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Chapter 15Nuclear Chemistry

• Radioactivity

• Nuclear Reactions

• Rates of Radioactive Decay

• Medical Applications of Isotopes

• Biological Effects of Radiation

• Nuclear Energy

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Radiation• Radiation

– Energy that comes from a source and travels through matter or space

– Two types of radiation:• Electromagnetic

– Includes light, gamma rays, and X-rays• Particulate

– Mass given off from unstable atoms with the energy of motion

– Ionizing radiation• Radiation of either type that can produce charged

particles in matter

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Radioactivity• Radioactive decay

– The spontaneous emission of electromagnetic or other types of radiation

• Radioactive atoms– Unstable atoms that give off excess

matter, energy, or both as ionizing radiation

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Nucleons• Nucleons

– General term used to describe nuclear particles, protons, and neutrons

– Remember:• Z signifies the atomic number, the number

of protons in the nucleus of an atom• N signifies the neutron number, the

number of neutrons in the nucleus of an atom

• Sum of N and Z is A (N+Z = A), the mass number

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Nuclides• Remember, isotopes are:

– Atoms with the same atomic number Z, but different neutron numbers N and mass numbers A

• Nuclides– Isotopes that exist for a measurable length of

time and have a defined energy state– An atom of a particular atomic number, mass

number and neutron number

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Band of Stability

• Of the more than 3,000 nuclides known, about 250 are stable

• The rest decompose over a period of time, emitting radiation in the process of creating new nuclides

• The stable nuclides have approximately equal numbers of protons and neutrons (N/Z ratio = 1) in the lighter elements (Z = 1 to 20) and more neutrons than protons in the heavier elements (N/Z ratio > 1).

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Figure 15.4

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Radiation• In a nuclear reaction, an emission of

radiation usually accompanies changes in the composition of the nucleus.

• Natural radiation associated with radioactive decay can be placed into three classes:– Alpha particles– Beta particles– Gamma rays

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Radiation• The three classes of natural radiation

behave differently in an electric field, as shown below:

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Figure 15.5

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Table 15.1 Properties of Types of Radiation

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Radiation Type

Notation Mass ChargePenetration

into Al

Alpha 4 2+ 0.01 mm

Beta (electron)

~0 1- 0.5-1.0 mm

Beta (positron)

~0 1+(Reacts with

electrons)

Gamma γ 0 0 50-110 mm

01

-01

242

42 He,

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Types of Radiation• Alpha particles

– Nuclei of helium-4 atoms– Contain 2 protons and 2

neutrons– Least harmful to animal

and human tissue• Gamma rays

– High energy electromagnetic radiation: energy without charge or mass

– Highest energy and most penetrating type of radiation

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Types of Radiation• Beta particles

– Small, charged particle that can be emitted from unstable atoms at speeds approaching the speed of light

– Penetrate through skin into tissue

– 2 types of beta particles:• Positron

– Same mass as an electron with an opposite charge

• Electron

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Nuclear Reactions• Two conditions must be met to

balance a nuclear equation:1. Conservation of mass number

2. Conservation of nuclear charge (atomic number)

• Examples:

15- n3 Pu U

Pa Th

Ra Th

10

23994

42

23892

-01-

23191

23190

42

22888

23290

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Alpha Particle Emission

42

22888

23290 Ra Th

• When a nucleus emits an alpha particle, it loses 2 protons and 2 neutrons, so its atomic number decreases by 2 and its mass number decreases by 4.

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Figure 15.7

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Beta Particle (Electron) Emission

01-23191

23190 Pa Th

• When a nucleus emits a beta particle (electron), its atomic number increases by 1 and its mass number remains unchanged.

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Figure 15.8

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Beta Particle (Positron) Emission

012311

2312 Na Mg

• When a nucleus emits a beta particle (positron), its atomic number decreases by 1 and its mass number remains unchanged.

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Figure 15.9

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Electron Capture

Li e Be 73

-01-

74

• A proton and an electron combine to form a neutron. The mass number stays the same, but the atomic number decreases by 1.

• Very few nuclides undergo this transformation.

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Gamma Ray Emission• In all nuclear reactions, the nucleus changes from a

state of higher energy to a state of lower energy.

• Gamma rays are pure electromagnetic energy.

• Results in no change in mass or atomic number. 99mTc 99Tc + γ

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Figure 15.10

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Practice – Nuclear Reactions• Fill in the appropriate nuclide for the

X in the following nuclear reactions:

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01

115

01-

23490

42

21482

B .3

Th .2

Pb 1.

X

X

X

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Practice Solutions – Nuclear Reactions

• Fill in the appropriate nuclide for the X in the following nuclear reactions:

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01

115

116

01-

23491

23490

42

21482

21884

B C .3

Pa Th .2

Pb Po 1.

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Nuclear Bombardment Reactions• Nuclei are hit with a beam of nuclei or nuclear

particles to trigger a nuclear reaction• Occurs when a nuclear reaction is not

spontaneous and is produced intentionally by artificial means

• Used to synthesize transuranium elements, those following uranium on the periodic table

• Some examples:

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n2 At Bi

n2 Tc H Mo

Np U n U

10

21185

42

20983

10

9743

21

9742

01-

23993

23992

10

23892

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Practice – Nuclear Bombardment Reactions

• Bombarding a bismuth-209 target with a beam of another nuclide produces bohrium-262 and a neutron. Identify the nuclide used in the bombardment, and write a balanced equation to describe this nuclear reaction.

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Practice – Nuclear Bombardment Reactions

• Bombarding a bismuth-209 target with a beam of another nuclide produces bohrium-262 and a neutron. Identify the nuclide used in the bombardment, and write a balanced equation to describe this nuclear reaction.

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n Bh Bi 10

262107

20983 X

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Practice Solutions – Nuclear Bombardment Reactions

For the mass number:209 + A = 262 + 1

A = 54For the atomic number:

83 + Z = 107 + 0Z = 24

n Bh Cr Bi 10

262107

5424

20983

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n Bh Bi 10

262107

20983 X

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Particle Accelerators• Particle accelerators are used for nuclear

bombardment reactions.• The synchroton, perhaps the most

successful accelerator, uses a circular path for the accelerating particles.

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Figure 15.13

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Spontaneous Nuclear Decay Reactions

• The tendency for the neutron/proton (N/Z) ratio to move toward the band of stability, explains the nuclear reactions of naturally radioactive nuclides.

• For every process except γ emission, the change that occurs for an unstable nuclide takes it closer to the observed band of stability.

• Radioactive nuclides convert spontaneously over time to form stable nuclides.

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Figure 15.14

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Reason for Nuclear

Instability

Radioactive Process

Emitted Radiation

Change in N/Z Ratio

Excess Mass Alpha decaySlight

increase

N/Z too high Beta decay Decrease

N/Z too lowPositron emission

Increase

N/Z too lowElectron capture

- Increase

Energetically

excitedGamma

emissionγ ray None

Table 15.2 Nuclear Instability

01

-01

42

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Practice – Predicting the Method of Decay• Predict the method of radioactive decay

of the unstable nuclide Neon-18.

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F Ne

positron. a and neutron a to proton

a converts positron a of emission The value. the

increase todecay eradioactiv undergoes nuclide

the so 1, of ratio ideal the than lower is ratio This

0.8 10

8

10

10-18

189

01

1810

:is nuclide this in ratio N/Z The

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Radioactive Decay Series• In heavier elements,

often the product of radioactive decay is itself radioactive.

• In such cases, a series of alpha and beta decay steps ultimately leads to a stable nuclide.

• Accounts for most of the radioactive decay among elements 83 through 92.

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Figure 15.15

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Detecting Radiation• Various instruments have been

developed to give speedier and more accurate measures of radiation intensity:– Geiger-Muller counter– Scintillation counter

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Figure 15.16

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Half-Life• The time required for half of a sample of

a nuclide to decay to a different nuclide

• It takes the same time for a fresh sample to decay to one-half the original number of atoms of that nuclide as it does one-half to decay to one-fourth and so on.

• The shorter the half-life of a nuclide, the more intense the radiation that it emits.

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Half-Life

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Practice – Half-Life• Gold-128 undergoes beta decay to

yield mercury-128:

The half-life is 2.7 days. What percentage of the gold-128 is left after 8.1 days?

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-01-

12881

12880 Hg Au

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Practice Solutions – Half-Life

The half-life is 2.7 days. What percentage of the gold-128 is left after 8.1 days?

8.1 days = 3 half-lives 2.7 days

If the initial amount is 100%, then the way to calculate the gold-128 remaining is:

After one ½ life – ½ (50%) of the gold-128 remains After two ½ lives – ¼ (25%) of the gold-128 remains

After three ½ lives – 1/8 (12.5%) of the gold-128 remains

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-01-

12881

12880 Hg Au

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Archeological Dating• Radio-carbon dating

– Using carbon-14 to measure time on an archeological scale

– As long as a plant or animal is alive, its carbon-14 content should match that in the atmosphere

– After it dies, its carbon-14 content decreases through beta decay:

– The half-life of the process is 5730 years.

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01-147

146 N C

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Practice - Archeological Dating• A leather strap found in an

archeological dig has a carbon-14 content that is 25% of what can be measured in living tissue. If the half-life of carbon-14 is 5730 years, how old is the leather?

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Practice Solutions - Archeological Dating

• A leather strap found in an archeological dig has a carbon-14 content that is 25% of what can be measured in living tissue. If the half-life of carbon-14 is 5730 years, how old is the leather?If the carbon-14 content in the leather strap is 25%, then 2 half-lives have progressed. The leather is:

2 half-lives x 5730 years = 1.146 x 104 years 1 half-life

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Medical Applications• Many medical applications exists

that use radioactivity:– Power generators

• Example: 238Pu is used to power pacemakers

– Medical diagnoses• Radioactive nuclides are used as tracers

to track movements of substances in chemical or biological systems

• Example: 99mTc is used to help doctors locate tumors

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Medical Applications– Positron Emission Topography

• A PET scan detects abnormalities in living tissues without disrupting the tissue.

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Medical Applications– Cancer therapy

• Radioactive nuclides, in much higher doses than those used for imaging, are used to treat cancerous tumors

• Cancer cells absorb nutrients containing gamma-emitting components, the gamma radiation becomes concentrated in the cancerous cells, destroying them in greater numbers than normal cells.

• Examples: 131I destroys thyroid tumors, 198Au used to treat lung cancer, 32P used for eye tumors

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Biological Effects of Radiation• Radiation can have one of four effects on

the functioning of a cell:1. The radiation can pass through the cell with no

damage.2. The cell can absorb the radiation and be

damaged, but it can subsequently repair the damage and resume normal functioning.

3. The cell can be damaged so severely that it cannot repair itself. New cells formed from this cell will be abnormal. This mutant cell can ultimately cause cancer if it continues to proliferate.

4. The cell can be so severely damaged that it dies. 15-

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Biological Effects of Radiation

• Radon– A rare noble gas which has also

been implicated as a possible cause of lung cancer

– Accumulates in houses from particular kinds of soils or rock strata

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Nuclear Energy• Fission

– Splitting of a heavy nucleus into two or more lighter nuclei and some number of neutrons

– Example:

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01-10

14058

9440

10

23592

10

14354

9038

10

23592

10

14156

9236

10

23592

6 n 2 Ce Zr n U

n3 Xe Sr n U

n3 Ba Kr n U

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Fission of Uranium-235

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Figure 15.22

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Practice – Fission Products

• When bombarded by a neutron, uranium-235 undergoes fission, emitting the nuclides tellurium-137 and zirconium-97. How many neutrons are emitted? Write a balanced nuclear equation to describe this fission process.

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Practice Solutions – Fission Products• When bombarded by a neutron, uranium-235

undergoes fission, emitting the nuclides tellurium-137 and zirconium-97. How many neutrons are emitted? Write a balanced nuclear equation to describe this fission process.Neutrons change the mass number by 1 but do not affect the atomic number. Adding the mass numbers on either side:

235 = 137 + 97 + AA = 1

So, only one nuclide is used.

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n Zr Te U 10

9740

13752

23592

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Chain Reactions• A reaction in which the product of one step is

the reactant in another step• In order for a chain reaction to sustain itself,

the amount and shape of the sample of fissionable material must be such that the neutrons will not escape due to energy that is higher than optimum for inducing further fission

• A chain reaction should maintain a constant rate

• Critical mass– The smallest amount of fissionable material

necessary to support a continuing chain reaction

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Plutonium

When nonfissionable U-238 captures a fast neutron, it eventually forms the fissionable nuclide of plutonium, Pu-239, which can support a chain reaction. Plutonium is a transuranium element, meaning that it has an atomic number greater than the 92 of uranium. The fissionable plutonium produced in a uranium-fueled reactor can be used as a fuel or in nuclear weapons.

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Fission Reactors• Nuclear power plants

use fission to produce electric energy

• If the chain reaction is going too quickly, movable control rods made of these elements are inserted into a core of uranium fuel in fission reactors

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Figure 15.23

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Fission Reactors

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A Nuclear World?Nuclear energy generates about 21 percent of the electricity produced in the United States. Questions of safety, costs, and nuclear waste disposal have halted construction of nuclear reactors in the United States.

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A Nuclear World?

Nuclear Power plants locations throughout the world.

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Fusion Reactions• Fusion

– Combination of light nuclei to form heavier nuclei

– A major fusion reaction occurs continuously in the Sun and other stars:

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01

42

32

11

32

21

11

01

21

11

11

01

42

11

He He H

He H H

H H H

2 He H 4

:steps several in occurs process This

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Fusion Reactor

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Figure 15.26

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Fusion Reaction Terms• Ignition temperature

– Temperature required to initiate a fusion reaction• Breeder reactors

– A reactor that produces fuel that can be used in other reactors

• Plasma– An ionized gas that must created and controlled at

temperatures of about 108 K – Melts most container material

• Until recently, fusion in reactors required more energy than was given off

• In order to achieve fusion, the gaseous reactants must be condensed to a small volume at high temperatures.

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Nuclear Fusion.

Nuclear fusion produces tremendous quantities of energy and has the potential of becoming the ultimate source of energy on earth.

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Practice – Fusion Reactions• What new element would form if two

oxygen-16 nuclei undergo fusion?

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Practice Solutions – Fusion Reactions

• What new element would form if two oxygen-16 nuclei undergo fusion?

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S O O 3216

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