Half-Life and Radioactive DecayMy Goals for this Lesson:

Distinguish between nuclear and chemical reactions. Compare and contrast different types of nuclear reactions. Describe and make predictions regarding half-life.I’m preparing to distinguish between nuclear and chemical reactions, compare and contrast

different nuclear reactions and understand half-life and how to predict it for a given situation.

Introduction Answer the following questions in your own words using what you learned from the introduction.

What is Carbon 14 dating? Who first developed the technique and when? What is Carbon 14?

Why can we use Carbon 14 to “date” once living remains? How do scientists do Carbon 14 dating? What assumption do scientists make in doing Carbon 14 dating? Who is Jim Chatters? Who is Kennewick Man? What does Carbon 14 dating have to do with them? Be sure to have watched the Discovery Education™ Streaming video to help you see how carbon 14 dating is used in science.

LessonFill in blanks using the Lesson.Nuclear reactions arise from an atom’s . Nuclear radiation occurs in the rocks and soil around us, in the air we breathe, and even in the food we eat. There are also nuclear reactions that are used by humans to our society. These reactions play an important role in medical diagnostics and radiation therapy and can also be used in nuclear power plants to convert nuclear energy to electricity. Reactions occurring in the are our planet’s most significant source of nuclear energy. Because of the , and sometimes

, roles that nuclear reactions and radiation play in the world around us, it is a good idea to have a basic understanding of nuclear reactions.

“Did you know?”What does this symbol mean?

Chemical vs. Nuclear ReactionsAtoms are made up of , , and . In the chemical reactions that we have studied throughout this course, the sharing or exchange of was involved in forming bonds and compounds while the and remained unchanged in the nuclei of the atoms.

However, some atoms have nuclei because the number of protons and neutrons are balance. Atoms with nuclei are ; they eventually break down into a different substance and release energetic particles, or radiation, in the process.

Radiation TherapyWhat does the lesson tell you about radiation therapy?

Alpha, Beta, and Gamma RaysFill in the blanks using the tabs in the lesson.

Alpha (α) RadiationAlpha radiation is made up of a stream of particles. Alpha particles are made up of protons and two neutrons released from the nucleus of the radioactive atom. This means that alpha particles have a charge, and that when an atom releases an alpha particle, its atomic number by two and its mass number decreases by four.

Alpha particles have a high amount of energy and can cause to surface materials such as skin and living tissue. However, alpha particles are relatively easy to against. They cannot normally penetrate materials such as paper or fabric. Also, as they travel through the air, the particles attract electrons and become helium atoms.

Beta (β) RadiationBeta radiation is made up of a stream of beta particles. Beta particles are fast-moving released from a nucleus when a neutron apart into one proton and one electron.

A negative beta particle, which is a very fast moving electron, is released when a neutron . A positive is left behind in the nucleus during this type of decay.

When the negative beta particle is released from the nucleus of an atom, the atom ends up with one more and one less .

Beta particles have a negative charge and they usually move than alpha particles. This means that beta particles are more to protect against than alpha particles; they can penetrate cloth and paper. Beta particles can penetrate deeply into skin and potentially or living cells. However, these particles cannot penetrate thin layers of materials such as and other metals. When beta particles are finally stopped by a substance, they are by the material, like any other electron.

Gamma (γ) RadiationGamma radiation can be given off during different types of nuclear decay. Gamma rays are a form of electromagnetic with a very high frequency and energy than ultraviolet light or X-rays.

Because gamma rays have high energy and no mass or charge, they can penetrate through materials. Gamma rays can cause much damage to living cells than alpha or beta particles. Only very materials, such as thick layers of lead, can stop gamma rays. This is why is commonly used as a shielding material in laboratories and hospitals where gamma radiation is present.

OverviewUse the Overview tab to complete this chart using your own words.

Alpha (α) Beta (β) Gamma (γ)DescriptionMovementImpact on humans

Radioactive DecayFill in the blanks and chart using the lesson.When a element’s nucleus decays and gives off an alpha or beta particle, the number of protons and neutrons inside the nucleus . When this happens, the atom becomes another element. This is what makes nuclear reactions than regular chemical reactions. In a nuclear reaction, the of the elements actually , because protons and neutrons are gained or lost by the atom over the course of the reaction.

Produces Leaves Behind Describe the energy released

Radioactive uranium-238Thorium-234

The decay of uranium-238 involves a series of alpha and beta decays that eventually produce .

The table shows the series of nuclear-decay reactions that occur. Take some time to examine how the atomic number and mass number changes for each reaction and identify them as alpha or beta decay.

Be sure to do the “Let’s Review” section on the Activity page.

Half-LifeFill in the blanks using the lesson.Radioactive isotopes at different rates, but the rates are all measured in terms of the substance’s . Half-life is the needed for of the radioactive atoms in a sample to decay. The half-life of a given is constant and is independent of conditions or the amount of atoms in the sample. This means that the half-life of a specific isotope will be the whether you have one million moles or one mole of the atoms.

The graph below represents the radioactive decay of a hypothetical element. Notice that the time it takes for the amount of radioactive material to decrease from 1.0 to 0.5 is the same as the amount of time it takes to decrease from 0.25 to 0.125. This time interval, marked as t½ on the graph, represents the half-life of this radioactive substance and is independent of the amount of radioactive substance remaining in the sample.

The graph represents the decay of a radioactive element and illustrates how the data can be used to determine the half-life of the substance. The amount of radioactive element remaining in the sample, measured in moles, percentage, grams, or radioactivity detected, is on the vertical y axis of the graph. Time, measured in seconds, minutes, days, or years, is on the horizontal x-axis.

The rate of radioactive decay, like any other rate, gives a curved line as the amount of the radioactive substance remaining decreases over time. The amount of time it takes for only half of the original sample to remain is equal to the amount it takes for the radioactive sample to decrease from one-half to one-quarter of the original amount. For example, if it took twenty days for the sample to decrease from one hundred moles to fifty moles of radioactive material, it would take another twenty days for the sample to decrease from fifty moles to twenty five moles of radioactive material remaining.

Radium-226 has a half-life of 1,620 years, which means that half of a given sample of radium-226 will decay into lead by the end of 1,620 years. In the next 1,620 years, half of the remaining sample will decay into lead, leaving one-fourth of the original amount of radium-226.

The half-lives of radioactive substances range from less than a millionth of a second to more than a billion years. Uranium-238 has a half-life of 4.5 billion years! So how can scientists measure half-lives that are that long? The answer is that they do not measure the actual half-life, but they can accurately measure the rate of the isotope’s decay using a radiation detector. The faster a substance decays, the more radiation per minute is detected, and the shorter the half-life of a given isotope. It is not necessary to wait through an entire half-life of a substance; the half-life can be calculated using the rate of decay that is observed.

Once we know the half-life of a substance, this information can be used to estimate the age of ancient remains. One of the most common examples of this is the use of carbon-14 dating to estimate the age of dead organisms or artifacts made of wood or cloth.

Radiation DetectorsUse the interactive section on Radiation Detectors to complete this chart.

Image Description

Be sure to do the “Let’s Practice” section on the Activity page.