Nuclear chemistry
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Nuclear Chemistry. Chapter 23. Radioactivity. Emission of subatomic particles or high-energy electromagnetic radiation by nuclei Such atoms/isotopes said to be radioactive. Its discovery. Discovered in 1896 by Becquerel Called strange, new emission uranic rays

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Nuclear chemistry

Nuclear Chemistry

Chapter 23


Radioactivity

Radioactivity

  • Emission of subatomic particles or high-energy electromagnetic radiation by nuclei

  • Such atoms/isotopes said to be radioactive


Its discovery

Its discovery

  • Discovered in 1896 by Becquerel

    • Called strange, new emission uranic rays

      • Because emitted from uranium

  • Marie Curie discovered two new elements both of which emitted uranic rays

    • Po & Ra

  • Uranic rays became radioactivity


Types of radioactivity

Types of radioactivity

  • Rutherford and Curie found that emissions produced by nuclei

  • Different types:

    • Alpha decay

    • Beta decay

    • Gamma ray emission

    • Positron emission

    • Electron capture


Isotopic symbolism

Isotopic symbolism

  • Remember from 139/141?

  • Let’s briefly go over it

  • Nuclide = isotope of an element

  • Proton = 11p

  • Neutron = 10n

  • Electron = 0-1e


Types of decay alpha decay

Types of decay: alpha decay

  • Alpha () decay

  • Alpha () particle: helium-4 bereft of 2e-

  • = 42He (don’t write He+2)

  • Parent nuclide  daughter nuclide + He-4

  • 23892U  23490Th + 42He

  • Daughter nuclide = parent nuclide atomic # minus 2

  • Sum of atomic #’s & mass #’s must be = on both sides of nuclear equation!


Alpha decay

Alpha decay

  • Has largest ionizing power

    • Ability to ionize molecules & atoms due to largeness of -particle

  • But has lowest penetrating power

    • Ability to penetrate matter

  • Skin, even air, protect against -particle radiation


Beta decay

Beta decay

  • Beta () decay

  • Beta () particle = e-

  • How does nucleus emit an e-?

  •  neutron changes into proton & emits e-

  •  10n  11p + 0-1e

  • Daughter nuclide = parent nuclide atomic number plus 1


Beta decay1

Beta decay

  • Lower ionizing power than alpha particle

  • But higher penetration power

  • Requires sheet of metal or thick piece of wood to arrest penetration

  •  more damage outside of body, but less in (alpha particle is opposite)


Gamma ray emission

Gamma ray emission

  • Gamma () ray emission

  • Electromagnetic radiation

  • High-energy photons

  • 00

  • No charge, no mass

  • Usually emitted in conjunction with other radiation types

  • Lowest ionizing power, highest penetrating power

    • Requires several inches lead shielding


Positron emission

Positron emission

  • Positron = antiparticle of e-

  •  same mass, opposite charge

  • (Collision with e- causes -ray emission)

  • Proton converted into neutron, emitting positron

  • 0+1e

  • 11p 10n + 0+1e

  • 3015P  3014Si + 0+1e

  • Atomic # of parent nuclide decreases by 1

  • Positrons have same ionizing/penetrating power as e-


Electron capture

Electron capture

  • Particle absorbed by, instead of ejected from, an unstable nucleus

  • Nucleus assimilates e- from an inner orbital of its e- cloud

  • Net result = conversion of proton into neutron

  • 11p+ 0-1e  10n

  • 9244Ru + 0-1e 9243Tc

  • Atomic # of parent nuclide decreases by 1


Problems

Problems

  • Write a nuclear equation for each of the following:

  • 1. beta decay in Bk-249

  • 2. electron capture in I-111

  • 3. positron emission in K-40

  • 4. alpha decay of Ra-224


The valley of stability

The valley of stability

  • Predicting radioactivity type

    • Tough to answer why one radioactive type as opposed to another

      • However, we can get a basic idea

  • Neutrons occupy energy levels

    • Too many lead to instability


Valley of stability

Valley of stability

  • In determining nuclear stability, ratio of neutrons to protons (N/Z) important

  • Notice lower part of valley (N/Z = 1)

  • Bi last stable (non-radioactive) isotopes

  • N/Z too high

    • Above valley: too many n, convert n to p

      • Beta-decay

  • N/Z too low

    • Below valley, too many p, convert p to n

      • Positron emission/e—capture and, to lesser extent, alpha-decay


Predict type of radioactive decay

Predict type of radioactive decay

  • 1. Mg-28

  • 2. Mg-22

  • 3. Mo-102


Magic numbers

Magic numbers

  • Actual # of n & p affects nuclear stability

    • Even #’s of both n & p give stability

      • Similar to noble gas electron configurations

        • 2, 10, 18, 36, etc.

  • Since nucleons (= n+p) occupy energy levels within nucleus

    • Magic numbers

      • N or Z = 2, 8, 20, 28, 50, 82, and N = 126


Radioactive decay series

Radioactive decay series


Detecting radioactivity

Detecting radioactivity

  • Particles detected through interactions w/atoms or molecules

  • Simplest  film-badge dosimeter

    • Photographic film in small case, pinned to clothing

    • Monitors exposure

      • Greater exposure of film  greater exposure to radioactivity


Geiger counter

Geiger counter

  • Emitted particles pass through Ar-filled chamber

    • Create trail of ionized Ar atoms

    • Induced electric signal detected on meter and then clicks

    • Each click = particle passing through gas chamber


Scintillation counter

Scintillation counter

  • Particles pass through material (NaI or CsI) that emits UV or visible light due to excitation

    • Atoms excited to higher E state

    • E releases as light, measured on meter


Radioactive decay kinetics

Radioactive decay kinetics

  • All radioactive nuclei decay via 1st-order kinetics

    •  rate of decay  to # of nuclei present

      • Rate = kN

  • Half-life = time taken for ½ of parent nuclides to decay to daughter nuclides


Decay of rn 220

Decay of Rn-220


Problem

Problem

  • Pu-236 is an -emitter w/half-life = 2.86 years. If sample initially contains 1.35 mg, what mass remains after 5.00 years?

  • How long would it take for 1.35 mg sample of Pu-236 above to decay to 0.100 mg?

    • Assume 1.35 mg/1 L air


Solution

Solution


Radiometric dating radiocarbon dating

Devised in 1949 by Libby at U of Chicago

Age of artifacts, etc., revealed by presence of C-14

C-14 formed in upper atmosphere via:

147N + 10n  146C + 11H

C-14 then decays back to N by -emission:

146C  147N + 0-1e; t1/2 = 5730 years

There is an approximately constant supply of C-14

Taken up by plants via 14CO2 & later incorporated in animals

Living organisms have same ratio of C-14:C-12

Once dead, no longer incorporating C-14  ratio decreases

5% deviation due to variance of atmospheric C-14

Bristlecone pine used to calibrate data

Carbon-dating good for 50,000 years

Radiometric dating: radiocarbon dating


Problem1

Problem

  • Artifact is found to have C-14 decay rate of 4.50 disintegration/min  g of carbon.

  • If living organisms have a decay rate of 15.3, how old is the artifact?

    • Given decay rate is  to amount of C-14 present.


Solution1

Solution


Radiometric dating uranium lead dating

Radiometric dating: uranium/lead dating

  • Relies on ratio of U-238:Pb-206 w/in igneous rocks (rocks of volcanic origin)

  • Measures time that has passed since rock solidified

    • t1/2 = 4.5 x 109 years


Example

Example

  • A meteor contains 0.556 g Pb-206 (to every 1.00 g U-238). Determine its age.


Solution2

Solution


Problem2

Problem

  • A rock from Australia was found to contain 0.438 g of Pb-206 to every 1.00g of U-238. Assuming that the rock did not contain any Pb-206 at the time of its formation, how old is the rock?


Solution3

Solution


Fission

Fission

  • Meitner, Strassmann, and Hahn discovered fission

    • Splitting of uranium-235

  • Instead of making heavier elements, created a Ba and Kr isotope plus 3 neutrons and a lot of energy

  • Sample rich in U-235 could create a chain rxn

  • To make a bomb, however, need critical mass = enough mass of U-235 to produce a self-sustaining rxn


Nuclear power

Nuclear power

  • In America, about 20% electricity generated by nuclear fission

  • Imagine:

  • Nuclear-powered car

  • Fuel = pencil-sized U-cylinder

  • Energy = 1000 20-gallon tanks of gasoline

  • Refuel every 1000 weeks (about 20 years)

  • !


Nuclear power plant

Nuclear power plant

  • Controlled fission through U fuel rods (3.5% U-235)

  • Rods absorb neutrons

  • Retractable

  • Heat boils water, making steam, turning turbine on generator to make electricity


Comparing

Comparing

  • Typical nuclear power plant makes enough energy for city of 1,000,000 people and uses about 50 kg of fuel/day

    • No air pollution/greenhouses gases

      • But, nuclear meltdown (overheating of nuclear core) is a potential threat

        • No problem!

      • Also, waste disposal

        • Location, containment problems


Comparing1

Comparing

  • Coal-burning power plant uses about 2,000,000 kg of fuel to make same amount of energy

    • But, releases huge amounts of SO2, NO2, CO2


Mass to energy

Mass to energy

  • E = mc2

  • Explains relationship between energy formation and matter loss

    • Amount of energy released in U-235 fission per atom of U-235 is 2.8 x 10-11 J

      • BUT, amount of energy released in U-235 fission per 1 mole of U-235 is 1.7 x 1013 J!

        • More than a million times more energy per mole than a chemical rxn!


Mass defect

Mass defect

  • Mass products < mass reactants

  • Difference in mass due to conversion of mass into energy

    • Called mass defect

  • Nuclear binding energy is energy corresponding to mass defect

    • Amount of energy required to break apart nucleus into nucleons (n + p)


Some more stuff

Some more stuff

  • Nuclear physicists use eV or MeV (mega eV) instead of joules

  • 1 MeV = 1.602 x 10-13 J

  • 1 amu = 931.5 MeV

    • Energy per nucleus and not per mole

  • To compare energy of 1 nucleus to another, calculate binding energy per nucleon

    • = nuclear binding energy of nuclide per #of nucleons (n + p) in nuclide

  • As binding energy per nucleon increases so does stability of species


Example1

Example

  • Calculate the mass defect and nuclear binding energy per nucleon (in MeV and in J) for:

  • 42He

    • Made from: 211H + 210n

  • 11H = 2 x 1.00783 amu

  • 210n = 2 x 1.00866 amu

    • Net mass = 4.03298 amu


Solution4

Solution


Problem3

Problem

  • Calculate the mass defect and nuclear binding energy per nucleon (in MeV) for C-16

    • Consider C-16 being made from 6 11H & 10 10n

      • C-16 mass = 16.014701 amu


Solution5

Solution


Curve of binding energy

Curve of binding energy

  • Measure of stability of nucleus (binding energy/nucleon)

    • Reaches max at Fe-56


Fusion

Fusion

  • 21H + 31H 42He + 10n

    • Ten times more energy per gram than fission


Transmutation

Transmutation

  • Transforming one element into another

  • In 1919, Rutherford bombarded N-17 to make O-17

  • The Joliot-Curie’s bombarded Al-27 to form P-30

  • In 1930’s, devices needed that could accelerate particles to high velocities:

    • Linear accelerator

    • Cyclotron


Linear accelerator

Linear accelerator

  • Charged-particle accelerated in evacuated tube

  • Alternating current causes particle to be pulled into next tube

  • Continues, allowing velocity = 90% speed of light!

  • 2 miles long 


Cyclotron

Cyclotron

  • Similar alternating voltage used

  • But applied between two semicircular halves of cyclotron

  • Particle spirals due to magnets

    • Hits target


Radiation on life

Radiation on life

  • 3 divisions

  • Acute radiation

  • Increased cancer risk

  • Genetic effects


The first

The first

  • Quickly dividing cells at greatest risk:

    • Intestinal lining

    • Immune response cells

  • Likelihood of death depends on dose & duration


The second

The second

  • Cancer = uncontrolled cell growth leading to tumors

    • Dose?

      • Unknown

  • Cancer is a murky illness


The third

The third

  • Causes genetic defects  teratogenic


How to measure radiation exposure

How to measure radiation exposure?

  • Decay events exposure

    • Curie (Ci)

      • 3.7 x 1010 decay events/second

        • Based on one gram of Ra-226 decay events/second

  • Or, amount of energy absorbed by body tissue

    • Gray (Gy)

      • 1 joule of energy absorbed/kg body tissue

    • Rad (radiation absorbed dose) = 0.01 Gy

      • 1 rad = 0.01 J/kg body tissue

  • SI unit

    • Becquerel (Bq)

      • 1 Bq = 1 decay/second

        • 1 Ci = 3.7 × 1010 Bq

  • All measure radiation but none account for amount of damage to life


Ah ha

Ah ha!

  • Biological effectiveness factor (RBE = relative biological effectiveness)

    • Dose in rads x RBE = dose in rems

  • Rems = (roentgen equivalent man)

    • Roentgen = amt of radiation producing 2.58 x 10-4 C of charge/kg air


Average american 360 mrem yr

Average American 360 mrem/yr


Good site let s take a look

Good site: let’s take a look

  • http://www.deq.idaho.gov/inl_oversight/radiation/radiation_guide.cfm


More facts

More facts

  • 20 rem

    • Decreased white blood cell count after instantaneous exposure

  • 100-400 rem

    • Vomiting, diarrhea, lesions, cancer-risk increase

  • 500-1000

    • Death w/in 2 months

  • 1000-2000

    • Death w/in 2 weeks

  • Above 2000

    • Death w/in hours


Diagnostic and therapeutic radiation

Diagnostic and therapeutic radiation

  • Radiotracer

    • Radioactive nuclide in brew to track movement of brew in body

  • Tc-99  bones

  • I-131  thyroid

  • Tl-201  heart

  • F-18  heart, brain

  • P-31  tumors


Nuclear chemistry

PET

  • Positron emission tomography

  • Shows both rate of glucose metabolism and structural features of imaged organ

  • F-18 emits positrons

    • Positron and e- produce two gamma rays

      • Rays detected

        • Imaged


Nuclear chemistry

PET


Radiotherapy

Radiotherapy

  • Using radiation to treat cancer

  • Depending on duration/dose can develop symptoms of radiation sickness

    • Vomiting, diarrhea, skin burns, hair loss


Other applications

Other applications

  • Irradiating foods

  • Nuking bugs like fruit flies and screw-worm flies


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