chem 163 chapter 24 spring 2009. natural radioactivity isotopes: – elements with varying number of...
TRANSCRIPT
CHEM 163
Chapter 24
Spring 2009
Natural Radioactivity• Isotopes: – elements with varying number of neutrons– Some have unstable nuclei– If emits radiation: radioisotope
• Nucleons: protons and neutrons
• Radiation– Unstable nucleus emits energy to become more stable– Types
• Alpha (α)• Beta (β)• Gamma (γ)
– “Nuclear Chemistry”2
Discovering Radiation
• Antoine-Henri Becquerel:– Ur-containing minerals expose photographic plate– Radiation creates an electric discharge in air
• Marie Curie– Thorium emits radiation– Intensity of radiation is directly porportional to
the concentration of the element in the mineral– Emission unaffected by conditions (i.e. T, P)– Discovered Polonium & Radium
Types of Radiation• Alpha particle– 2 protons and 2 neutrons– Helium nucleus (not atom)
4
He42
• Beta particle– 1 electron
e01
• Gamma rays– Energy only (no mass)
00
Radioactive Decay
• Alpha Emitters
5
nucleus eradioactiv ),,(radiation nucleus new
U23892 He4
2 nucleus new Th23490
Element with 2 fewer protons than U?
• Beta Emitters
C146 e01- nucleus new N14
7Element with 1 more
proton than C?
Mass = 4 less
Mass = same• Gamma Emitters
Tc9943 γ Tc99
43Same elements, same number of neutrons
Beta Decay• - decay– negatron emission
• Positron emission– + decay
• Electron capture (EC)
0111
10 pn
0110
11 np
nep 10
01
11
Positron• Opposite of an electron • “antimatter”
• Electron + positron gamma rays
7
00
01
01 2 ee
e01
Stability & Nuclear Structure• Strong force:– Holds nucleus together– Only at short distances– 137X stronger than repulsive force
• Elements with even Z have larger number of stable nuclides (set of protons + neutrons)
• Many stable nuclides have even N and even Z
Nuclear Stability • Band of Stability : N = Z• All Z > 83 are unstable
Mode of DecayDecay towards band of stability (Z = N)
1. Neutron-rich nuclides: – High N/Z– - decay
2. Proton-rich nuclides:1. Low N/Z2. + emission
3. Heavy nuclides1. Z > 832. α decay
May occur in steps:Disintegration series
(decay series)
Rate of Decay: Activity• Activity: number of nuclear disintegrations per second
tΑ
N
Units:• curie (Ci)
# of disintegrations per second for 1 g of radium= 3.70 x 1010 d/s
• becquerel (Bq)1 disintegration / second
kNDecay constant
Half-Life of a RadioisotopeHalf-life:
amount of time it takes for half of a sample to decay
12
Naturally-occurring isotopes tend to have long half-lives
21t
kt
2ln21
Radioisotopic Dating• 12C / 14C ratio in an organism is same as its
environment when it is alive• 14C decays over time (t½ = 5730 years)
• 12C/14C ratio increases over time
t
o
A
A
kt ln
1
living organism
object to date
Producing Isotopes• Transmutation: changing one element into another• Notation:
reactant nucleus (particle in, particle out) product nucleus
• Bombard stable nucleus with alpha particles (or neutrons or protons) radioactive isotope
• Particle Accelerators– Use electric/magnetic fields to accelerate particles
14
ON 178
11
42
147 p
O , N 1714 p
Effects of Nuclear Radiation• Chernobyl Disaster– April 26, 1986– Steam explosion at a nuclear power plant– Radioactive particles escaped into air• Ukraine, Belarus, Russia• “clouds” to Europe and eastern US
• Medicinal Uses– Cancer treatment
Radioactive emissions can effect electrons of nearby atoms
radiation ionizingatom -eion
Measuring Energy Absorbed by TissueMeasure # of cation-electron pairs created in living tissue• Gray (Gy)
1 J absorbed per kg body tissue
• Radiation Absorbed Dose (rad)
• Radiation Equivalent in humans (rem)Also called equivalent dose
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damage biological factor (rad) dose absorbed
β and γ = 1
Protons & neutrons ~ 10
α ~ 20
• Sievert (Sv): 1 Sv = 100 rem
1 rad = 0.01 Gy
Radiation Protection: Shielding• Alpha particles:– Travel short distances (a few cm)– Can be blocked with a sheet of paper, clothing, skin– More harmful if ingested or inhaled
• Beta particles:– Travel meters– Penetrate 4-5 mm into body tissue; burn skin– Lab coats and gloves needed
• Gamma rays:– Pass through body tissue– Lead or concrete shielding
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Exposure to RadiationRadiation Sickness
< 25 rem can’t usually be detected
> 100 rem: radiation sickness• Nausea, vomiting, fatigue• Lower white-cell count
~ 300 rem: • Diarrhea, hair loss, infection• Total depletion of white cells
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LD50 = 500 rem
Radiation Protection Rules• Time:– Exposure is proportional to exposure time– A certain amount of radiation is emitted every minute. – Example:• 30 minutes near radiation means double the exposure of
15 minutes near radiation.
• Distance:– Exposure drops with distance – Example: • Double the distance from source means ¼ the exposure• Triple the distance from source means 1/9 the exposure
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Molecular InteractionsIonizing radiation creates free radicals
Free radicals: molecular or atomic species with one or more unpaired electrons
γOH2 eOH2
-OHH OHOH3
+ H2O + H2O
Double-bonds are susceptible to reaction with free radicals.
Applications of IsotopesRadioactive tracers: emit radiation to show presence
Uses:• Study material flow
– Flow of water from land to bodies of water
• Activation analysis– Neutrons bombard nonradioactive sample, creates radioisotopes– Isotopes decay characteristically – Determine elements present
• Medical Diagnosis• Determine reaction pathways
)(6O)(OHC)(O6H)(CO6 2612622 gslg
Determination of Reaction Pathways)(OH)(2I)(IO 24 laqaq )(OH2)(IO)(I --
32 aqaqs
Is the iodine in IO3- from the I- or IO4
-?
Use 131I- Which product is radioactive? 131I2
Or use 131IO4- Which product is radioactive? 131IO3
-
I- is oxidized; IO4- is reduced
Calvin Cycle:• Photosynthesis• 13 steps • 14C tracer in CO2
Medical Diagnosis• “25% of US hospital admissions are for
diagnoses based on data from radioisotopes.”• 131I:– Iodine incorporated into thyroid gland at known rate– Drink radioisotope; detect amount in thyroid
• Technetium-99:– Imaging of thyroid, heart, lungs, and liver
• Fe-59:– Measure blood flow (incorporated into hemoglobin)
Medical DiagnosisPET scans:• Brain structure and function• Biological substance made using radioisotope– Emits positrons
• Inject into body; taken up by brain• Each positron combines with e-• Detect gamma radiation
Positron-Emission Tomography
00
01
01 2 e
15O measures blood flow18F bonded to glucose measures glucose uptakeGlucose labeled with C-11 is used to detect brain damage
Other Applications of Ionizing Radiation• Used to kill cancer cells– Radiation interferes with cell division
(occurs more rapidly in cancer cells)– Gamma emitters• 90Sr and 198Au; pituitary and breast cancer• 60Co; brain cancer
• Irradiation of food – Kills microorganisms– FDA approved in 2003
• Sterilize insects
Fission & FusionFission: splitting atoms Fusion: combining nuclei
Mass and Energy:• Conservation of Mass• Conservation of Energy• Mass and energy are interconvertibleTotal quantity of mass-energy in the universe is constant
2mcE 2mcE 2c
Em
rxtprod mmm Typically very small ∆m in chemical reactions
Mass Changes in Nuclear Reactions• Lots of E needed to separate nucleus• mnucleus is less than the combined masses of its nucleons
• ∆m is nearly 10 million times greater than typical chemical processes
C12 n6p6 10
11
H atoms1.007825 amu
1.008665 amu
Mass of products = 6*1.008665 amu6*1.007825 amu+
Mass of reactant =
12.098940 amu
12.000000 amu
∆m =12.098940 – 12.000000 = 0.098940 amu / 12C= 0.098940 g / mol 12C
Nuclear Binding Energy∆m = 0.098940 g / mol 12C = 9.8940 x 10-5 kg / mol 12C
2mcE
285 /109979.2108940.9 smkg
J/mol108921.8 12
Nuclear binding energy
Nuclear binding energy:• Energy required to break 1 mol of atoms into neutrons and
H atoms (individual nucleons)• Positive value: E is absorbed
Nuclear Binding Energy Units• Joule: – Used for binding energy of a mole of nuclei
• electron volt (eV):– Used for binding energy of a single nucleus
1 eV = 1.602 x 10-19 J• mega-electron volts (MeV):
1 MeV = 106 eV = 1.602 x 10-13 J
E-to-m conversion: 1 amu = 931.5 MeV
Larger binding energy per nucleon More stable nuclide
Fission & Fusion• Fission: splitting atoms– Some mass is converted to energy
– Resulting nuclei are also unstable…splits again– “chain reaction”
• Fusion: – Two small nuclei combine– Mass is converted to LOTS of energy
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U n 23592
10 Ba Kr 142
569136 n3 1
0 energy
Back to Binding EnergyMax Ebinding / nucleon for mass number = 60
mass number > 60
mass number < 60
fission
fusion
Fission235U can split in many ways
1. Neutron bombardment 2. 236U splits (10-14 s) Energy is released(2.1 x 1013 J / mol U)
Chain reaction!
1 billion times more energy than burning ½ lb of coal!
“daughter nuclei”
Fission Chain Reaction!
Uncontrolled v. Controlled Fission
Atomic Bomb!• TNT explosion bring together enough
fissionable material to start chain reaction.
Nuclear Energy Reactors• Important source of electricity – Sweden (50%); France (80%); US (20%)
• Thermal pollution• Nuclear waste disposal
– long half-lives of fission products
Nuclear Power PlantsGenerate heat Produce steam Turn turbine Electricity!(reactor core)
Nuclear Fusion?
• Produces lots of Energy• Has no radioactive byproducts• Not totally uncommon reactants
Main Issue:• Need 108 K to make deuterium and tritium react– Use plasma (can’t really be contained)– Use lasers to heat reactants
HH 31
21 nHe 1
042
1.7 x 109 kJ/mol