Chapter

Chapter

Chemistry: A Molecular Approach, 1st Ed. Nivaldo Tro Chapter 19 Radioactivity and Nuclear Chemistry Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA 2008, Prentice Hall The Discovery of Radioactivity Antoine-Henri Becquerel designed an experiment to determine if phosphorescent minerals also gave off X-rays Tro, Chemistry: A Molecular Approach 2

The Discovery of Radioactivity Becquerel discovered that certain minerals were constantly producing penetrating energy rays he called uranic rays like X-rays but not related to fluorescence Becquerel determined that all the minerals that produced these rays contained uranium the rays were produced even though the mineral was not exposed to outside energy Energy apparently being produced from nothing?? Tro, Chemistry: A Molecular Approach 3

The Curies Marie Curie used electroscope to detect uranic rays in samples Discovered new elements by detecting their rays radium named for its green phosphorescence polonium named for her homeland Since these rays were no longer just a property of uranium, she renamed it radioactivity Tro, Chemistry: A Molecular Approach 4 Electroscope

+ ++ ++ + When charged, the metal foils spread apart due to like charge repulsion Tro, Chemistry: A Molecular Approach When exposed to ionizing radiation, the radiation knocks electrons off the air molecules, which jump onto the foils and discharge them,

causing them to drop down. 5 Other Properties of Radioactivity radioactive rays can ionize matter cause uncharged matter to become charged basis of Geiger Counter and electroscope radioactive rays have high energy radioactive rays can penetrate matter radioactive rays cause phosphorescent chemicals to glow basis of scintillation counter Tro, Chemistry: A Molecular Approach 6 Types of Radioactive Rays Rutherford discovered there were three

types of radioactivity alpha rays () have a charge of +2 c.u. and a mass of 4 amu what we now know to be helium nucleus beta rays () have a charge of -1 c.u. and negligible mass electron-like gamma rays ( form of light energy (not particle like and) Tro, Chemistry: A Molecular Approach 7 Rutherfords Experiment ++++++++++++

-------------- Tro, Chemistry: A Molecular Approach 8 Penetrating Ability of Radioactive Rays 0.01 mm

1 mm 100 mm Pieces of Lead Tro, Chemistry: A Molecular Approach 9 Facts About the Nucleus Every 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 different atomic masses

Isotopes are identified by their mass number (A) mass number = number of protons + neutrons Tro, Chemistry: A Molecular Approach 10 Facts About the Nucleus The number of neutrons is calculated by subtracting the atomic number from the mass number The nucleus of an isotope is called a nuclide less than 10% of the known nuclides are nonradioactive, most are radionuclides Each nuclide is identified by a symbol Element -Mass Number = X-A mass number Element atomic number Tro, Chemistry: A Molecular Approach

A ZX 11 Radioactivity Radioactive nuclei spontaneously decompose into smaller nuclei Radioactive decay We say that radioactive nuclei are unstable The parent nuclide is the nucleus that is undergoing radioactive decay, the daughter nuclide is the new nucleus that is made Decomposing involves the nuclide emitting a particle and/or energy All nuclides with 84 or more protons are radioactive

Tro, Chemistry: A Molecular Approach 12 Important Atomic Symbols Particle Symbol proton p+ neutron n0 electron

e- alpha beta positron Nuclear Symbol 1 1 1

1 H p 1 0 0 1 n 4 2 e 4 2 He 0

1 0 1 0 1 0 1 e e 13

Transmutation Rutherford discovered that during the radioactive process, atoms of one element are changed into atoms of a different element - transmutation Daltons Atomic Theory statement 3 bites the dust in order for one element to change into another, the number of protons in the nucleus must change Tro, Chemistry: A Molecular Approach 14 Nuclear Equations we describe nuclear processes with nuclear equations use the symbol of the nuclide to represent the nucleus atomic numbers and mass numbers are conserved use this fact to predict the daughter nuclide if you know parent and emitted particle

Tro, Chemistry: A Molecular Approach 15 Alpha Emission an particle contains 2 protons 4 2 and 2 neutrons 4 2 He helium nucleus

most ionizing, but least penetrating loss of an alpha particle means atomic number decreases by 2 mass number decreases by 4 222 Ra 88 Tro, Chemistry: A Molecular Approach 4 He 2

218 Rn 86 16 Tro, Chemistry: A Molecular Approach 17 Beta Emission a particle is like an electron moving much faster produced from the nucleus 0 1 0

1 e when an atom loses a particle its atomic number increases by 1 mass number remains the same in beta decay, a neutron changes into a proton 234 90 Th Tro, Chemistry: A Molecular Approach 0 1 e

234 91 Pa 18 Tro, Chemistry: A Molecular Approach 19 Gamma Emission 0 0 gamma () rays are high energy photons of light no loss of particles from the nucleus

no change in the composition of the nucleus Same atomic number and mass number least ionizing, but most penetrating generally occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange Tro, Chemistry: A Molecular Approach 20 Positron Emission positron has a charge of +1 c.u. and negligible mass 0 0

anti-electron 1 1 when an atom loses a positron from the nucleus, its e mass number remains the same atomic number decreases by 1 positrons appear to result from a proton changing into a neutron 22 11 Na

Tro, Chemistry: A Molecular Approach 0 1 e 22 10 Ne 21 Tro, Chemistry: A Molecular Approach 22 0 1 occurs when an inner orbital electron is pulled

into the nucleus no particle emission, but atom changes Electron Capture e same result as positron emission proton combines with the electron to make a neutron mass number stays the same atomic number decreases by one 92 44

Ru 92 44 Tro, Chemistry: A Molecular Approach 0 1 92 43 e Ru Tc 92 43

Tc 23 Particle Changes Beta Emission neutron changing into a proton 1 1 0 0 n 1p 1 Positron Emission proton changing into a neutron 1 1p 1 0

0 n 1 Electron Capture proton changing into a neutron 1 1p Tro, Chemistry: A Molecular Approach 0 -1e 1 0n 24 25

Nuclear Equations in the nuclear equation, mass numbers and atomic numbers are conserved we can use this fact to determine the identity of a daughter nuclide if we know the parent and mode of decay Tro, Chemistry: A Molecular Approach 26 Ex 19.2b - Write the Nuclear Equation for Positron Emission From K-40 1) Write the nuclide symbols for both the starting radionuclide and the particle 40 19

K 40 K 0 1 positron e Tro, Chemistry: A Molecular Approach 27 Ex. 19.2b - Write the Nuclear Equation for Positron Emission From K-40 2) Set up the equation emitted particles are products captured particles are reactants

40 19 K Tro, Chemistry: A Molecular Approach 0 1 A Z e X 28 Ex. 19.2b - Write the Nuclear Equation for Positron Emission From K-40 3) Determine the mass number and atomic

number of the missing nuclide mass and atomic numbers are conserved 40 K 19 Tro, Chemistry: A Molecular Approach 0 e 1 40

X 18 29 Ex. 19.2b - Write the Nuclear Equation for Positron Emission From K-40 4) Determine the element from the atomic number 40 19 K Tro, Chemistry: A Molecular Approach 0 1

e 40 18 Ar 30 Practice - Write a nuclear equation for each of the following alpha emission from U-238 beta emission from Ne-24 positron emission from N-13 electron capture by Be-7 Tro, Chemistry: A Molecular Approach 31 Practice - Write a nuclear equation for

each of the following alpha emission from U-238 238 4 234 92 U 2 He 90Th beta emission from Ne-24 24 0 24 10 Ne -1 e 11 Na positron emission from N-13 13 0 13 7 N 1 e 6 C electron capture by Be-7 7 0

7 4 Be 1e 3 Li Tro, Chemistry: A Molecular Approach 32 What Causes Nuclei to Break Down? the particles in the nucleus are held together by a very strong attractive force only found in the nucleus called the strong force acts only over very short distances the neutrons play an important role in stabilizing the nucleus, as they add to the strong force, but dont repel each other like the protons do Tro, Chemistry: A Molecular Approach 33 N/Z Ratio

the ratio of neutrons : protons is an important measure of the stability of the nucleus if the N/Z ratio is too high neutrons are converted to protons via decay if the N/Z ratio is too low protons are converted to neutrons via positron emission or electron capture or via decay though not as efficient Tro, Chemistry: A Molecular Approach 34 Valley of Stability for Z = 1 20, stable N/Z 1 for Z = 20 40, stable N/Z approaches 1.25 for Z = 40 80, stable N/Z approaches 1.5

for Z > 83, there are no stable nuclei Tro, Chemistry: A Molecular Approach 35 Ex 19.3b Determine the kind of radioactive decay that Mg-22 undergoes Mg-22 Z = 12 N = 22 12 = 10 N/Z = 10/12 = 0.83 from Z = 1 20, stable nuclei have N/Z 1 since Mg-22 N/Z is low, it

should convert p+ into n0, therefore it will undergo positron emission or electron capture Tro, Chemistry: A Molecular Approach 36 Magic Numbers besides the N/Z ratio, the actual numbers of protons and neutrons effects stability most stable nuclei have even numbers of protons and neutrons only a few have odd numbers of protons and neutrons if the total number of nucleons adds to a magic number, the nucleus is more stable

same idea as the electrons in the noble gas resulting in a more stable electron configuration most stable when N or Z = 2, 8, 20, 28, 50, 82; or N = 126 Tro, Chemistry: A Molecular Approach 37 Decay Series in nature, often one radioactive nuclide changes in another radioactive nuclide daughter nuclide is also radioactive all of the radioactive nuclides that are produced one after the other until a stable nuclide is made

is called a decay series to determine the stable nuclide at the end of the series without writing it all out 1. 2. 3. count the number of and decays from the mass no. subtract 4 for each decay from the atomic no. subtract 2 for each decay and add 1 for each Tro, Chemistry: A Molecular Approach 38 U-238 Decay Series

or

Tro, Chemistry: A Molecular Approach or other combinations 39 Detecting Radioactivity To detect something, you need to identify what it does Radioactive rays can expose light-protected photographic film Use photographic film to detect its presence film badges Tro's Introductory Chemistry, Chapter 17 40

Detecting Radioactivity Radioactive rays cause air to become ionized An electroscope detects radiation by its ability to penetrate the flask and ionize the air inside A Geiger-Mller Counter works by counting electrons generated when Ar gas atoms are ionized by radioactive rays Tro's Introductory Chemistry, Chapter 17 41 Detecting Radioactivity Radioactive rays cause certain chemicals to give off a flash of light when they strike the chemical A scintillation counter is able to count the number of flashes per minute 42

Natural Radioactivity there are small amounts of radioactive minerals in the air, ground, and water even in the food you eat! the radiation you are exposed to from natural sources is called background radiation Tro, Chemistry: A Molecular Approach 43 Rate of Radioactivity it was discovered that the rate of change in the amount of radioactivity was constant and different for each radioactive isotope change in radioactivity measured with Geiger counter counts per minute

each radionuclide had a particular length of time it required to lose half its radioactivity a constant half-life we know that processes with a constant half-life follow first order kinetic rate laws rate of change not affected by temperature means that radioactivity is not a chemical reaction! Tro, Chemistry: A Molecular Approach 44 Kinetics of Radioactive Decay Rate = kN N = number of radioactive nuclei t1/2 = 0.693/k the shorter the half-life, the more nuclei decay

every second we say the sample is hotter Nt rate t ln kt ln N0 rate 0 Tro, Chemistry: A Molecular Approach 45 Half-Lives of Various Nuclides Nuclide Half-Life Type of Decay Th-232

1.4 x 1010 yr alpha U-238 4.5 x 109 yr alpha C-14 5730 yr beta Rn-220 55.6 sec

alpha Th-219 1.05 x 106 sec alpha Tro, Chemistry: A Molecular Approach 46 Pattern for Radioactive Decay Tro, Chemistry: A Molecular Approach 47 Half-Life

half of the radioactive atoms decay each halfRadioactive Decay life percentage of original sample 100 90 80 70 60 50 40 30 20 10 0 0 1

2 3 4 5 6 time (half-lives) 7 8 9 10

48 Pattern for Radioactive Decay Tro, Chemistry: A Molecular Approach 49 Radon in the U.S. Tro, Chemistry: A Molecular Approach 50 Ex.19.4 If you have a 1.35 mg sample of Pu-236, calculate the mass that will remain after 5.00 years Given: mass Pu-236 = 1.35 mg, t = 5.00 yr, t1/2 = 2.86 yr Find: mass, mg Concept Plan:

Relationships: t1/2 k t 1 Solve: 2 0.693 k + m0, t mt

ln Nt kt N0 N t 0.693 lnt kt N0 k 1 2 kt 01.35 .693mg e 0.2423 yr -51.00 yr Nk t 0N.693 e

0 0.2423 yr t N 0.402 mg2.86 yr -1 t 1 2 Check: units are correct, the magnitude makes sense since it is less than the original mass for longer than 1 half-life Tro, Chemistry: A Molecular Approach

51 Object Dating mineral (geological) compare the amount of U-238 to Pb-206 compare amount of K-40 to Ar-40 archaeological (once living materials) compare the amount of C-14 to C-12 C-14 radioactive with half-life = 5730 yrs. while substance living, C-14/C-12 fairly constant CO2 in air ultimate source of all C in organism atmospheric chemistry keeps producing C-14 at the nearly the same rate it decays once dies C-14/C-12 ratio decreases limit up to 50,000 years

Tro, Chemistry: A Molecular Approach 52 Radiocarbon Dating C-14 Half-Life = 5730 yrs % C-14 (relative to living organism) Number of Half-Lives Time (yrs) 100.0 0

0 50.0 1 5,730 25.00 2 11,460 12.50 3 17,190

6.250 4 22,920 3.125 5 28,650 1.563 6 34,380 Tro, Chemistry: A Molecular Approach

53 Radiocarbon Dating % C-14 (compared to living organism) Objects Age (in years) 100% 0 90% 870 80% 1850

60% 4220 50% 5730 40% 7580 25% 11,500 10% 19,000

5% 24,800 1% 38,100 54 Ex.19.4 An ancient skull gives 4.50 dis/mingC. If a living organism gives 15.3 dis/mingC, how old is the skull? Given: ratet = 4.50 dis/mingC, ratet = 15.3 dis/mingC Find: time, yr Concept Plan: Relationships: t1/2

k t 1 Solve: 2 + rate0, ratet 0.693 k ln t rate t kt rate0

rate t 0.693 ln t 12 rate 0 kt k dis 4.50 rate min gC 0.693t 0.693 ln 4 -1 ln dis k rate

1 . 2 0 9 10 yr 15.3 min gC 4 0 t 5 t 730 yr 1 . 0

10 yr -4 -1 1 k2 1.209 10 yr Check: units are correct, the magnitude makes sense since it is less than 2 half-lives Tro, Chemistry: A Molecular Approach

55 Nonradioactive Nuclear Changes a few nuclei are so unstable that if their nucleus is hit just right by a neutron, the large nucleus splits into two smaller nuclei - this is called fission small nuclei can be accelerated to such a degree that they overcome their charge repulsion and smash together to make a larger nucleus - this is called fusion both fission and fusion release enormous amounts of energy fusion releases more energy per gram than

fission Tro, Chemistry: A Molecular Approach Lise Meitner 56 Tro, Chemistry: A Molecular Approach 57 Fission Chain Reaction a chain reaction occurs when a reactant in the process is also a product of the process in the fission process it is the neutrons so you only need a small amount of neutrons to start the chain

many of the neutrons produced in fission are either ejected from the uranium before they hit another U-235 or are absorbed by the surrounding U-238 minimum amount of fissionable isotope needed to sustain the chain reaction is called the critical mass Tro, Chemistry: A Molecular Approach 58 Tro, Chemistry: A Molecular Approach 59 Tro, Chemistry: A Molecular Approach 60

Fissionable Material fissionable isotopes include U-235, Pu-239, and Pu-240 natural uranium is less than 1% U-235 rest mostly U-238 not enough U-235 to sustain chain reaction to produce fissionable uranium, the natural uranium must be enriched in U-235 to about 7% for weapons grade to about 3% for reactor grade Tro, Chemistry: A Molecular Approach 61 Nuclear Power Nuclear reactors use fission to generate electricity About 20% of U.S. electricity The fission of U-235 produces heat

The heat boils water, turning it to steam The steam turns a turbine, generating electricity Tro, Chemistry: A Molecular Approach 62 Nuclear Power Plants vs. Coal-Burning Power Plants Use about 50 kg of fuel to generate enough electricity for 1 million people No air pollution Use about 2 million kg

Tro, Chemistry: A Molecular Approach of fuel to generate enough electricity for 1 million people Produces NO2 and SOx that add to acid rain Produces CO2 that adds to the greenhouse effect 63 Nuclear Power Plants - Core the fissionable material is stored in long tubes, called fuel rods, arranged in a matrix subcritical

between the fuel rods are control rods made of neutron absorbing material B and/or Cd neutrons needed to sustain the chain reaction the rods are placed in a material to slow down the ejected neutrons, called a moderator allows chain reaction to occur below critical mass Tro, Chemistry: A Molecular Approach 64 Pressurized Light Water Reactor design used in U.S. (GE, Westinghouse) water is both the coolant and moderator water in core kept under pressure to keep it from boiling fuel is enriched uranium subcritical

containment dome of concrete Tro, Chemistry: A Molecular Approach 65 Tro, Chemistry: A Molecular Approach 66 Containment Building PLWR Turbine Condenser Boiler Core

Cold Water 67 PLWR - Core Control Rods Hot Water Fuel Rods Cold Water 68

Concerns About Nuclear Power core melt-down water loss from core, heat melts core China Syndrome Chernobyl waste disposal waste highly radioactive reprocessing, underground storage? Federal High Level Radioactive Waste Storage Facility at Yucca Mountain, Nevada transporting waste how do we deal with nuclear power plants that are no longer safe to operate? Yankee Rowe

Tro, Chemistry: A Molecular Approach 69 Where Does the Energy from Fission Come From? during nuclear fission, some of the mass of the nucleus is converted into energy E = mc2 each mole of U-235 that fissions produces about 1.7 x 1013 J of energy a very exothermic chemical reaction produces 106 J per mole Tro, Chemistry: A Molecular Approach 70 Mass Defect and Binding Energy

when a nucleus forms, some of the mass of the separate nucleons is converted into energy the difference in mass between the separate nucleons and the combined nucleus is called the mass defect the energy that is released when the nucleus forms is called the binding energy 1 MeV = 1.602 x 10-13 J 1 amu of mass defect = 931.5 MeV the greater the binding energy per nucleon, the more stable the nucleus is Tro, Chemistry: A Molecular Approach 71 72

Nuclear Fusion Fusion is the combining of light nuclei to make a heavier one The sun uses the fusion of hydrogen isotopes to make helium as a power source Requires high input of energy to initiate the process Because need to overcome repulsion of positive nuclei Produces 10x the energy per gram as fission No radioactive byproducts Unfortunately, the only currently working application is the H-bomb Tro, Chemistry: A Molecular Approach

73 Fusion Tro, Chemistry: A Molecular Approach 74 Tokamak Fusion Reactor Tro, Chemistry: A Molecular Approach 75 Artificial Transmutation bombardment of one nucleus with another causing new atoms to be made can also bombard with neutrons

reaction done in a particle accelerator linear cyclotron Tc-97 is made by bombarding Mo-96 with deuterium, releasing a neutron 96 42 2 1 Mo H Tro, Chemistry: A Molecular Approach 97 43

Joliot-Curies 1 0 Tc n 76 Linear Accelerator Tro, Chemistry: A Molecular Approach 77 Cyclotron Tro, Chemistry: A Molecular Approach 78

Biological Effects of Radiation Radiation is high energy, energy enough to knock electrons from molecules and break bonds Ionizing radiation Energy transferred to cells can damage biological molecules and cause malfunction of the cell Tro, Chemistry: A Molecular Approach 79 Acute Effects of Radiation High levels of radiation over a short period of time kill large numbers of cells From a nuclear blast or exposed reactor core Causes weakened immune system and lower ability to absorb nutrients from food

May result in death, usually from infection Tro, Chemistry: A Molecular Approach 80 Chronic Effects Low doses of radiation over a period of time show an increased risk for the development of cancer Radiation damages DNA that may not get repaired properly Low doses over time may damage reproductive organs, which may lead to sterilization Damage to reproductive cells may lead to a genetic defect in offspring Tro, Chemistry: A Molecular Approach 81

Measuring Radiation Exposure the curie (Ci) is an exposure of 3.7 x 1010 events per second no matter the kind of radiation the gray (Gy) measures the amount of energy absorbed by body tissue from radiation 1 Gy = 1 J/kg body tissue the rad also measures the amount of energy absorbed by body tissue from radiation 1 rad = 0.01 Gy a correction factor is used to account for a number of factors that affect the result of the exposure this biological effectiveness factor is the RBE, and the result is the dose in rems rads x RBE = rems rem = roentgen equivalent man

Tro, Chemistry: A Molecular Approach 82 Factors that Determine Biological Effects of Radiation 1. The more energy the radiation has, the larger its effect can be 2. The better the ionizing radiation penetrates human tissue, the deeper effect it can have Gamma >> Beta > Alpha 3. The more ionizing the radiation, the larger the effect of the radiation Alpha > Beta > Gamma 4. The radioactive half-life of the radionuclide 5. The biological half-life of the element 6. The physical state of the radioactive material

Tro, Chemistry: A Molecular Approach 83 Tro, Chemistry: A Molecular Approach 84 Biological Effects of Radiation The amount of danger to humans of radiation is measured in the unit rems Dose (rems) Probable Outcome 20-100 decreased white blood cell count; possible increased cancer risk

100-400 radiation sickness; increased cancer risk 500+ death Tro, Chemistry: A Molecular Approach 85 Medical Uses of Radioisotopes, Diagnosis radiotracers certain organs absorb most or all of a particular element can measure the amount absorbed by using tagged isotopes of the element and a Geiger counter

use radioisotope with short half-life use radioisotope low ionizing beta or gamma Tro, Chemistry: A Molecular Approach 86 Nuclide Iodine-131 Iron-59 Molybdenum-99 Phosphorus-32 Strontium-87 Technetium-99 Tro, Chemistry: A Molecular Approach Half-life 8.1 days

45.1 days 67 hours 14.3 days 2.8 hours 6 hours Organ/System thyroid red blood cells metabolism eyes, liver bones heart, bones, liver, lungs 87 Bone Scans Tro, Chemistry: A Molecular Approach

88 Medical Uses of Radioisotopes, Diagnosis PET scan positron emission tomography F-18 in glucose brain scan and function Tro, Chemistry: A Molecular Approach 89 Medical Uses of Radioisotopes, Treatment - Radiotherapy cancer treatment cancer cells more sensitive to radiation than healthy cells

brachytherapy place radioisotope directly at site of cancer teletherapy use gamma radiation from Co-60 outside to penetrate inside IMRT radiopharmaceutical therapy use radioisotopes that concentrate in one area of the body Tro, Chemistry: A Molecular Approach 90 Gamma Ray Treatment Tro, Chemistry: A Molecular Approach 91

Intensity-Modulated Radiation Therapy use precisely controlled xray from a linear accelerator to irradiate a malignant tumor designed to conform to the 3-D shape of the tumor Tro, Chemistry: A Molecular Approach 92 Nonmedical Uses of Radioactive Isotopes smoke detectors Am-241 smoke blocks ionized air, breaks circuit insect control sterilize males

food preservation radioactive tracers follow progress of a tagged atom in a reaction chemical analysis neutron activation analysis Tro, Chemistry: A Molecular Approach 93 Nonmedical Uses of Radioactive Isotopes authenticating art object many older pigments and ceramics were made from minerals with small amounts of radioisotopes crime scene investigation measure thickness or condition of industrial materials

corrosion track flow through process gauges in high temp processes weld defects in pipelines road thickness Tro, Chemistry: A Molecular Approach 94 Nonmedical Uses of Radioactive Isotopes agribusiness develop disease-resistant crops trace fertilizer use

treat computer disks to enhance data integrity nonstick pan coatings photocopiers to help keep paper from jamming sterilize cosmetics, hair products, and contact lens solutions and other personal hygiene products Tro, Chemistry: A Molecular Approach 95

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