HChemChapter18NuclearChemistry

Copyright©2004 by Houghton Mifflin
Company. All rights reserved.
1
Radioactivity and
Nuclear energy
Chapter 18

Wooden
artifacts such
as this dragon
figurehead
from a Viking
Ship can be
dated from
their carbon-
14 content.
Source:
Leslie Garland/Leslie
Garland Picture
Library/Alamy Images

Copyright©2004 by Houghton
Mifflin Company. All rights reserved.
3
Facts About the Nucleus
• Very small volume compared to volume of atom
• Essentially entire mass of atom
– Very dense
• Composed of protons and neutrons that are tightly
held together
– Nucleons
• Every atom of an element has the same number of
protons
– Atomic Number
• Isotopes are atoms of the same elements that have
different masses
– Different numbers of neutrons
– Mass Number = number of protons + neutrons

4
Facts About the Nucleus
• The number of neutrons is calculated by
subtracting the atomic number from the
mass number
• The nucleus of an isotope is called a
nuclide
– less than 10% of the known nuclides are
nonradioactive, most are radionuclides
• Each nuclide is identified by a symbol
– Element -Mass Number = X-Z
Element AX
Z
mass number
atomic number =

Mifflin Company. All rights
reserved.
5

6

7
Radioactivity
• Radioactive nuclei spontaneously decompose into
smaller nuclei
– Radioactive decay
– We say that radioactive nuclei are unstable
• Decomposing involves the nuclide emitting a particle
and/or energy
• During radioactive decay, atoms of one element are
changed into atoms of a different element
– In order for one element to change into another, the
number of protons in the nucleus must change
– All nuclides with 84 or more protons are radioactive
• We describe nuclear changes with using nuclear
equations
– atomic numbers and mass numbers are conserved

reserved.
8

9
Bone scintigraph of a patient's
cranium following administration of the
radiopharmaceutical Technetium-99.
Source: Kopal/Mediamed Publiphoto/Photo Researchers, Inc.

reserved.
10

222
88 ® +
234
90 ® + -
Th e 234Pa
11
alpha decay
• an a particle contains 2 protons and 2 neutrons
– helium nucleus
• loss of an alpha particle means
– atomic number decreases by 2
– mass number decreases by 4
Ra He 216Rn
86
42
beta decay • a b particle is like an electron
moving much faster
found in the nucleus
• when an atom loses a b particle its
atomic number increases by 1
mass number remains the same
• in beta decay a neutron changes into a proton
91
0
1

12
gamma emission
• Gamma (g) rays are high energy
photons
• Gamma emission occurs when the
nucleus rearranges
• No loss of particles from the nucleus
• No change in the composition of the
nucleus
– Same atomic number and mass number
• Generally occurs whenever the
nucleus undergoes some other type
of decay

reserved.
13
Penetration Power Comparison

22
11 ® + +
200
80 + ® -
14
positron emission
• positron has a charge of +1 c.u. 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
• positrons appear to result from a proton
changing into a neutron
Na e 22Ne
10
0
1
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
Hg e 200Au
79
0
1

238
92 + ® +
15
Artificial Nuclear
Transformation
• Nuclear transformation involves
changing one element into another by
bombarding it with small nuclei, protons
or neutrons
• reaction done in a particle accelerator
– linear
– cyclotron
• made-made transuranium elements
U X 4 n 246Cf
98
1
0
12
6

16

17
Figure 18.2: A schematic
representation of a Geiger –
Müller counter.

18
Detecting Radioactivity
• To detect something, you need to identify
something it does
• radioactive rays cause air to become ionized
• Geiger-Müller Counter works by counting
electrons generated when Ar gas atoms are
ionized by radioactive rays
• radioactive rays cause certain chemicals to
give off a flash of light when they strike the
chemical
• a scintillation counter is able to count the
number of flashes per minute

19
Half-Life
• Not all radionuclides in a sample decay at once
• The length of time it takes one-half the
radionuclides to decay is called the half-life
• Even though the number of radionuclides changes,
the length of time it takes for half of them to decay
does not
– the half-life of a radionuclide is constant
• Each radionuclide has its own, unique half-life
• The radionuclide with the shortest half-life will have
the greater number of decays per minute
– For samples of equal numbers of radioactive
atoms

Researcher
taking a bone
sample for
carbon-14
dating at an
archeological
site in Egypt. Source:
Mark W.
Philbrick/BYU

21
Half-Life
• half of the radioactive atoms decay each
half-life
Radioactive Decay
100
90
80
70
60
50
40
30
20
10
0
0 1 2 3 4 5 6 7 8 9 10
time (half-lives)
percentage of original sample

22

23
Object Dating
• mineral (geological)
– compare the amount of U-238 to Pb-206
– compare amount of K-40 to Ar-40
• archeological (once living materials)
– compare the amount of C-14 to C-12
– C-14 radioactive with half-life = 5730 yrs.
– while living, C-14/C-12 fairly constant
• CO2 in air ultimate source of all C in body
• atmospheric chemistry keeps producing C-14 at the same rate it
decays
– once dies, C-14/C-12 ratio decreases
– limit up to 50,000 years

Medical Uses of Radioisotopes,
24
Diagnosis
• radiotracers
– certain organs absorb most or all of a particular
element
– can measure the amount absorbed by using
tagged isotopes of the element and a Geiger
counter
– use radioisotope with short half-life
– use radioisotope low ionizing
• beta or gamma

Figure 18.3: After consumption of
Na131I, the patient’s thyroid is scanned
for radioactivity levels to determine the
25
efficiency of iodine absorption.

26

27
Other Nuclear Changes
• a few nuclei are so unstable, that if their
nucleus is hit just right by a neutron, the
large nucleus splits into two smaller
nuclei - this is called fission
• small nuclei can be accelerated to such
a degree that they overcome their
charge repulsion and are smashed
together to make a larger nucleus - this
is called fusion
• both fission and fusion release
enormous amounts of energy

Figure 18.4: Upon capturing a neutron, the U
235
nucleus undergoes fission 92
to produce two
lighter nuclides, more neutrons (typically three),
28
and a large amount of energy.

29
Fissionable Material
• fissionable isotopes include U-235, Pu-239,
and Pu-240
• natural uranium is less than 1% U-235
– rest mostly U-238
– not enough U-235 to sustain chain reaction
• fission produces about 2.1 x 1013 J/mol of U-
235
– 26 million times the energy of burning 1 mole
CH4
• to produce fissionable uranium the natural
uranium must be enriched in U-235

30
Fission Chain Reaction
• a chain reaction occurs when a reactant is
also a product
– in the fission process it is the neutrons
– only need a small amount of neutrons to keep
the chain going
• many of the neutrons produced in the fission
are either ejected from the uranium before
they hit another U-235 or are absorbed by the
surrounding U-238
• minimum amount of fissionable isotope
needed to sustain the chain reaction is called
the critical mass

Figure 18.5:
Representation
of a fission
process in
which each
event produces
two neutrons
that can go on
to split other
nuclei, leading
to a
self-sustaining
chain reaction.

32
Nuclear Power Plants
• use fission of U-235 or Pu-240 to make heat
• heat picked up by coolant and transferred to
the boiler
• in the boiler the heat boils water, changes it to
steam, which turns a turbine, which generates
electricity
• the fission reaction takes place in the reactor
core

Figure 18.6: A schematic diagram
33
of a nuclear power plant.

34

35

36
Nuclear Power Plants - Core
• the fissionable material is stored in long
tubes arranged in a matrix called fuel rods
– subcritical
• between the fuel rods are control rods made
of neutron absorbing material
– B and/or Cd
– neutrons needed to sustain the chain reaction
• the rods are placed in a material used to slow
down the ejected neutrons called a
moderator
– allows chain reaction to occur below critical mass

The core of
a nuclear
power
plant.

Figure 18.7:
A schematic
of a reactor
core.

39
Breeder Reactor
• Design common in Europe
• Makes its own fuel by converting U-238 to Pu-
239
• Use liquid sodium as a moderator
• Use water filled radiator to transfer heat to
boiler
• Plutonium highly toxic and spontaneously
combusts in air

Figure 18.9: A
schematic diagram
for the tentative plan
for deep
underground
isolation of nuclear
waste.

41
Nuclear Fusion
• Fusion is the process of combining two
light nuclei to form a heavier nucleus
• The sun’s energy comes from fusion of
hydrogen to produce helium
• Releases more energy per gram than
fission
• Requires high temperatures and large
amounts of energy to initiate, but should
continue if you can get it started

A portion of
the Cygnus
Loop
supernova
remnant
taken by
the Hubble
Space
Telescope.

A solar
flare
erupts
from the
surface of
the sun.
Source:
NASA

44
Factors that Determine
Biological Effects of Radiation
The more energy the radiation has the larger its effect
can be
The better the ionizing radiation penetrates human
tissue, the deeper effect it can have
– Gamma Beta Alpha
The more ionizing the radiation, the more effect the
radiation has
– Alpha Beta Gamma
The radioactive half-life of the radionuclide
° The biological half-life of the element
± The physical state of the radioactive material
• The amount of danger to humans of radiation is
measured in the unit rems

Figure 18.8:
Radioactive
particles and
rays vary
greatly in
penetrating
power.

46
Somatic Damage
• Somatic Damage is damage which has
an impact on the organism
– Sickness or Death
• May be seen immediately or in the future
– Depends on the amount of exposure
– Future effects include cancer

47

48
Genetic Damage
• Genetic Damage occurs when the
radiation causes damage to reproductive
cells or organs resulting in damage to
future offspring

49

Figure 18.1: The decay series from
50
U to Pb.
238
92
206
82

HChemChapter18NuclearChemistry

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