Chapter 19 Radioactivity and Nuclear Chemistry

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Chapter 19Radioactivity and Nuclear Chemistry
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GOALS Types of radioactivity Identify
radioactive nuclides Nuclear equations Binding
energy per nucleon units Kinetics of
radioactive decay
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Facts About the Atomic Nucleus

• Every atom of an element has the same number of
  protons (ve)
• atomic number (Z)
• Atoms of the same elements can have different
  numbers of neutrons (no charge)
• Isotopes atoms of the same element, having same
  atomic number, Z, but different mass number, A
  (diff no. of neutrons).
• Isotopes are identified by their mass number (A)
• mass number number of protons neutrons

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Facts About the Atomic Nucleus

• mass number number of protons neutrons
• neutrons mass number number of protons
• The nucleus of an isotope is called a nuclide
• Each nuclide is identified by a symbol

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Nuclide Symbols

• Boron-10 (105B) has 5 p and 5 n
• Boron-11 (115B) has 5 p and 6 n

• Oxygen-16 (168O) has 8 p and 8 n
• Oxygen-17 (178O) has 8 p and 9 n
• Oxygen-18 (188O) has 8 p and 10 n

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The Discovery of Radioactivity

• Becquerel discovered that certain minerals were
  constantly producing penetrating energy rays he
  called uranic rays (1896)
• Marie Curie discovered 2 new elements (Po, Ra)
  which also
• emitted uranic rays.
• Curie changed term uranic rays to radioactivity
  (present in elements other than uranium).
• Some nuclei are unstable they emit particles
  and/or electromagnetic radiation spontaneously.
  This is radioactivity.

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Types of Radioactive Rays

• Rutherford discovered there were 3 types of
  radioactivity
• 2 additional types were later discovered.
• alpha (a) beta (b) decay, gamma ray (g) then
  positron emission, and electron capture.

Another type of radioactivity (nuclear
transmutation) results from the bombardment of
nuclei (heavy) by neutrons, protons or other
nuclei (lighter).
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Penetrating Ability of Radioactive Rays
0.01 mm 1 mm 100 mm
Pieces of lead
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Important Atomic Symbols
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Nuclear Equations

• nuclear processes are described using nuclear
  equations
• use the symbol of the nuclide to represent the
  nucleus
• atomic numbers and mass numbers are conserved
• use this to predict identity of daughter nuclide
  if parent and emitted particle are known

emitted particle product
captured particle reactant
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Alpha Emission

• an ? particle contains 2 protons and 2 neutrons
• most ionizing, but least penetrating
• loss of an alpha particle means
• atomic number decreases by 2
• mass number decreases by 4

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Beta Emission

• An unstable nucleus emits an electron
• when an atom loses a ? particle its
• atomic number increases by 1
• mass number remains the same
• in beta decay, a neutron changes into a proton

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If californium-251 decays by successive a, a, ß
emissions, what nucleus is produced?
a) b) c)
d)
e)  
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Gamma Emission

• gamma (g) rays are high energy photons of light
• least ionizing, but most penetrating
• generally occurs after the nucleus undergoes some
  other type of decay and the remaining particles
  rearrange

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Positron Emission

• The positron has a charge of 1 and negligible
  mass
• anti-electron
• when an atom loses a positron from the nucleus,
  its
• mass number remains the same
• atomic number decreases by 1
• A positron appears to result from a proton
  changing into a neutron

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Electron Capture

• occurs when an inner orbital electron is pulled
  into the nucleus
• no particle emission, but atom changes
• same result as positron emission
• proton combines with the electron to make a
  neutron
• mass number stays the same
• atomic number decreases by one

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Summary of Decay Processes
(Table 19.1 pg 871)
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Write the nuclear equation for positron emission
from K-40

• a) Write the nuclide symbols for both the
  starting radionuclide and the particle

b) Set up the equation (emitted particles are
products captured particles are reactants)
c) Determine the mass number and atomic number
of the missing nuclide (mass and atomic numbers
are conserved)
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Write the nuclear equation for positron emission
from K-40

• 4) Determine the element from the atomic number

Q. In a decay series, U-238 emits 8 alpha
particles and 6 beta particles. What nuclide is
formed?
Mass dec by 32 charge 6 -16
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Write a nuclear equation for each of the following
alpha emission from U-238
beta emission from Ne-24
positron emission from N-13
electron capture by Be-7
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Stability of Nuclei
- stable isotopes fall in a very narrow range
called the island of stability.
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What Causes Nuclei to Break Down?

• the particles in the nucleus are held together by
  a very strong attractive force found in the
  nucleus called the strong force
• acts only over very short distances
• the neutrons play an important role in
  stabilizing the nucleus, as they add to the
  strong force, but do not repel each other like
  the protons do

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Neutron to Proton (N/Z) Ratio

• the ratio of neutrons protons is an important
  measure of the stability of the nucleus
• if the N/Z ratio is too high (neutron rich)
  neutrons are converted to protons via b decay
• if the N/Z ratio is too low (proton rich)
  protons are converted to neutrons via positron
  emission or electron capture
• or via a decay though not as efficient

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Valley (Island) of Stability (Plot of Neutrons
vs
Protons)
for Z 1 ? 20 (H - Ca), stable N/Z 1
for Z 20 ? 40, stable N/Z approaches 1.25
for Z 40 ? 80, stable N/Z approaches 1.5
Heavy nuclei for Z gt 83, there are no stable
nuclei
low N/Z
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Determine the kind of radioactive decay that
Mg-22 undergoes

• Mg-22
• Z 12 (protons)
• N 22 12 10 (neutrons)
• N/Z 10/12 0.83
• from Z 1 ? 20, stable nuclei have N/Z 1
• Mg-22 has low N/Z it should convert 11p into
  10n, therefore it will undergo positron emission
  or electron capture

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Determine the kind of radioactive decay that N-18
undergoes

• N-18
• Z 7 (protons)
• N 18 7 11 (neutrons)
• N/Z 11/7 1.57
• from Z 1 ? 20, stable nuclei have N/Z 1

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Q. Which of the following will undergo beta
decay?
16O, 20F, 13N
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Magic Numbers
besides the N/Z ratio, the numbers of protons and
neutrons effects stability
most stable nuclei have even numbers of protons
and neutrons
only a few have odd numbers of protons and
neutrons
if the total number of nucleons adds to a magic
number, the nucleus is more stable (compare
electrons in noble gases)

• most stable when N or Z 2 (He), 8 (O), 20 (Ca),
  28 (Ni), 50 (Sn), 82 (Pb)

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Binding Energy, Eb
-All atoms are a little lighter than they are
really supposed to be. Missing mass ?m mass
defect. -This missing mass is converted to
energy, and released when 1 mole of atoms is
formed from its subatomic particles
(protons neutrons electrons). -Energy
holds the nucleus together.
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Calculating Binding Energy, Eb
Eb is the energy required to separate the nucleus
of an atom into protons, neutrons, electrons.
For stability, Eb gt electrostatic
repulsive forces between protons. In deuterium,
21H 21H ? 11p 10n Eb 2.15 ? 108 kJ/mol
21H Eb per mol nucleon Eb/2 nucleons 1.08
? 108 kJ/mol nucleons
Also, calc Eb per nucleon (?6.022 ? 1023
nucleons)
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Calculating Binding Energy, Eb
For deuterium, 21H 21H ? 11p
10n Actual mass of 21H 2.01410 g/mol (given or
PT) Mass of proton 1.007825 g/mol Mass of
neutron 1.008665 g/mol Theoretical mass
2.016490 g/mol Mass
defect (missing mass) 2.016490
2.01410 0.00239 g/mol
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Calculate Binding Energy, Eb
Mass defect 0.00239 g/mol
(0.00239?1000) kg/mol
2.39 ? 10-6 kg/mol
From Einsteins equation Eb (?m)c2 2.39 ?
10-6 kg ? (3.00 108 m/s)2 2.15 1011
kg?m2/s2 (but 1 kg?m2/s2 1 J)
2.15 ? 1011 J/mol ?1000 J 2.15 ? 108 kJ/mol
Two nucleons for deuterium, 21H ? 11p
10n
Eb /mol nucleon 1.08 ? 108 kJ/mol nucleons
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Calculating Binding Energy, Eb
For I-127, 12753I 53p 74n (i.e. 127
nucleons) Actual mass of 12753I 126.9045 g/mol
(given or PT) 53 protons 53?1.007825 g/mol
53.41473 g/mol 74 neutrons 74 ? 1.008665 g/mol
74.64121 g/mol Theoretical mass defect
128.05594 g/mol Mass
defect (128.05594 -126.9045) g/mol
1.1514 g/mol 1.1514 ? 10-3 kg/mol
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Calculate Binding Energy, Eb
Eb 1.1514 ? 10-3 kg/mol? (3.00 108 m/s)2
1.04 1014 kg?m2/s2 (but 1 kg?m2/s2
1 J) 1.04 1014 J/mol
Eb /mol nucleon 1.04 1014 J/ (127 nucleons)
8.19 1011 J
Eb /nucleon 8.191011 J ? (6.022 1023)
1.36 10-12 J
Also, can express Eb in MeV 1 MeV 1.602
10-13 J
Eb /nucleon ? MeV
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Plot of Eb vs Mass -the greater the binding
energy per nucleon, the more stable the nucleus is
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Nuclear Fission
The splitting of a heavy unstable nucleus of an
atom into two or more fragments Pu, U
Th! -induced reaction to produce energy!
Energy released ? 16,800,000,000 kJ/mol
(235 g Uranium)
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Nuclear Fusion
Light nuclei fuse to generate heavier nuclei
(more stable)
Free of long-lived radioactive waste.
More difficult to achieve. Nuclei must travel at
v. large KEs at each other.
More destructive than fission bombs (WWII)!
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Kinetics of Radioactive Decay
Rate kN N number of radioactive nuclei
the shorter the half-life, the more nuclei decay
every second (sample is hot!)
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Starting with a 1.35 mg sample of Pu-236,
calculate the mass that will remain after 5.00
years
mass Pu-236 1.35 mg, t 5.00 yr, t1/2 2.86
yr mass, mg
Given Find
Concept Plan Relationships
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Starting with a 1.35 mg sample of Pu-236,
calculate the mass that will remain after 5.00
years
Solve
units are correct, the magnitude makes sense
since it is less than ½ the original mass for
longer than 1 half-life
Check
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An ancient skull gives 4.50 dis/mingC. If a
living organism gives 15.3 dis/mingC, how old is
the skull? 14C-t1/2 5730 yr
ratet 4.50 dis/mingC, ratet 15.3
dis/mingC time, yr
Given Find
Concept Plan Relationships
Solve
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An ancient skull gives 4.50 dis/mingC. If a
living organism gives 15.3 dis/mingC, how old is
the skull? 14C-t1/2 5730 yr
Solve
units are correct, the magnitude makes sense
since it is less than 2 half-lives
Check
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An artifact containing carbon taken from the tomb
of a king of ancient Egypt gave 8.1 dpm/gC. How
old is the artifact? Carbon from a living
organism gives 15.3 dis/mingC 14C-t1/2 5730
yr.
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Artificial Nuclear Reactions

• bombardment of one nucleus with another (2H, 4He,
  10B, 12C) causing new atoms to be made
• can also bombard with neutrons protons
• reaction done in a particle accelerator
• linear
• cyclotron
• Tc-97 is made by bombarding Mo-96 with
  deuterium, releasing a neutron

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Artificial Nuclear Reactions
Reactions using neutrons are called n,g reactions
because a g ray is usually emitted. Radioisotopes
used in medicine are often made by n,g reactions.

• An example of a n,g reaction is production of
  radioactive 31P for use in studies of P uptake in
  the body.
• 3115P 10n ? 3215P g

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Transuranium Elements

• Elements beyond 92 (transuranium) made starting
  with an n, g reaction
• 23892U 10n ? 23992U g
• 23992U ? 23993Np 0-1b
• 23993Np ? 23994Pu 0-1b

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Q. 56Fe when bombarded with deuterium, produces
54Mn and one other particle. Write a balanced
equation for the reaction identify the other
particle.
5626He 21H ? 5425Mn ?
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Medical Uses of Radioisotopes
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Nonmedical Uses of Radioactive Isotopes

• smoke detectors
• Am-241
• smoke blocks ionized air, breaks circuit
• insect control
• sterilize males
• food preservation
• radioactive tracers
• follow progress of a tagged atom in a reaction

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Nonmedical Uses of Radioactive Isotopes

• authenticating art object
• many older pigments and ceramics were made from
  minerals with small amounts of radioisotopes
• crime scene investigation
• measure thickness or condition of industrial
  materials
• corrosion
• track flow through process
• gauges in high temp processes
• weld defects in pipelines
• road thickness