.

Once again, the group will have to complete a response letter indicating the elemental isotope they feel should be used on each of the new items. The new data only has 2 items, so students must complete a 4 paragraph response essay. Collect student essays and review responses as a class. The MEA has been completed.


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  • If using block scheduling, this allows you approximately 1 hour of the same class day to review concepts with the class, do reading on related topics, watch an educational video, or any other desired lesson reinforcement. Supplemental Reading The BrainPop website has numerous information associated with the videos presented in the lesson plan. Advanced students may be able to learn from this information. Answering these questions should aid you in selecting an elemental isotope to be used in dating these objects: Ground sloth During what time eras or epochs do we think the giant ground sloth lived?

    About how many years ago was that? Stone slab from Antarctica What type of rock is the stone slab from Antarctica? In what eon were most of the rocks nearby this location formed? Petrified trees Although petrified trees used to be alive, what are they made of when we find them in the present day?

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    In what era did most of the trees from the park in Arizona live? Ground sloth The Pleistocene and Holocene epochs The Holocene is the recent past going back to about 11, years age. The Pleistocene goes from 11, Rubidium and Uranium; they are not accurate at dating objects less than 10 million years old. Stone slab from Antarctica Igneous Archean Eon 2. Only works on matter that was once living.

    Also, C dating only works on objects up to , years old. Only works on matter that was once living The minerals that replaced the tree were never living Also, C dating only works on objects up to , years old. Reading Passage 1 See attached Client Letter 1. Attention Geologists, The Natural History Foundation NHF is a company that works to ensure the accuracy and validity of scientific research and claims. The three items that must be dated are as follows: Thank you for your help!

    What is the client asking your team to do? What are the three objects being dated by the company? What things do you need to include in your response? Sample Answers to Readiness Questions: Our client is a company called the National History Foundation. Our client wants us to select an elemental isotope to use for radioactive dating of three different objects.

    The three objects are the remains of a giant ground sloth, a large stone found in Antarctica, and a petrified tree. In our response, we need to complete a 5 paragraph essay that explains which elemental isotope we feel should be used to date each of the three objects. We also have to explain why we selected the isotope for each object. Letter Template 1 No template is provided with this lesson providing a template would essentially negate the Language Arts standard, as students would not be developing the essay independently. The section has been copied below: Reading Passage 2 See attached file Client Letter 2.

    Attention Geologists, The Natural History Foundation NHF has completed our dating process using the elemental isotopes you suggested with excellent results! The two items that must be dated are as follows: Letter Template 2 No template is provided with this lesson providing a template would essentially negate the Language Arts standard, as students would not be developing the essay independently.

    Additional Instructions or Materials Be sure students have access to a simple geologic time scale. Reflection question 2 Were you able to use the same reasoning to date the second two objects as you used on the first objects? Do you think the obsidian hand-axe was crafted by an Australopithecus? Did you have any trouble deciding which method to use for the rock from Australia? Why or why not?

    How Carbon-14 Dating Works

    As indicated in the Instructional Suggestions section: If focusing more on the mathematics of this lesson, it is suggested that extra time and attention be placed on proper calculations of the half-lives, perhaps doing a few extra examples. A follow-up lesson involving reinforcement of multiplication of fractions and decimals could be helpful.

    It could be presented at any point you feel comfortable before, during, or after the completion of the MEA. There are excellent videos available online that discuss much of this information.

    Irradiation vs Contamination | Teach Nuclear

    When it decays it forms thorium which is also unstable. Finally, after a series of radioactive isotopes are formed it becomes lead, which is stable. The age of the rock can be calculated if the ratio of uranium to lead is known. As the rock gets older the proportion of lead increases. If half of the uranium has turned into lead the rock will be million years old.