Transcript Alpha decay

Nuclear Chemistry
The weird world of the nucleus
Isotope Refresher


Atoms of the same
element all have the
same number of
protons
Atoms of the same
element may have
different neutrons and
therefore mass
numbers (p+ + n0)
235
92
U
Nuclear Instability
Not all combinations of protons and
neutrons are created equal
 Some are more unstable than others.
 If they are unstable they will do one of the
following:

Radioactive decay
 Nuclear fission
 Nuclear fusion

Nuclear “Reactions”
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Nuclear “reactions” must still be balanced.
U  He  Th
238
92
4
2
234
90
Notice 238 = 4 + 234 “Mass balanced”
 Notice 92 = 2 + 90 “Charge balanced”
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Radioactive Decay
Radioactive Decay

Radioactive decay – the nucleus of an
atom undergoes a change so that it is no
longer the same element

Decay is a totally random event. Nothing
has an effect when an atom decays
Two Main Types of Radioactive Decay
1. Alpha decay
2. Beta decay
Alpha Decay
Alpha decay – emission of an alpha
particle from the nucleus
 Alpha (α) particle - a helium-4 nucleus
4
2
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He
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Uranium-235, “enriched uranium,” decays
by alpha decay
U  He  Th
235
92
4
2
231
90
Beta Decay
Beta decay – emission of a beta particle
from the nucleus
0 
 Beta (β) particle – electron
1

e

Thorium-231 decays by beta decay
0 
1
Th e  Pa
231
90
231
91
Practice Decay Reactions

Beta decay by cesium-137
0 
1
Cs e  Ba
137
55

137
56
Alpha decay by polonium-210
210
84
Po He  Pb
4
2
206
82
Decay Series

Some atoms require
multiple decays to
become stable

Most decay series of
naturally occurring
isotopes end in lead.
Half Life
Half Life

Half life – the amount of time it takes for ½
of a radioactive isotope to decay into
something else.

Notice the atoms don’t disappear, they just
change their identity.
Half Life Problem
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A 100.0g radioactive
sample decays for 5
hours. Only 12.5g of
the original isotope
remains after 5 hours.
How long is the half
life?
3 half lives = 5 hours
5hours/3half lives =
1.67hours/halflife
Number
of half
lives
Mass of
sample
Time
0
100.0g
0
1
50.0g
2
25.0g
3
12.5g
5 hours
Half Life
Problem
Selenium-72 has a half
life of 8.40 days.
How much of a
450.0g sample of
selenium-72 will
remain after 42.0
days?
Number
of half
lives
Mass of
sample
Time
0
450.0g
0
1
225.0g
8.40 days
2
112.5g
16.8 days
3
56.25g
33.6 days
25.2 days
4
28.13g
33.6 days
5
14.06g
42.0 days
Radiocarbon Dating Technique

Uses the known half life of C-14 to
estimate death of organic matter

Based on the known ratio of C-14 to C-12
Nuclear Fission and Fusion
Nuclear Fission

Nuclear fission – one atom’s nucleus splits
apart.
U  n Ba  Kr 3 n
235
92
1
0
141
56
92
36
1
0
A neutron strikes a nucleus causing it to
split into small pieces
 Releases lots of energy.
 Extra neutrons are also produced.
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Nuclear Fission

The extra neutrons can cause a chain reaction
Uses of Nuclear Fission

Nuclear power
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Energy produced by
fission is transferred
as heat to a coolant
Nuclear Power
Uses of Nuclear Fission
Mushroom cloud from Nagasaki
Uses of Nuclear Fission
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Nuclear weaponry
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“Atomic bomb”
“A-bomb”
Runaway fission
reaction resulting in
an explosion.
Nuclear Fusion

Fusion – combining of nuclei
0 
1
1
1
H H H e
2
1
H  H  He
1
1
3
2
2
1
1
1
3
2
He  He  He  H  H
3
2
4
2
1
1
1
1
Above reactions describe fusion in the Sun.
 Releases more energy than fission.
 Also requires very high temperatures.

Uses of Nuclear Fusion
Uses of Nuclear Fusion

Fusion weapons

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Hydrogen bomb
“H-bomb”
Thermonuclear bomb
Uses a fission reaction to
start the fusion reaction
“A-bomb” is the detonator for
an “H-bomb”
Radiation
3 Primary Types of Radiation
Alpha radiation – α – alpha particles
1.
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Low energy
Result of alpha decay
Low penetration
Stop with a sheet of paper
Beta radiation – β –beta particles (electrons)
2.
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Higher energy
Result of beta decay
Some penetration ability
Stop with several sheets of aluminum foil
Gamma radiation – γ –high energy gamma rays
3.
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Very high energy
Can result from any type of decay
Very highly penetrating and damaging
Stop with several centimeters of lead or very thick concrete
3 Primary Types of Radiation
Sources of Human Exposure