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  • 804 Chapter 25

    Nuclear Chemistry

    CHAPTER 25

    What Youll LearnYou will trace the history ofnuclear chemistry from dis-covery to application.

    You will identify types ofradioactive decay and solvedecay rate problems.

    You will describe the reac-tions involved in nuclearfission and fusion.

    You will learn about appli-cations of nuclear reactionsand the effects of radiationexposure.

    Why Its ImportantFrom its role in shaping worldpolitics to its applications thatproduce electrical power anddiagnose and treat disease,nuclear chemistry has pro-found effects upon the worldin which we live.

    Many medical diagnostic testsand treatments involve the use ofradioactive substances. Here, aradioactive substance known as aradiotracer is used to illuminatethe carotid artery, which runsthrough the neck into the skull.

    Visit the Chemistry Web site atchemistrymc.com to find linksabout nuclear chemistry.

    http://chemistrymc.com

  • 25.1 Nuclear Radiation 805

    Objectives List the founding scientists

    in the study of radioactivityand state their discoveries.

    Identify alpha, beta, andgamma radiation in termsof composition and keyproperties.

    VocabularyradioisotopeX ray

    Section 25.1 Nuclear Radiation

    You may recall from Chapter 4 that the nuclei of some atoms are unstableand undergo nuclear reactions. In this chapter you will study nuclear chem-istry, which is concerned with the structure of atomic nuclei and the changesthey undergo. An application of a nuclear reaction is shown in the photo ofthe human neck and skull. Table 25-1 offers a comparison of chemical andnuclear reactions.

    DISCOVERY LAB

    Materials

    28 dominotiles (1 set)

    stopwatch

    Chain Reactions

    When the products of one nuclear reaction cause additionalnuclear reactions to occur, the resulting chain reaction canrelease large amounts of energy in a short period of time. Exploreescalating chain reactions by modeling them with dominoes.

    Procedure

    1. Obtain a set of domino tiles.

    2. Stand the individual dominoes on end and arrange them so that whenthe first domino falls, it causes the other dominoes to fall in series.

    3. Practice using different arrangements until you determine how tocause the dominoes to fall in the least amount of time.

    4. Time your domino chain reaction. Compare your time with those ofyour classmates.

    Analysis

    What arrangement caused the dominoes to fall in the least amount oftime? Do the dominoes fall at a steady rate or an escalating rate?What happens to the domino chain reaction if a tile does not contactthe next tile in the sequence?

    Characteristics of Chemical and Nuclear Reactions

    Chemical reactions

    1. Occur when bonds are broken and formed.

    2. Atoms remain unchanged, though they may be rearranged.

    3. Involve only valence electrons.

    4. Associated with small energy changes.

    5. Reaction rate is influenced by temperature, pressure, concentration, and catalysts.

    Nuclear reactions

    1. Occur when nuclei emit particlesand/or rays.

    2. Atoms are often converted intoatoms of another element.

    3. May involve protons, neutrons, and electrons.

    4. Associated with large energychanges.

    5. Reaction rate is not normallyaffected by temperature, pressure,or catalysts.

    Table 25-1

  • The Discovery of RadioactivityIn 1895, Wilhelm Roentgen (18451923) found that invisible rays were emit-ted when electrons bombarded the surface of certain materials. The emittedrays were discovered because they caused photographic plates to darken.Roentgen named these invisible high-energy emissions X rays. As is true inmany fields, Roentgens discovery of X rays created excitement within the

    scientific community and stimulated further research. At that time, Frenchphysicist Henri Becquerel (18521908) was studying minerals that

    emit light after being exposed to sunlight, a phenomenon calledphosphorescence. Building on Roentgens work, Becquerel

    wanted to determine whether phosphorescent minerals also emitted X rays. Becquerel accidentally discovered that phosphorescent

    uranium saltseven when not exposed to lightproduced spontaneousemissions that darkened photographic plates. Figure 25-1 shows the darken-ing of photographic film that is exposed to uranium-containing ore.

    Marie Curie (18671934) and her husband Pierre (18591906) tookBecquerels mineral sample (called pitchblende) and isolated the componentsemitting the rays. They concluded that the darkening of the photographicplates was due to rays emitted specifically from the uranium atoms presentin the mineral sample. Marie Curie named the process by which materials giveoff such rays radioactivity; the rays and particles emitted by a radioactivesource are called radiation.

    The work of Marie and Pierre Curie was extremely important in estab-lishing the origin of radioactivity and the field of nuclear chemistry. In 1898,the Curies identified two new elements, polonium and radium, on the basisof their radioactivity. Henri Becquerel and the Curies shared the 1903 NobelPrize in Physics for their work. Marie Curie also received the 1911 NobelPrize in Chemistry for her work with polonium and radium. Figure 25-2shows the Curies at work in their laboratory.

    Types of RadiationWhile reading about the discovery of radioactivity, several questions may haveoccurred to you. Which atomic nuclei are radioactive? What types of radia-tion do radioactive nuclei emit? It is best to start with the second questionfirst, and explore the types of radiation emitted by radioactive sources.

    806 Chapter 25 Nuclear Chemistry

    Figure 25-2

    Both Pierre and Marie Curieplayed important roles in founding the field of nuclearchemistry. Marie Curie went onto show that unlike chemicalreactions, radioactivity is notaffected by changes in physicalconditions such as temperatureand pressure. She is the onlyperson in history to receiveNobel Prizes in two different sci-encesphysics in 1903, andchemistry in 1911.

    Figure 25-1

    A sample of radioactive uranium-containing ore exposes photo-graphic film.

  • As you may recall, isotopes are atoms of the same element that have differ-ent numbers of neutrons. Isotopes of atoms with unstable nuclei are calledradioisotopes. These unstable nuclei emit radiation to attain more stable atomicconfigurations in a process called radioactive decay. During radioactive decay,unstable atoms lose energy by emitting one of several types of radiation. Thethree most common types of radiation are alpha (), beta (), and gamma ().Table 25-2 summarizes some of their important properties. Later in this chap-ter youll learn about other types of radiation that may be emitted in a nuclearreaction.

    *(1 MeV 1.60 1013 J)

    Ernest Rutherford (18711937), whom you know of because of his famousgold foil experiment that helped define modern atomic structure, identifiedalpha, beta, and gamma radiation when studying the effects of an electric fieldon the emissions from a radioactive source. As you can see in Figure 25-3,alpha particles carry a 2 charge and are deflected toward the negativelycharged plate. Beta particles carry a 1 charge and are deflected toward thepositively charged plate. In contrast, gamma rays carry no charge and are notaffected by the electric field.

    25.1 Nuclear Radiation 807

    Figure 25-3

    The effect of an electric field onthree types of radiation is shownhere. Positively charged alphaparticles are deflected towardthe negatively charged plate.Negatively charged beta parti-cles are deflected toward thepositively charged plate. Betaparticles undergo greater deflec-tion because they have consider-ably less mass than alphaparticles. Gamma rays, whichhave no electrical charge, arenot deflected.

    Properties of Alpha, Beta, and Gamma Radiation

    Property Alpha () Beta () Gamma ()

    Composition Alpha particles Beta particles High-energyelectromagneticradiation

    Description of Helium nuclei Electrons photonsradiation 42He 01 00

    Charge 2 1 0

    Mass 6.64 1024 kg 9.11 1028 kg 0

    Approximate 5 MeV 0.05 to 1 MeV 1 MeVenergy*

    Relative Blocked Blocked Not completelypenetrating by paper by metal foil blocked bypower lead or concrete

    Table 25-2

    Positive plate

    SlotLead block

    Radioactivesource

    Gamma rays(no charge)

    Zinc sulfidecoated screen

    Betaparticles

    (1 charge)

    Alphaparticles

    (2 charge)Negative plate

    LAB

    See page 964 in Appendix E forModeling Radiation Penetration

  • An alpha particle () has the same composition as a helium nucleustwoprotons and two neutronsand is therefore given the symbol 42He. The chargeof an alpha particle is 2 due to the presence of the two protons. Alpha radi-ation consists of a stream of alpha particles. As you can see in Figure 25-4,radium-226, an atom whose nucleus contains 88 protons and 138 neutrons,undergoes alpha decay by emitting an alpha particle. Notice that after thedecay, the resulting atom has an atomic number of 86, a mass number of 222,and is no longer radium. The newly formed radiosiotope is radon-222.

    In examining Figure 25-4, you should note that the particles involved arebalanced. That is, the sum of the mass numbers (superscripts) and the sum ofthe atomic numbers (subscripts) on each side of the arrow are equal. Also notethat when a radioactive nucleus emits an alpha particle, the product nucleushas an atomic number that is lower by two and a mass number that is lowerby four. What particle is formed when polonium-210 (21084Po) undergoes alphadecay?

    Because of their mass and charge, alpha particles are relatively slow-moving compared with other types of radiation. Thus, alpha particles are notvery penetratinga single

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