View
270
Download
5
Category
Preview:
Citation preview
11
Chemistry: Atoms FirstSecond Edition
Julia Burdge & Jason Overby
Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chapter 20
Nuclear Chemistry
M. Stacey Thomson
Pasco-Hernando State College
What Is Radioactivity?What Is Radioactivity?• Radioactivity is the release of tiny, high-
energy particles or gamma rays from an atom
• Particles are ejected from the nucleus
2Tro: Chemistry: A Molecular Approach
Nuclear DecayNuclear DecaySome nuclei are unstable, and will,
over time, emit particles and/or electromagnetic radiation until they become stable.
The spontaneous emission of particles or electromagnetic radiation is known as radioactivity.
All elements with Z > 83 are radioactive.
Tro: Chemistry: A Molecular Approach
3
4
Nuclei and Nuclear ReactionsNuclei and Nuclear Reactions20.1
During a nuclear reaction, the products and reactants will contain different elements as the nuclei change.
There are several types of particles or forms of electromagnetic radiation that may be emitted during a nuclear reaction.
You should learn the names, symbols and the mass number and charge of each of the particles.
5
Nuclei and Nuclear ReactionsNuclei and Nuclear Reactions
The symbols for subatomic particles include:
11H
11pproton
10nneutron
01e
01electron
01e
01positron
42
42Heα particle
6
Nuclei and Nuclear ReactionsNuclei and Nuclear Reactions
In balancing a nuclear reaction, simply balance the total of all atomic numbers and total of all mass numbers for the products and reactants.
42He212
84Po 20882Pb +
Mass number:
Atomic number:
212 208 + 4 = 212
84 82 + 2 = 84
7
Example 20.1
Identify the missing species X in each of the following nuclear equations:
(a)
(b)
(c)
XPb Po 20882
21284
01-9038 X Sr
01-188 O X
8
Nuclear StabilityNuclear Stability20.2
Review the information from Chapter 2 on Nuclear stability.
Principle factor for nuclear stability is neutron-to-protonratio (n/p)
There are more stabile nuclei with 2, 8, 20, 50, 82, or 126 protons or neutrons
More with even #’s All with atomic number > 83 are radioactive All isotopes of Tc and Pm are radioactive
9
Nuclear StabilityNuclear Stability
The figure shows the number of neutrons vs. the number of protons in various isotopes.
Stable nuclei are located in an area of the graph known as the belt of stability.
Most radioactive nuclei lie outside the belt.
Above the belt of stability, the nuclei have higher neutron-to-proton ratio.
10
Types of Nuclear DecayTypes of Nuclear Decay
Above the belt, isotopes decay by:
beta emission
11Tro: Chemistry: A Molecular Approach
12
Types of Nuclear Decay Types of Nuclear Decay
Below the belt, isotopes decay by:
positron emission
electron capture
A Low Neutron to Proton RatioA Low Neutron to Proton Ratio
If the N/Z ratio is too low, and the nuclide has too many protons. These nuclides tend to undergo either positron emission or electron capture.
13Tro: Chemistry: A Molecular Approach
Positron EmissionPositron Emission
14Tro: Chemistry: A Molecular Approach
Positrons result from a proton changing into a neutron.
A positron has a charge of +1 and negligible mass.anti-electron
Positron EmissionPositron Emission
15Tro: Chemistry: A Molecular Approach
• When an atom loses a positron from the nucleus, itsmass number remains the sameatomic number decreases by 1
16Tro: Chemistry: A Molecular Approach
Electron CaptureElectron Capture
17Tro: Chemistry: A Molecular Approach
Electron capture occurs when an inner orbital electron is pulled into the nucleus. Aproton combines with the electron to make a neutron. This decreases the atomic number, and increases the N/Z ratio.
Electron CaptureElectron Capture
18Tro: Chemistry: A Molecular Approach
As a result of electron capture: mass number stays the same atomic number decreases by one
Particle Changes
19Tro: Chemistry: A Molecular Approach
Alpha (α) EmissionAlpha (α) Emission
Many nuclides that are too heavy to be stable (Z>83) undergo alpha emission.
An particle contains 2 protons and 2 neutrons, and is the same as a helium nucleus.
Tro: Chemistry: A Molecular Approach
20
21Tro: Chemistry: A Molecular Approach
Alpha EmissionAlpha Emission
22Tro: Chemistry: A Molecular Approach
Loss of an alpha particle means:atomic number decreases by 2mass number decreases by 4
As a result, the N/Z ratio increases.
Gamma (γ) EmissionGamma (γ) Emission
During a nuclear reaction, high energy electromagnetic radiation, called gamma rays, is often emitted.
Generally occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange.
Tro: Chemistry: A Molecular Approach
23
Gamma EmissionGamma Emission
24Tro: Chemistry: A Molecular Approach
• Gamma () rays are high energy photons of light
• No loss of particles from the nucleus• No change in the composition of the nucleus
same atomic number and mass number
Other Properties of Other Properties of RadioactivityRadioactivity
25Tro: Chemistry: A Molecular Approach
• Radioactive rays can ionize mattercause uncharged matter to become chargedbasis of Geiger Counter and electroscope
• Radioactive rays have high energy
• Radioactive rays can penetrate matter
• Radioactive rays cause phosphorescent chemicals to glowbasis of scintillation counter
Penetrating Ability of Radioactive Rays
0.01 mm 1 mm 100 mm
Pieces of Lead
26Tro: Chemistry: A Molecular Approach
Ionizing Ability of Radiation
Tro: Chemistry: A Molecular Approach
27
Highly energetic radiation interacts with molecules and atoms by ionizing them. This can have serious biological effects on cells in living systems. Cell damage, or abnormal cell replication can occur.
α particles are highly ionizing, but not very penetrating. They can be stopped by a sheet of paper, clothing, or air. As a result, they are not very damaging unless ingested or breathed into the lungs.
β particles have lower ionizing power, but are more penetrating. A sheet of metal or a thick piece of wood will stop them.
Ionizing Ability of Radiation
Tro: Chemistry: A Molecular Approach
28
γ rays have the lowest ionizing power, but are the most penetrating. Several inches of lead or slabs of concrete are needed to stop gamma rays.
29
Nuclear Binding EnergyNuclear Binding Energy
A quantitative measure of nuclear stability is the nuclear binding energy.The nuclear binding energy is the energy required to break up a nucleus into its component protons and neutrons.
The difference between the mass of an atom and the sum of the masses of its protons, neutrons, and electrons is called the mass defect.
The measured mass of 19F = 18.99840 amu
mass of 9 protons = 9 x 1.007825 amu = 9.070425 amumass of 9 electrons = 9 x 5.4858x10-4 amu = 0.0049372 amumass of 10 neutrons = 10 x 1.008665 amu = 10.08665 amu
The calculated mass of 19F = 19.16201 amu
Mass defect of 19F = 19.16201 amu – 18.99840 amu = 0.16361 amu
30
Nuclear Binding Energy Nuclear Binding Energy
The loss in mass is converted to energy and can be quantified with Einstein’s mass-energy equivalence relationship.
ΔE = energy of product – energy of reactant Δm = mass of product – mass of reactant
For 19F, Δm = 18.99840 – 19.16201 amu = –0.16361 amu
ΔE = (Δm)c2
kgxamux
kgamum 28
261071682
1002214186
1163610
.
..
31
Nuclear Binding Energy Nuclear Binding Energy
The loss in mass is converted to energy and can be quantified with Einstein’s mass-energy equivalence relationship.
ΔE = (–2.7168 x 10–28 kg)(2.99792458 x 108 m/s)2
ΔE = (Δm)c2
ΔE = –2.4417 x 10–11 kg (m/s)2
ΔE = –2.4417 x 10–11 J
32
Nuclear Binding Energy Nuclear Binding Energy
Plot of nuclear binding energy per nucleon versus mass number.
33
Nuclear RadioactivityNuclear Radioactivity
The disintegration of a radioactive nucleus often is the beginning of a radioactive decay series, which is a sequence of nuclear reactions that ultimately result in the formation of a stable isotope.
The beginning radioactive isotope is called the parent and the product isotope is called the daughter.
20.3
34
Kinetics of Radioactive DecayKinetics of Radioactive Decay
All radioactive decays obey first-order kinetics.
The corresponding half-life of the reaction is given by:
0
ln tNkt
N
1/2
0.693t
k
35
Nuclear RadioactivityNuclear Radioactivity
A piece of linen cloth found at an ancient burial site is found to have a 14C activity of 4.8 disintegrations per minute. Determine the age of the cloth.
Assume that the carbon-14 activity of an equal mass of living flax (the plant from which linen is made) is 14.8 disintegrations per minute. The half-life of carbon-14 is 5715 years.
SolutionStep 1:Determine the rate constant from the equation below:
1/2
0.693t
k
4 1
1/2
0.693 0.6931.21 10 yr
5715 yrk
t
36
Dating Based on Radioactive DecayDating Based on Radioactive Decay
A piece of linen cloth found at an ancient burial site is found to have a 14C activity of 4.8 disintegrations per minute. Determine the age of the cloth.
Assume that the carbon-14 activity of an equal mass of living flax (the plant from which linen is made) is 14.8 disintegrations per minute. The half-life of carbon-14 is 5715 years.
SolutionStep 2:Use the equation below to calculate time:
t = 1.0 x 104 yr
4 14.8ln 1.21 10 yr
14.8t
0
ln tNkt
N
37
Worked Example 20.3
Strategy The activity of a radioactive sample is proportional to the number of radioactive nuclei. Thus, we can use ln([A]t/[A]0) = –kt with activity in place of concentration:
ln = –kt
To determine k, though, we must solve t½ = 0.693/k, using the value of t½ for carbon-14 (5715 years) given in the problem statement.
A wooden artifact is found to have a 14C activity of 9.1 disintegrations per second. Given that the 14C activity of an equal mass of fresh-cut wood has a constant value of 15.2 disintegrations per second, determine the age of the artifact. The half-life of carbon-14 is 5715 years.
14C activity in artifact14C activity in fresh-cut wood
38
Nuclear transmutation differs from radioactive decay in that transmutation is brought about by the collision of two particles.
Particle accelerators made it possible to synthesize the so-called transuranium elements, elements with atomic numbers greater than 92.
Nuclear TransmutationNuclear Transmutation20.4
147N
42
178O 1
1p+ +
39
Nuclear TransmutationNuclear Transmutation
Write an equation for the process represented by:
SolutionStep 1:Determine the bombarding particle and the emitted particle:
Step 2:Write the equation:
AgpPd 10947
10646 ),(
AgpPd 10947
10646 ),(
emitted particlebombarding particle
p Ag He Pd 11
10947
42
10646
40
Worked Example 20.5
Strategy The species written first is a reactant. The species written last is a product. Within the parentheses, the bombarding particle (a reactant) is written first, followed by the emitted particle (a product).
Write the balanced nuclear equation for the reaction represented by where d represents a deuterium nucleus.
Mn)Fe(d, 5425
5626
42
5425
21
5625 Mn H Fe
Solution The bombarding and emitted particles are represented by and , respectively.
H21 42
Think About It Check your work by summing the mass numbers and atomic numbers on both sides of the equation.
41
Nuclear TransmutationNuclear Transmutation
Schematic of a cyclotron particle accelerator.
42
Nuclear TransmutationNuclear Transmutation
Section of a particle accelerator.
43
Nuclear FissionNuclear Fission
Nuclear fission is the process in which a heavy nucleus (mass number > 200) divides to form smaller nuclei and one or more neutrons.
20.5
23592U
10n 90
38Sr 103 n+ +
14354 Xe +
44
Nuclear FissionNuclear Fission
Relative yields of the products resulting from the fission of 235U as a function of mass number.
45
Nuclear FissionNuclear Fission
235U is capable of a self-sustaining sequence of nuclear fission known as a nuclear chain reaction.
46
Nuclear FissionNuclear Fission
The minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction is the critical mass.
47
Nuclear FissionNuclear Fission
Schematic of a nuclear fission reactor.
48
Nuclear FusionNuclear Fusion
Nuclear fusion is the process of combining small nuclei into larger ones.
The following reactions are believed to take place in the sun:
Because fusion reactions take place at very high temperatures, they are often called thermonuclear reactions.
20.6
49
Nuclear FusionNuclear Fusion
Promising fusion reactions include:
Due to the high temperaturerequirements, containmentis an issue.
50
Nuclear FusionNuclear Fusion
A promising design employs high power lasers
A small scale fusion reaction was carried out at the Lawrence Livermore National Laboratory (right)
Technical difficulties still need to be overcome before it can be put to practical use
51
Use of Isotopes: Chemical AnalysisUse of Isotopes: Chemical Analysis
Radioactive and stable isotopes have applications in science for molecular structure determination.
20.7
Two proposed structures for thiosulfate ion:
By using radioactive sulfer-35 isotope, the isotope acts as a “label” for the S atoms.
Based on studies, structure 2 has been confirmed.
1)
2)
52
Isotopes in MedicineIsotopes in Medicine
Radioactive and stable isotopes have applications in science and medicine.
Radioactive isotopes are used as tracers.
Use of tracers for diagnosis include:
Sodium-24 – blood flow Iodine-131 –thyroid conditions Iodine-123 – brain imaging
53
Biological Effects of RadiationBiological Effects of Radiation
The fundamental unit of radioactivity is the curie (Ci)
1 Cu = 3.70 x 1010 disintegrations per second.
20.8
54
Biological Effects of RadiationBiological Effects of Radiation
A common unit for the absorbed dose of radiation is the rad (radiation absorbed dose).
1 rad = 1 x 105 J/g of tissue irradiated
The rem (roentgen equivalent for man) is determined from the number of rads:
Number of rems = number of rads x 1 RBE
RBE = the relative biological effectivness.
55
Biological Effects of RadiationBiological Effects of Radiation
Recommended