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11.3: Lesson 11.2 Radioactive Decay

Difficulty Level: At Grade Created by: CK-12

Key Concept

Radioactive decay is the process in which unstable nuclei emit particles and energy and change to different elements. Types of decay include alpha, beta, and gamma decay. They differ in what is emitted, how far it can travel, and what it can penetrate. The rate of radioactive decay, measured by the half-life, varies from one radioisotope to another.


  • AAAS.6-8.4.D.2; AAAS.6-8.4.F.9

Lesson Objectives

  • Identify three types of radioactive decay.
  • Define the half-life of a radioisotope.
  • Explain radioactive dating.

Lesson Vocabulary

  • half-life: length of time it takes for half of a given amount of a radioisotope to decay
  • radioactive dating: method of determining the age of fossils or rocks that is based on the rate of decay of radioisotopes
  • radioactive decay: process in which the unstable nucleus of a radioisotope becomes stable by emitting charged particles and energy and changing to another element

Teaching Strategies

Introducing the Lesson

Introduce radioactive decay by probing students’ understandings of the term decay. Play a word association game, using decay as the prompt. Call on each student in turn to say the first word that comes to mind when they hear the word decay. (Possible responses might include rot, break down, spoil and fall apart.) When no new responses are forthcoming, explain to the class that they will learn how unstable nuclei decay when they read this lesson. Challenge them to predict what decay means in this context.


Use the activity at the URL below to introduce the three types of radioactive decay. With the activity, students can explore how decay leads to the transmutation of elements. It requires them to use mathematical reasoning and graphing skills and will give them an understanding of radioactive decay at the conceptual level. The URL provides all necessary worksheets and other materials for students as well as teacher notes and sample answers.

Differentiated Instruction

Work with students to make a compare/contrast table for alpha, beta, and gamma decay. A sample is shown in the Table below.

Type of Decay What Is Emitted? Is a New Element Produced?
Alpha alpha particle (2 protons + 2 neutrons) & energy yes
Beta beta particle (1 electron) & energy yes
Gamma energy (only) no


Ask one or more students to make a Jeopardy quiz game on lesson content. First, they should prepare a list of at least 20 answers that require contestants to generate the correct questions. Here are two possible examples:

Answer: Tiny particle emitted by uranium-238 when it changes to thorium-234
Question: What is an alpha particle?
Answer: Rays of energy that can be stopped only be several meters of concrete
Question: What are gamma rays?

After the students have created the list of answers (including the number of points for each one based on difficulty), have them lead the class in playing the game. Contestants can be individual students, partners, or small groups. If you wish, you can have the students use the Flash Jeopardy game generator at this URL below to create an online version of the game: http://www.superteachertools.com/jeopardy/.

Science Inquiry

Assign the half-life and radioactive dating game lab at the first URL below. The lab worksheet guides students in using the simulation at the second URL below. In the lab, students investigate the half-lives of carbon-14 and uranium-238 and use radioactive dating to estimate the ages of virtual fossils and rocks. In the analysis section, students apply the concepts of half-life and radioactive dating to solve additional problems.

Common Misconceptions

The URL below provides an alternative radioactive dating activity to the one described just above. In this “Date a Rock!” activity, students use simulated rock samples, each one containing a different proportion of a parent radioisotope and its daughter product. By counting atoms and using knowledge of the half-life of the parent radioisotope, students estimate the ages of the rock samples. The Web site includes a materials list, overhead transparencies, teaching strategy, pre- and post-lab assessments, extensions, and other resources.

Life Science Connection

Discuss the importance of radioactive dating to our understanding of how living things have evolved. The URL below provides additional information that you may want to share with students.

Reinforce and Review

Lesson Worksheets

Copy and distribute the lesson worksheets in the CK-12 Physical Science for Middle School Workbook. Ask students to complete the worksheets alone or in pairs to reinforce lesson content.

Lesson Review Questions

Have students answer the Review Questions listed at the end of the lesson in the FlexBook® student edition.

Lesson Quiz

Check students’ mastery of the lesson with Lesson 11.2 Quiz in CK-12 Physical Science for Middle School Quizzes and Tests.

Points to Consider

Radioactive decay releases energy. Other types of nuclear reactions also release energy.

  • What other types of reactions might the nucleus undergo?
  • What might be the pros and cons of using nuclear reactions for energy?

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