Chapter 20Nuclear Chemistry

HISTORY Radioactivity was discovered by Henry Bequerel

in 1896 by observing uranium salts emit energy. Madame Curie and her husband extended

Bequerel’s work on radioactivity Curie was the first to use Radioactivity to describe

the spontaneous emission of alpha, beta, and gamma particles from an unstable nucleus.

Both Curies suffered the effects of radiation poisoning.

Rutherford took over and bombarded gold with alpha particles

Identification of Radiation

Penetrating Effects

Nuclear Chemistry• Nuclear chemistry is the study of reactions that

involve changes in the nuclei of atoms.

• Radioactive decay is the spontaneous disintegration of alpha, beta, and gamma particles.

• Radioactive decay follows first-order kinetics.

NUCLEAR STABILITY

• Kinetic stability describes the probability that a nucleus will undergo decomposition to form a different nucleus (radioactive decay)

• Thermodynamic Stability - the potential energy of a particular nucleus as compared with the sum of the potential energies of its component protons and neutrons. (Binding energy)

Stability radioactive decay Light elements are stable if the neutrons

and protons are equal, i.e. 1:1 ration, heavier nuclides require a ratio of 1:1.5

Magic numbers of protons and neutrons seem to exist, much like 8 electrons to make elements and ions Nobel.

Magic Numbers• Even number protons and neutrons are stable

compared to odd ones• Magic numbers (protons or neutrons)

2,8,20,28,50,82 and 126• For example tin has 10 stable isotopes, even

number, but on either side of elemental tin indium and antimony have only two stable isotopes

• Nuclei with magic numbers of both protons and neutrons are said to be “doubly magic” and even more stable i.e. Helium-4 two protons and two neutrons and Pb-208 with 82 protons and 126 neutrons

• Could be shells for nucleons, like electrons

Number of stable nuclides related to numbers of protons and neurons

Stability radioactive decay

Nuclear Stability

Stability radioactive decay• Nucleons are protons and neutrons• The strong nuclear force keeps the nucleus

together by overcoming the repulsive force of the protons.

• Neutrons are present to help dissipate the repulsive forces between the protons

• As the atomic number (number of protons) increases, so does the number of neutrons to shield the repulsion of the protons

• All nuclides with 84 or more protons are unstable and radioactive. This means the strong force is only strong enough neutralize the force of 84 protons.

Decay Series

The Half-Lives of Nuclides in the 238

92U Decay Series

Carbon Radioactive Decay Products

NameMass (amu)

Mode(s) of Decay

Half-LifeNatural

Abundance

Carbon-10 Positron Emis. 19.45 s

Carbon-11 Positron Emis. 20.3 min

Carbon-12 12.00000 (Stable) 98.89 %

Carbon-13 13.00335 (Stable) 1.11 %

Carbon-14 Decay 5730 y

Carbon-15 Decay 2.4 s

Carbon-16 Decay 0.74 s

Types of Radioactive DecayDecay processes

Neutron-rich nuclei, converts a neutron to a proton, thus lowering the neutron/proton ration

Neutron-poor nuclei, net effect of converting a proton to a neutron thus causing an increase neutron/proton ratio

Heavy nuclei, Z>200 just unstable regardless of the neutron/proton ratio, just too many positive protons

Decay Types

Alpha particle emitters (Mass number changes)

Nuclei with atomic mass number>200 The daughter nuclei contains two fewer

protons and two fewer neutrons than the parent

U-238, Th-230

Types of Radioactive DecayDecay processes

Neutron-rich nuclei, converts a neutron to a proton, thus lowering the neutron/proton ration

Neutron-poor nuclei, net effect of converting a proton to a neutron thus causing an increase neutron/proton ratio

Heavy nuclei, Z>200 just unstable regardless of the neutron/proton ratio, just too many positive protons

Decay TypesBeta particle decay

Too many neutrons Atomic number increases, thus more

protons Neutron splits into a proton and electron

called transmutation. n → P + β Examples Th-234, I-131

10

11

o-1

Th90

234 e0-1Pa

23491 +

Decay Types

Electron CaptureNeutron-poor nuclides

Electron in an inner shell reacts with a proton 1

1P+ 0-1β → 1

0nA

ZX + 0-1β → A

Z-1X’ + x-ray

No change in mass number

Example iron-55

Decay Types

Positron Emission

Neutron-poor nuclides

Same mass as an electron, but opposite charge, the positron emission is opposite beta decay

11P → 1

0n + 0+1β

AZX → A

Z-1X’ + 0+1β

Example C-11

Decay TypesElectron Capture

Neutron-poor nuclides

Electron in an inner shell reacts with a proton 1

1P+ 0-1β → 1

0n A

ZX + 0-1β → A

Z-1X’ + x-ray

No change in mass number

Example iron-55

Decay Types

Gamma Emission 00γ

Many nuclear decay daughters are in an elevated, or excited, energy state

These meta stable isotopes emit gamma rays to lower their potential energy

This emission can be instantaneous, or delayed for sever hours

Te-99m has a half life of about 6 hours 98

43Tc* → 9843Tc + 0

0γ

Decay Types

Spontaneous Fission Very massive nuclei Z > 103 Usually large amounts of energy are

released Usually neutrons are released Example: 254

98Cf → 11846Pd + 132

52Te + 4 10n

Various Types of Radioactive Processes Showing the Changes That Take Place in the Nuclides

Decay Types

Radioactive Decay

Radiochemical Dating• n = t/t1/2

t - time, t1/2 - time for a half-life, and n - the number of half-lives

• At/Ao = 0.5n

Ao - amount initially present, At - amount at time t, and n - the number of half-lives

• If we know what fraction of sample is left (At/Ao) and its half-life (t1/2), we can calculate how much time has elapsed.

Radiocarbon Dating of Artifacts

Calibration Curves

Kinetics of Radioactive Decay• Radioactive decay is a first order process, but

using atoms instead of concentration• Radioactive decay rates

Activity is defined as the number of nuclei that decay per unit time

A = -ΔN/Δt, the units are usually disintegrations per second or minute (dps), dpm

The activity is directly proportional to the number of atoms, thus A(Rate)=kN

• From Che162 we know the first order rate law is lnN/N0 = -kt

• Also t1/2 ln1/2N0/N0 = -kt1/2 → t1/2 = 0.693/k

Example problem Fort Rock Cave in Oregon is the site where archaeologists discovered several Indian sandals, the oldest ever found in Oregon. Analysis of the 14C/12C ratio of the sandals gave an average decay rate of 5.1 dpm per gram of carbon. Carbon found in living organisms has a C-14/C-12 ratio of 1.3 X 10-12, with a decay rate of 15 dpm/g C. How long ago was the sage brush in the sandals cut? The half life of carbon-14 is 5730 years. Note dpm is disintegrations per minute

Sample Problem Solution• First calculate the rate constant k from the half-life: k=0.693/5730 = 0.000121 yr-1

• Substitute into the first order rate equation.

• ln(N/N0) = kt

t = ln(N/N0)/k = ln(15/5.1)/0.000121

t = 8910 years old sandals

Practice

A mammoth tusk containing grooves made by a sharp stone edge (indicating the presence of humans or Neanderthals) was uncovered at an ancient camp site in the Ural Mountains in 2001. The 14C/12C ratio in the tusk was only 1.19% of that in modern elephant tusks. How old is the mammoth tusk?

Practice Radioactive radon-222 decays with a loss of one

particle. The half-life is 3.82 days. What percentage of the radon in a sealed vial would remain after 7.0 days?

Nuclear Transformations• Rutherford (1919) was the first to carry out a

bombardment reaction, when he combined an alpha particle with nitrogen-14, creating oxygen-17 and a proton

• The next successful bombardment reaction was done 14 years later when Aluminum-27 to make phosphorus-30 and a neutron

• If the bombarding particle has a positive charge then repulsion by the nucleus hinders the process, thus particle accelerators are required.

• Cyclotron and linear accelerator pg850• Neutrons, do not suffer from the repulsive effect• Synthetic elements have been made, called

transuranium elements

Cyclotron Nuclear reactions can be induced by accelerating a particle and colliding it with the nuclide.

An Aerial View of Fermilab, a High Energy Particle Accelerator Cyclotron.

Cyclotron

The Accelerator Tunnel at Fermilab

Linear Accelerator

Linear Accelerator Cyclotron

Detection and Uses of Radioactivity• Geiger counter, high energy from radioactive

substances ionizes the Ar, thus allowing a current to flow. The more ions the more current, thus more radioactive

• Scintillation counter, measures the amount of light given off by a phosphor such as ZnS, which is measured by a photometer

• Badges

Geiger CounterOne can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample.The ionizing radiation creates ions, which conduct a current that is detected by the instrument

Geiger Counter

Thermodynamic Stability• This is done by comparing the mass of the individual protons

and neutrons to the mass of the nucleus itself. The difference in mass is called the mass defect (Δm), which when plugged into E = mC2, or ΔE = ΔmC2 for change in energy

• The mass of an atom is always less than the mass of the subatomic particles

Protium is the only exception, since there is no defect The other isotopes of hydrogen deuterium and tritium have

defects Mass of neutron = 1.008665 amu Mass of proton = 1.007276 amu Mass of electron = 0.0005446623 amu , note mass of

electron is not really necessary in calculations since it subtracts out when finding the difference

Subatomic Particles

Particle Mass(g) Charge

Electron(e) 9.11 x 10-28 -1

Proton(p) 1.67 x 10-24 +1

Neutron(n) 1.67 x 10-24 0

Particle 6.64 x 10-24 +2

Positron 9.11 x 10-28 +1

Thermodynamic Stability

• Just like a molecule is more stable that its atoms, an nucleus in more stable than its individual atoms.

• Energy changes for nuclear process are extremely large when compared to normal chemical and physical changes, thus very valuable energy source.

• Normal units are expressed per nucleon, in MeV (million electron volts)

• MeV = 1.60 X 10-13 J OR amu = 931 MeV• All nuclei have different relative stabilities, see

figure 18.9

Sample problem: • Calcualte the changes in mass (in amu) and energy (in J/mol

and eV/atom) that accompany the radioactive decay of 238U to 234Th and an alpha particle. The alpha particle absorbs two electrons from the surrounding matter to form a helium atom.

Solution (Note: AMU = g/mole)

Δm = mass prod. – mass react.

Δm = (mass 234Th + mass 42He) - mass238U

Δm = (234.43601 + 4.002603) - 238.050788 =

-0.004584 amu or -4.584X10-6kg

ΔE = mC2 ↔ ΔE =( -4.584X10-6kg)(2.998X108m/s)2

=-4.120X1011 j/mole

ΔE = -0.004584 amu X 931 MeV/amu

Divide by the mass number to get energy per nucleon, called binding energy

Practice What is the binding energy of 60Ni? The mass of a

60Ni atom is 59.9308 amu. The mass of an electron is 9.10939 x 10-31 kg and 1 amu is 1.66054 x 10-

27 kg.

Thermodynamic StabilityRevisiting the graph on page 988

Notice that Iron is the most stable nuclide ∆E is negative when a process goes from a

less stable to a more stable state In nuclear reactions more stable nuclei can

be achieved by combining nuclei (fusion) or splitting a nucleus (fission)

Lighter elements typically undergo fusion, while elements heavier than iron undergo fission.

Thermodynamic Stability• For lighter elements, fusion processes lead to

nuclei with greater binding energy, whereas heavy elements are formed through other processes.

Artificial Elements• Scientists have been transmuting elements

since 1919 when oxygen-17 and hydrogen-1 were produced from nitrogen-14 and particles.

147N + 4

2He 178O + 1

1H

• Artificial transmutation requires bombardment with high velocity particles.

• Alpha particles are positivly charged so how do they strike the nucleus, since the nucleus is positivly charged?

Energy in Nuclear Reactions

• In the types of chemical reactions we have encountered previously, the amount of mass converted to energy has been minimal.

• However, these energies are many thousands of times greater in nuclear reactions.

Energy in Nuclear ReactionsFor example, the mass change for the decay of 1 mol of uranium-238 is −0.0046 g.

The change in energy, E, is then

E = (m) c2

E = (−4.6 10−6 kg)(3.00 108 m/s)2

E = −4.1 1011 J

Fission Process• Discovered in the 1930’s when U-235 was

bombarded with neutrons• Neutrons, due to their neutral charge do not require

accelerators• 11n + 235

92U → 14156Ba + 92

36Kr + 3 11n

This process delivers 2.1X1013J/mole, compared to 8.0X105j of energy for the combustion of methane

About 26 million times more energy Another splitting process produces the elements Te-137

and Zr-97, with two neutrons There are 200 different isotopes of 35 different element

produced, thus the nucleus fragments in many different ways

Fission Process

• Since neutrons are produced, then it is possible to have a self-sustaining reaction

If the average production of neutrons is less than one, the reaction is called subcritical

If the neutron production is equal to one then it is called critical

If the neutron production is greater than one then the reaction is called super-critical

To achieve the a critical state, then a critical mass is required

If the mass is too small then the neutrons escape before splitting other nuclei

Nuclear Fission• How does one tap all that energy?

• Nuclear fission is the type of reaction carried out in nuclear reactors.

Nuclear Fission

Bombardment of the radioactive nuclide with a neutron starts the process.Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.

Nuclear Fission

If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.

Nuclear Fission

Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: critical mass.

Nuclear Reactors• In nuclear reactors the heat generated by

the reaction is used to produce steam that turns a turbine connected to a generator

Nuclear ReactorsThe reaction is kept in check by the use of control rods.These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.

Fusion Process

• Combining of nuclei, such as the reaction the occurs on the sun

• Problem is that the nuclei are positive in charge, thus high temperatures (4X 107K) necessary to give the nuclei the correct amount of kinetic energy to overcome the repulsion

Electric current heat Laser heat

• Because to the high temperature then what about containment?

Nuclear FusionFusion would be a superior method of generating power.The good news is that the products of the reaction are not radioactive.The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins

Hydrogen Fusion• Heavier elements formed through the process

of fusion.

11H + 1

1H 12H + 1

0e (positron)

11H + 1

2H 23He

2 23He 2

4He + 2 11H

Effects of Radiation• What happens when one is exposed to

radiation?• Somatic damage is damage to the organism

itself• Genetic damage is damage to the genetic

machinery, RNA DNA for example• Damage depends on the following factors

Quantities of RadiationUnit Parameter Description

Curie (Ci)Level of

Radioactivity3.7x1010 nuclear disintegrations/s

Becquerel (B)*Level of

Radioactivity1 disintegration/s

Gray (Gy)Ionizing Energy

Absorbed1 Gy = 1 J/kg of tissue

mass

Sievert (Sv)Amount of Tissue

Damage1Sv = 1Gy x RBE**

*SI unit of radioactivity **Relative Biological Effectiveness

Damage Factors

• The energy of the radiation, measured in rads ( radiation absorbed dose), where one rad = 10-2 J of energy deposited per kg of tissue

Since different radioactive particles do different kinds of damage the rad is not the best way to consider the effects

• Penetrating ability of the radiation Gamma highly penetrating, since electromagnetic

energy consisting of photons Beta particles penetrate up to one cm Alpha particles are stopped by the skin

Damage Factors• Ionizing ability of the radiation

Gamma radiation only occasionally ionize Alpha particles, highly ionizing and leave a

trail of damage, since it is an ion itself, it will strip electrons from other substances

• Chemical properties of the radiation source. Inert nuclides such as the noble gases pass

through the body A radioactive substance such as iodine, can

be concentrated in a specific location of the body. For iodine it is the thyroid.

rem = rads X RBE

About REM• rem is the radiation equivalent in man• rbe is the relative effectiveness of the radiation

in causing biologic damage, which is one for betta and gamma, and 20 for alpha

• Alpha particles have a higher rbe than beta and gamma, since the helium nuclei is much larger.

Acute Effects of Single Whole-Body Doses of Ionizing Radiation

Dose(REM) Toxic Effect

0.05-0.25No acute effect, possible carcinogenic

or mutagenic damage to DNA

0.25-1.0Temporary reduction in white blood cell

count

1.0-2.0Radiation sickness: fatigue, vomiting, diarrhea, impaired immune system

2.0-4.0Severe radiation sickness: intestinal bleeding, bone marrow destruction

4.0-10.0Death, usually through infection, within

weeks

>10.0 Death within hours

Typical Radiation Exposures for a Person Living in the United States (1 millirem = 10-3 rem)

Sources of Radiation

Biological Effect of Radiation

Radon Gas Release from Rocks

Radiation TherapyNuclide Radiation Half-Life Treatment

32P 14.3 d Leukemia Therapy

60Co 5.3 yr External Cancer Therapy

123I 13.3 yr Thyroid Therapy

131Cs 9.7 days Prostate cancer therapy

192Ir 74 d Coronary disease

Medical Imaging Radionuclides

NuclideRadiation Emitted

Half-Life (hr)

Use

99mTc 6.0Bones, Circulatory system, Various

Organs

67Ga 78Tumors in the Brain and Other

Organs

201Tl 73 Coronary Arteries, Heart Muscle

123I 13.3Thyroxin Production in Thyroid

Gland

Synthesis of the elements in stars• Stars are formed from the gravitational attraction of interstellar

dust, mostly hydrogen• The density gradually increases reaching a density of about

100g/mL, with a temperature of about 1.5X107 K• At this point hydrogen begins to fuse into He-4, releasing

energy, like our sun• The overall reaction is 4 protium atoms combining to make

helium and 2 beta particles plus 2 photons of gamma radiation

• The helium then concentrates in the core of the star, thus increasing the density and temperature, thus becoming a red giant star

Synthesis of the elements in stars• At a temperature of about 2X108 K, the helium nuclei

begin to fuse producing Be-8• Be-8 is unstable due to low neutron numbers, and

absorbs alpha particles creating C, O, Ne, Mg• The next stage is formation of a red supergiant

star, where Na, Si, S, Ar, and Ca are produced• Next in the progressions is the formation of a

massive red supergiant star, where Fe and Ni are formed by proton-neutron exchange reactions

Finally the supernova is produced where elements with Z>28 being formed by multiple neutron captures

Red Giant

Supernova

ChemTour: Half-Life

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Students develop and test their understanding of the concepts of half-life and carbon dating by manipulating interactive graphs and working Practice Exercises.

ChemTour: Fusion of Hydrogen

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This ChemTour demonstrates the process by which hydrogen nuclei fuse to form helium nuclei. This nuclear reaction fuels the sun and stars and is the first step in the synthesis of heavier elements.

ChemTour: Modes of Radioactive Decay

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This ChemTour presents animated explanations of alpha decay, beta decay, positron emission, and electron capture.

ChemTour: Balancing Nuclear Reactions

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This quantitative exercise teaches nuclear equation balancing through worked examples and Practice Exercises.

ChemTour: Half-Life

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Students develop and test their understanding of the concepts of half-life and carbon dating by manipulating interactive graphs and working Practice Exercises.

ChemTour: Fusion of Hydrogen

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This ChemTour demonstrates the process by which hydrogen nuclei fuse to form helium nuclei. This nuclear reaction fuels the sun and stars and is the first step in the synthesis of heavier elements.

ChemTour: Modes of Radioactive Decay

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This ChemTour presents animated explanations of alpha decay, beta decay, positron emission, and electron capture.

ChemTour: Balancing Nuclear Reactions

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This quantitative exercise teaches nuclear equation balancing through worked examples and Practice Exercises.

End Chapter #20Nuclear Chemistry