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

16

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

17

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

18

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

19

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

30

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