1 chapter 26 nuclear chemistry. 2 chapter goals 1. the nucleus 2. neutron-proton ratio and nuclear...

1

Chapter 26

Nuclear Chemistry

2

Chapter Goals

1. The Nucleus2. Neutron-Proton Ratio and Nuclear Stability3. Nuclear Stability and Binding Energy4. Radioactive Decay5. Equations for Nuclear Reactions6. Neutron-Rich Nuclei (Above the Band of

Stability)7. Neutron-Poor Nuclei (Below the Band of

Stability)8. Nuclei with Atomic Number Greater than 839. Detection of Radiation

3

Chapter Goals

10. Rates of Decay and Half-Life

11. Disintegration Series

12. Uses of Radionuclides

13. Artificial Transmutations of Elements

14. Nuclear Fission

15. Nuclear Fission Reactors

16. Nuclear Fusion

4

Comparison Of Chemical and Nuclear Reactions

Nuclear Reactions1 Elements may be

converted from one element to another.

2 Particles within the nucleus, such as protons and neutrons, are involved in reactions.

Chemical Reactions1 No new elements can

be produced, only new chemical compounds.

2 Usually only the outer most electrons participate in reactions.

5

Comparison Of Chemical and Nuclear Reactions

Nuclear Reactions3 Release or absorb

immense amounts of energy, typically 1000 times more.

4 Rates of reaction are not influenced by external factors.

Chemical Reactions3 Release or absorb

much smaller amounts of energy.

4 Rates of reaction depend on factors such as concentration, pressure, temperature, and catalysts.

6

Beginning of Nuclear Science In 1896, Henri Becqurel accidentally

discovered radioactivity in U salts. In 1898, Marie and Pierre Curie discovered

two new radioactive elements in U mine residue. Po and Ra

In 1898, Ernest Rutherford discovered that radioactivity has two distinct forms. and radiation

7

Fundamental Particles of Matter

PARTICLE MASS (amu) CHARGE

Electron(e-)

0.0005458 1-

Proton(p or p+)

1.0073 1+

Neutron(n or n0)

1.0087 none

8

The Nucleus

The nucleus consists of protons and neutrons in a very small volume. Protons and neutrons are made of other

fundamental particles called quarks. Nuclei have a diameter of approximately 10-12 cm Nuclei have a density of approximately 2 x 1014 g/cm3. The strong nuclear force binds the nucleus together at

extremely short distances of 10-13 cm

9

Neutron-Proton Ratio and Nuclear Stability

Terminology used in nuclear chemistry.

1. Nuclides denotes different nuclei.

2. Isotopes are nuclei that have the same number of protons but different neutron numbers.

Isotopes are the same element. Experimentally, it can be shown that nuclei have a

preference for even numbers of protons and neutrons The next table is all of the nonradioactive nuclides broken

into various combinations of protons and neutrons.

10


Proton Number Neutron Number Number of NuclidesEven Even 157Even Odd 52Odd Even 50Odd Odd 4

11


Special stability is associated with certain proton and neutron numbers due to shell effects in nuclei similar to the

s, p, d, and f shells in atoms These proton and neutron numbers

are called “Magic Numbers.” Magic numbers are:

2 8 20 28 50 82 126

12


Example nuclides with magic numbers of nucleons includes:

nucleus magicdoubly a O

nucleus magicdoubly a He

816

8

242 nucleus magicdoubly a He242

nucleus magicdoubly a Ca



204020

816

8

242





284820

204020

816

8

242

nucleus magicsingly a Sn





70120

50

284820

204020

816

8

242

nucleus magicdoubly a Pb

nucleus magicsingly a Sn





12620882

7012050

284820

204020

816

8

242

13

Nuclear Stability and Binding Energy

The mass deficiency or mass defect of a nucleus is determined in this fashion.

The mass defect is the mass of the nuclear particles that has been used to bind the nucleus in the nuclear binding energy or strong nuclear force.

atom of mass actuale andn ,p all of masses of summ -

14


Due to the Einstein relationship, we can calculate the nuclear binding energy for a nucleus.

2

2

2

cm Energy Binding

or

cmE

mc E

15


Example 26-1: Calculate the mass deficiency for 39K. The actual mass of 39K is 39.32197 amu per atom.

amu 0.0005458 of mass a haselectron 1

amu 1.0087 of mass a hasneutron 1

amu 1.0073 of mass a hasproton 1

electrons 19 and neutrons 20 protons, 19 hasK 39

16


Example 26-1: Calculate the mass deficiency for 39K. The actual mass of 39K is 39.32197 amu per atom.

amu 32307.39

amu 0104.0 amu 1740.20 amu 1387.19

amu 0.000545819 amu 1.008720 amu 1.0073 19

is electrons and neutrons, protons, theof masses theof sum The

amu 0.000545819 amu 1.008720 amu 1.0073 19

:is electrons and neutrons, protons, of masses theof sum The

amu 00110.0m

amu 39.32197-amu 32307.39 m

17


Example 26-2: Calculate the nuclear binding energy of 39K in J/mol of K atoms. 1 J = 1 kg m2/s2.

amu

g24mol

amu20

molatoms23

atomamu

10661.110624.6

10022.6 0.00110=m

18



molkg6

molg

amug24

molamu20

molatoms23

atomamu

1010.1 00110.0

10661.110624.6

10022.6 0.00110=m

19



E = mc2 kgmol

ms

kgmol

ms

kg ms mol

Jmol

2

2

2

2

110 10 3 00 10

110 10 9 00 10

9 90 10 9 90 10

6 8 2

6 16

10 10

. .

. .

. .

20

Radioactive Decay

Nuclei whose neutron-to-proton ratio lies outside the belt of stability experience spontaneous radioactive decay.

Decay type depends on where the nuclei is positioned relative to the band of stability.

Radioactive particles are emitted with different kinetic energies. Energy change is related to the change in binding

energy from reactant to products.

21

Radioactive Decay

22

Equations for Nuclear Reactions

Two conservation principles hold for nuclear reaction equations.

The following principles hold for all nuclear reactions.

1. The sum of the mass numbers of the reactants equals the sum of the mass numbers of the products.

2. The sum of the atomic numbers of the reactants equals the sum of the atomic numbers of the products.

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Equations for Nuclear Reactions

For the general reaction:

The two conservation principles demand

1. M1 = M2 + M3

and

2. Z1 = Z2 + Z3

Where the M's are mass numbers, And the Z's are atomic numbers.

YRQ 3

3

2

2

1

1

MZ

MZ

MZ

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Neutron Rich Nuclei (Above the Band of Stability)

These nuclei have too high a ratio of neutrons to protons.

Decays must lower this ratio and include: beta emission neutron emission

Beta emission is associated with the conversion of a neutron to a proton;

01n p1

110

25


Beta emission simultaneously decreases the number of neutrons (by one) and increases the number of protons (by one). Efficiently changes the neutron to proton ratio.

Examples of beta emission:

614

714

-10C N+ 6

14 714

-10

89226

-10

C N +

Ra Ac +

88226

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Neutron emission does not change the atomic number, but it decreases the number of neutrons. The product isotope is less massive by the mass of 1

neutron. Examples of neutron emission

nI+I

n+ NN10

13653

13753

10

167

177

n+ NN 10

167

177

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Neutron Poor Nuclei (Below the Band of Stability)

These nuclides have too low a ratio of neutrons to protons.

Nuclear radioactive decays must raise this ratio

The possible decays include: 1. electron capture

2. positron emission

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29


Electron capture involves the capture of an electron in the lowest energy level in the atom by the nucleus. conversion of a proton to a neutron

CleAr

n e p3717

01-

3718

10

01-

11

n e p 10

01-

11

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A positron has the mass of an electron but has a positive charge. The symbol is 0

+1e. Positron emission is associated with the

conversion of a proton into a neutron.

n e p 10

01

11

Ar eK

n e p3918

01

3919

10

01

11

N eO

Ar eK

n e p

157

01

158

3918

01

3919

10

01

11

31

Nuclei with Atomic Number Greater than 83

Alpha emission occurs for some nuclides, especially heavier ones.

Alpha () particles are helium nuclei, 42He, containing two protons and two

neutrons. Alpha emission increases the neutron-to-

proton ratio.

He HgPb 42

20080

20482

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All nuclides having atomic numbers greater than 83 are beyond the belt of stability and are radioactive. Many of these isotopes decay by emitting

alpha particles.

He ThU 42

23490

23892

33


The transuranium elements (Z>92) also decay by nuclear fission in which the heavy nuclide splits into nuclides of intermediate mass and neutrons.

n4 Mo BaCf 10

10642

14256

25298

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Detection of Radiation

Present radiation detection schemes depend on the fact that particles and radiations emitted by radioactive decay are energetic and some carry charges.

1. Photographic Detection Radioactivity affects photographic plates or

film as does ordinary light. Medical and dental x-ray photographs are

made using this technique.

35


2. Fluorescence Detection Fluorescent substances absorb energy from

high energy rays and then emit visible light. A scintillation counter is an instrument

using this principle.

36


3. Cloud Chambers contain air saturated with a vapor.

Radioactive decay particles emitted ionize the air molecules in the chamber.

The vapor condenses on these ions. Then the ion tracks are photographed.

37


Diagram of a Simple Cloud Chamber

38


A Cloud Chamber Photo from a Large Detector.

39


4. Gas Ionization Counters The ions produced by ionizing radiation are

passed between high voltage electrodes causing a current to flow between the electrodes and the current is amplified.

This is the basis of operation of gas ionization counters such as the Geiger-Mueller counter.

40


Schematic of Geiger Counter

41


Picture of a Geiger Counter

42

Rates of Decay and Half-Life

The rates of all radioactive decays are independent of temperature and obey first order kinetics.

The same relationships developed in Chapter 16 apply here as well.

k t aA

Aln

or Ak decay of Rate

0

43


For counting radioactive decay the relationship changes just slightly:

k t aN

Nln

or Ak decay of Rate

0

44


The half-life, t1/2, is related to the rate constant by the simple relationship:

k a

0.693

k a

2ln t

21

45


Example 26-3: How much 60Co remains 15.0 years after it is initially made? 60Co has a half-life of 5.27 years.

k t

0

0

0

A

A

k tA

Aln

case for thisk t k t aA

Aln

e1-y 132.0k

y 27.5

0.693

t

0.693 k

k a

0.693

k a

2ln t

21

21

97.1

y 15.0y 132.0

k t0

0.1A

0.1A

AA1-

e

e

eremains 13.9%or 0.139 A

.139)0( 0.1A

46

Disintegration Series

Some nuclides are so far away from the belt of stability, that it takes many nuclear disintegrations (a series of them) to attain nuclear stability.

Table 26-4 in the textbook outlines in detail three of these disintegration series: The 238U, 235U and 232Th series:

47

Disintegration Series

48

Uses of Radionuclides

Radioactive Dating Radiocarbon dating can be used to

estimate the ages of items of organic origin.

14C is produced continuously in the upper atmosphere by the bombardment of 14N by cosmic-ray neutrons:

p C n N 11

146

10

14

49


14C atoms react with O2 to form CO2

The CO2 then is incorporated into plant life by photosynthesis.

After the organism dies the 14C content decreases via radioactive decay The 14C half-life is 5730 years.

01-147

146 N C

50


The potassium-argon and uranium-lead methods are used for dating older objects.

Potassium-argon method relies on the electron capture decay of 40K to 40Ar

y103.1t

Ar K 9

01

4018

4019

21

e

51


The uranium-lead method relies on the alpha decay of 238U to 234Th.

y105.4t

He Th U9

42

23490

23892

21

52


Example 26-4: Estimate the age of an object whose 14C activity is only 55% that of living wood.

1. Determine the rate constant for 14C.

14 y1021.1y 5730

693.0

t

0.693k

k

693.0

k a

693.0t

21

21

53


2. Determine the age of the object.

ty1021.1 1.82ln

ty1021.1 55%

100%ln

case in thisk t k t aA

Aln

14

14

0

y 4940 t

y1021.1

0.598t

ty1021.1 0.598

14

14

54

Artificial Transmutations of Elements

Bombardment of a nuclide with a nuclear particle can make an unstable compound nucleus that decays to a new nuclide by emission of a different particle.

The rules for balancing equations for nuclear reactions which were presented in the section on radioactivity still hold.

55


Bombardment with Positive Ions If the bombarding particle is positively charged, it must be accelerated with sufficient energy to overcome

the coulomb repulsion of the positive nucleus bombarding particles penetrate the nucleus

Particle accelerators such as cyclotrons or linear accelerators are used for this.

56


n TcH Mo 10

9743

21

9642

Example reactions are:

57


At n 3He Bi

n TcH Mo210

8510

42

20983

10

9743

21

9642

58


? n 3 H Th

At n 3He Bi

n TcH Mo

10

11

23090

21085

10

42

20983

10

9743

21

9642

59


Pa n 3 H Th

At n 3He Bi

n TcH Mo

22891

10

11

23090

21085

10

42

20983

10

9743

21

9642

60


Neutron Bombardment Because neutrons have no charge, there is no coulomb repulsion to their nuclear penetration,

so they do not have to be accelerated. Nuclear reactors are often used as neutron sources.

61


Neutrons with large kinetic energy are called fast neutrons. Slow neutrons ("thermal neutrons") have had their excess energy decreased by collisions with moderators

Common moderators are hydrogen, deuterium, or the hydrogen atoms in paraffin. Slow neutrons are more likely to be captured by target nuclei.

62


reaction n, Hgn Hg 00

20180

10

20080

63


reaction n, He H n Li

reaction n, Hg n Hg42

31

10

63

00

20180

10

20080

64

Nuclear Fission Some nuclides with atomic numbers greater than 80 are

able to undergo fission. These nuclei split into nuclei of intermediate masses and emit

one or more neutrons.

Some fissions are spontaneous while others require activation by neutron bombardment.

Enormous amounts of energy are released in these fissions. Some of the numerous possible fission paths for 235U (after

bombardment by a neutron) are:

65

Nuclear Fission

energy n 2 Sr Xe

energy n 2 Rb Cs

energy n 3 Kr Ba

energy n 3 Br La

energy n 4 Zn Sm

Un U

10

9038

14454

10

9037

14455

10

9336

14056

10

8735

14657

10

7230

16062

23692

10

23592

66

Nuclear Fission

Fission is energetically favorable for elements with Z greater than 80 The product nuclides are more stable (near

the high part of the nuclear binding energy curve).

67

Nuclear Fission Reactors

Electricity can be generated from steam heated by nuclear fission reactions.

Greatest danger of nuclear reactors is core meltdown.

There have been two very serious nuclear reactor accidents:

1. Three Mile Island in PA.

2. Chernobyl in the Ukraine.

68

Nuclear Fission ReactorsDescription of Nuclear Reactors

Light Water Reactors use normal water as the coolant and moderator.

Typical Reactor Fuels are: 235UO2

239Pu

Moderator is the material that slows neutrons from fast to thermal. Commonly used moderators are graphite, water, heavy

water.

69

Nuclear Fission Reactors Control Rods are usually made of boron which is an

efficient neutron absorber. Control rods remove neutrons and slow the chain reaction.

Cooling Systems The reactor core must be cooled to remove the heat from

the nuclear reactions. Some possible coolants are:

water - both normal and heavy helium liquid sodium

70

Nuclear Fission Reactors Shielding provides workers and public

with protection from radiation. Lead and concrete are commonly used in

commercial reactors.

71

Nuclear Fusion Fusion, the merging of light nuclei to make

heavier nuclei, is favorable for very light atoms. Extremely high energies or temperatures are

necessary to initiate fusion reactions. The energy source for stars is fusion.

The fusion reaction in main sequence stars is:

energy n He H H 10

42

31

21

72

Nuclear Fusion

Fusion is the most energetic process in nature. Fusion has produced all of the chemical

elements beyond H and He up to Fe. Fusion is a potential energy source for

humans. Thermonuclear or hydrogen bombs

have been in existence since the 1950’s.

73

Nuclear Fusion

Controlled nuclear fusion is a very real possibility. Nuclear fusion must occur at temperatures of 10

million oC. Fusion reactors must contain this temperature

and not melt! Some fusion reactors exist around the world

However at present none can generate a sustainable fusion reaction.

Potential energy source for the 21st Century.

74

Synthesis Question

How are thermonuclear or fusion reactors designed so that the hot plasma, temperature of approximately 10 x 106 degrees, does not touch the sides of the reactor? The reactor would melt if the plasma were to touch the sides.

75

Synthesis Question

Most fusion reactors use intense magnetic fields to confine the hot plasma in the center of the reactor away from the walls.

76

Group Question

Stars are enormous thermonuclear fusion reactors generating enormous amounts of heat and energy. What keeps stars from blowing themselves apart? How do they remain stable for millions and billions of years?

77

End of Chapter 26

Nuclear science has been one of the driving forces of science in the 20th Century.

1 chapter 26 nuclear chemistry. 2 chapter goals 1. the nucleus 2. neutron-proton ratio and nuclear...

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