chapter 17 radioactivity and nuclear chemistry

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Chapter 17 Radioactivity and Nuclear Chemistry. The Discovery of Radioactivity. Antoine-Henri Becquerel designed an experiment to determine if phosphorescent minerals also gave off X-rays. - PowerPoint PPT Presentation

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  • Chapter 17Radioactivityand NuclearChemistry

    Tro's Introductory Chemistry, Chapter 17

  • The Discovery of RadioactivityAntoine-Henri Becquerel designed an experiment to determine if phosphorescent minerals also gave off X-raysBequerel discovered that certain minerals were constantly producing penetrating energy rays he called uranic rays (like X-rays, but not related to fluorescence)Bequerel determined that all the minerals that produced these rays contained uraniumthe rays were produced even though the mineral was not exposed to outside energyEnergy apparently being produced from nothing??

    Tro's Introductory Chemistry, Chapter 17

  • The CuriesMarie Curie used electroscope to detect uranic rays in samplesDiscovered new elements by detecting their raysradium named for its green phosphorescence polonium named for her homelandSince these rays were no longer just a property of uranium, she renamed it radioactivity

    Tro's Introductory Chemistry, Chapter 17

  • What isRadioactivity?release of tiny, high energy particles from an atomparticles are ejected from the nucleusradioactive rays can ionize mattercause uncharged matter to become chargedbasis of Geiger Counter and electroscoperadioactive rays have high energyradioactive rays can penetrate matterradioactive rays cause phosphorescent chemicals to glowbasis of scintillation counter

    Tro's Introductory Chemistry, Chapter 17

  • Rutherfords Experiment++++++++++++--------------Rutherford discovered there were three types of radioactivityalpha rays (a)have a charge of +2 c.u. and a mass of 4 amuwhat we now know to be helium nucleusbeta rays (b)have a charge of -1 c.u. and negligible masselectron-likegamma rays (g)form of light energy (not particle like a & b)

    Tro's Introductory Chemistry, Chapter 17

  • Penetrating and Ionizing Ability0.01 mm 1 mm 100 mmPieces of Leadpenetrating abilitya < b < g

    ionizing abilitya > b > g

    Tro's Introductory Chemistry, Chapter 17

  • Facts About the NucleusVery small volume compared to volume of the atomEssentially entire mass of atom; very denseComposed of protons and neutrons (nucleons) that are tightly held togetherEvery atom of an element has the same number of protons (atomic number, Z)Atoms of the same elements can have different numbers of neutrons (isotopes)Isotopes are identified by their mass number (A)mass number = number of protons + neutronsThe nucleus of an isotope is called a nuclideless than 10% of the known nuclides are non-radioactive, most are radionuclides

    Tro's Introductory Chemistry, Chapter 17

  • RadioactivityRadioactive nuclei spontaneously decompose into smaller nucleiRadioactive decayWe say that radioactive nuclei are unstableThe parent nuclide is the nucleus that is undergoing radioactive decay, the daughter nuclide is the new nucleus that is madeDecomposing involves the nuclide emitting a particle and/or energyAll nuclides with 84 or more protons are radioactive

    Tro's Introductory Chemistry, Chapter 17

  • TransmutationRutherford discovered that during the radioactive process, atoms of one element are changed into atoms of a different element - transmutationDaltons Atomic Theory statement 3in order for one element to change into another, the number of protons in the nucleus must change

    Tro's Introductory Chemistry, Chapter 17

  • Nuclear EquationsWe describe nuclear processes with using nuclear equationsuse the symbol of the nuclide to represent the nucleus in the nuclear equation, mass numbers and atomic numbers are conservedwe can use this fact to determine the identity of a daughter nuclide if we know the parent and mode of decay

    Tro's Introductory Chemistry, Chapter 17

  • Alpha emissionan particle contains 2 protons and 2 neutronshelium nucleusloss of an alpha particle meansatomic number decreases by 2mass number decreases by 4

    Tro's Introductory Chemistry, Chapter 17

  • Beta emissiona particle is like an electronmoving much fasterproduced from the nucleuswhen an atom loses a particle itsatomic number increases by 1mass number remains the samein beta decay, a neutron changes into a proton

    Tro's Introductory Chemistry, Chapter 17

  • Gamma emissionGamma (g) rays are high energy photons of lightNo loss of particles from the nucleusNo change in the composition of the nucleusSame atomic number and mass numberGenerally occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange

    Tro's Introductory Chemistry, Chapter 17

  • Positron emissionpositron has a charge of +1 c.u. and negligible massanti-electronwhen an atom loses a positron from the nucleus, itsmass number remains the sameatomic number decreases by 1positrons appear to result from a proton changing into a neutron

    Tro's Introductory Chemistry, Chapter 17

  • Important Atomic Symbols

    ParticleSymbolNuclear Symbolprotonp+neutronn0electrone-alphaabetab, b-positronb, b+

  • What Kind of Decay?

    Tro's Introductory Chemistry, Chapter 17

  • Practice - Write a nuclear equation for each of the followingalpha emission from Th-238beta emission from Ne-24positron emission from N-13

    Tro's Introductory Chemistry, Chapter 17

  • Detecting RadioactivityTo detect something, you need to identify something it doesRadioactive rays can expose light-protected photographic filmUse photographic filmto detect its presence film badges

    Tro's Introductory Chemistry, Chapter 17

  • Detecting RadioactivityRadioactive rays cause air to become ionizedAn electroscope detects radiation by its ability to penetrate the flask and ionize the air insideGeiger-Mller Counter works by counting electrons generated when Ar gas atoms are ionized by radioactive rays

    Tro's Introductory Chemistry, Chapter 17

  • Natural Radioactivitythere are small amounts of radioactive minerals in the air, ground and watereven in the food you eat!the radiation you are exposed to from natural sources is called background radiation

    Tro's Introductory Chemistry, Chapter 17

  • Half-Lifethe length of time it takes for half of the parent nuclides in a sample to undergo radioactive decay each radioactive isotope decays at a unique ratesome fast, some slownot all the atoms of an isotope change simultaneouslymeasured in counts per minute, or grams per time

    NuclideHalf-LifeType of DecayTh-2321.4 x 1010 yralphaU-2384.5 x 109 yralphaC-145730 yrbetaRn-22055.6 secalphaTh-2191.05 x 106 secalpha

    Tro's Introductory Chemistry, Chapter 17

  • Decay of Au-198

    Sheet1

    time (in half-lives)percentage of sample

    0100.00

    150.00

    225.00

    312.50

    46.25

    53.13

    61.56

    70.78

    80.39

    90.20

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    percentage of sample

    time (half-lives)

    percentage

    Radioactive Decay

    Chart1

    60000

    46417.5627683828

    35909.8355559459

    27780.7840986737

    21491.9381609179

    16626.7231433101

    12862.8660856187

    9951.0482318418

    7698.3900985453

    5955.6750935787

    4607.4653747381

    3564.4552205842

    2757.5553989426

    2133.3166802952

    1650.3893485407

    1276.7841863026

    987.753501823

    764.151836176

    591.1677636712

    457.3427796142

    353.8122863235

    &A

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    radioactivity (counts per min.)

    Decay of Au-198half-life = 2.7 days

    Sheet2

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    060000

    146418

    235910

    327781

    421492

    516627

    612863

    79951

    87698

    95956

    104607

    113564

    122758

    132133

    141650

    151277

    16988

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  • Example It takes 3 half-lives, or 5.7 yrs, to reach 0.225 mol How long does it take for a 1.80 mol sample of Th-228 to decay to 0.225 mol (half-life = 1.9 yrs.)

  • Decay Seriesin nature, often one radioactive nuclide changes in another radioactive nuclidedaughter nuclide is also radioactiveall of the radioactive nuclides that are produced one after the other until a stable nuclide is made is called a decay seriesto determine the stable nuclide at the end of the series without writing it all outcount the number of a and b decaysfrom the mass no. subtract 4 for each a decayfrom the atomic no. subtract 2 for each a decay and add 1 for each b

    Tro's Introductory Chemistry, Chapter 17

  • Radioisotope Datingmineral (geological)compare the amount of U-238 to Pb-206compare amount of K-40 to Ar-40archeological (once living materials)compare the amount of C-14 to C-12C-14 radioactive with half-life = 5730 yrs.while substance living, C-14/C-12 fairly constantCO2 in air ultimate source of all C in bodyatmospheric chemistry keeps producing C-14 at the same rate it decaysonce dies C-14/C-12 ratio decreaseslimit up to

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