Transcript Nuclear Equations, Radioactivity, and Fission/Fusion

Nuclear
Equations,
Radioactivity, and
Fission/Fusion
1
Facts About the Nucleus
 Very small volume compared to
volume of the whole atom
 Essentially entire mass of atom
 Very dense
 Composed of protons and
neutrons that are tightly held
together
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Nucleons
Facts About the Nucleus, Con’t
Every atom of an element has the same number
of protons; equal to the atomic number
 Atoms of the same elements can have different
numbers of neutrons.
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Isotopes
Different atomic masses
Isotopes are identified by their mass number
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Mass number = number of protons + neutrons
Facts About the Nucleus, Con’t
The number of neutrons is calculated by
subtracting the atomic number from the mass
number.
 The nucleus of an isotope is called a nuclide.
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Over 90% of isotopes are radioactive.
Therefore, their nucleus is called a radionuclide
Each nuclide is identified by a symbol.
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Element − mass number.
Uranium  238
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Radioactivity

Radioactive nuclei (radionuclides) spontaneously
decompose into smaller nuclei
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Is called Radioactive Decay
We say that radioactive nuclei are unstable
The parent nuclide is the nucleus that is undergoing
radioactive decay; the daughter nuclide are the new
nuclei that are made
Decomposing involves the nuclide emitting a particle
(α, β, etc.) and/or energy
All nuclides with 84 or more protons are radioactive
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Transmutation

Rutherford discovered that during the
radioactive process, atoms of one element are
changed into atoms of a different element—
transmutation.
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In order for one element to change into another,
the number of protons in the nucleus must
change.
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Chemical Processes vs.
Nuclear Processes

Chemical reactions involve changes in the
electronic structure of the atom.
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Nuclear reactions involve changes in the
structure of the nucleus.
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Atoms gain, lose, or share electrons.
No change in the nuclei occurs.
When the number of protons in the nucleus
changes, the atom becomes a different element.
Nuclear Equations
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We describe nuclear processes using nuclear
equations.
Use the symbol of the nuclide to represent the
nucleus.
Atomic numbers and mass numbers are
conserved.
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Use this fact to predict the daughter nuclide if you
know parent and emitted particle.
Alpha decay:
238
92
U
234
90
Th +
4
2
He
SAME ON BOTH SIDES
Mass numbers: 238
Atomic numbers: 92
Beta decay:
234
90
Th 
234
91
Pa +
0
-1
e
SAME ON BOTH SIDES
Mass numbers: 234
Atomic numbers: 90
What Kind of Decay and How Many Protons and
Neutrons Are in the Daughter?
11 p+
9 n0
Alpha emission giving a daughter nuclide with
9 protons and 7 neutrons.
10
What Kind of Decay and How Many Protons and
Neutrons Are in the Daughter?, Continued
9 p+
12 n0
Beta emission giving a daughter nuclide with
10 protons and 11 neutrons.
11
What Kind of Decay and How Many Protons and
Neutrons Are in the Daughter?, Continued
5 p+
4 n0
Positron emission giving a daughter nuclide with
4 protons and 5 neutrons.
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Nuclear Equations
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In the nuclear equation, mass numbers and
atomic numbers are conserved.
We can use this fact to determine the identity of
a daughter nuclide if we know the parent and
mode of decay.
Practice—Write a Nuclear Equation for Each of
the Following:
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Alpha emission from Th-238.
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Beta emission from Ne-24.
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Positron emission from N-13.
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Practice—Write a Nuclear Equation for Each of
the Following, Continued:
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Alpha emission from Th-238
238 U  4 He  234Th
92
2
90
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Beta emission from Ne-24
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Positron emission from N-13
24 Ne  0 β  24Na
10
-1
11
13 N  0 β  13C
7
1
6
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Detecting Radioactivity
To detect when a phenomenon is present,
you need to identify what it does:
Radioactive rays can expose light-protected
photographic film.
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1.
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Use photographic film to detect the presence of
radioactive rays — film badges.
Detecting Radioactivity, Con’t
2. Radioactive rays cause air to become ionized.
 An electroscope detects radiation by its ability to
penetrate the flask and ionize the air inside.
 Geiger-Müller counter works by counting
electrons generated when Ar gas atoms are
ionized by radioactive rays.
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Detecting Radioactivity, Con’t
3. Radioactive rays cause certain chemicals to give
off a flash of light when they strike the chemical.
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A scintillation counter is able to count the number of
flashes per minute
Able to measure alpha and beta particles only
Natural Radioactivity
There are small amounts of radioactive
minerals in the air, ground, and water.
 It’s even in the food you eat!
 The radiation you are exposed to from
natural sources is called background
radiation.
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Half-Life
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Each radioactive isotope decays at a unique
rate.
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Some fast, some slow.
Not all the atoms of an isotope change
simultaneously.
Rate is a measure of how many of them change in a
given period of time.
Measured in counts per minute, or grams per time.
The length of time it takes for half of the parent
nuclides in a sample to undergo radioactive
decay is called the half-life.
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Half-Lives of Various Nuclides
Nuclide
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Half-life
Type of decay
Th-232
1.4 x 1010 yr
Alpha
U-238
4.5 x 109 yr
Alpha
C-14
5730 yr
Beta
Rn-220
55.6 sec
Alpha
Th-219
1.05 x 10–6 sec
Alpha
Half-Life
Half of the radioactive atoms decay each half-life.
Radioactive decay
100
Percentage of original sample
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90
80
70
60
50
40
30
20
10
0
0
22
1
2
3
4
5
6
Time (half-lives)
7
8
9
10
Decay of Au-198
half-life = 2.7 days
Radioactivity (cpm.)
60000
50000
40000
30000
20000
10000
0
0
2
4
6
8
10
12
14
Time (days)
23
16
18
20
22
How Long Is the Half-Life of this
Radionuclide?
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Practice—Radon-222 Is a Gas that Is Suspected
of Causing Lung Cancer as It Leaks into Houses.
It Is Produced by Uranium Decay. Assuming No
Loss or Gain from Leakage, if There Is 1024 g of
Rn-222 in the House Today, How Much Will
There be in 5.4 Weeks?
(Rn-222 Half-Life Is 3.8 Days.)
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Practice—Radon-222 Is a Gas that Is Suspected of Causing Lung
Cancer as It Leaks into Houses. It Is Produced by Uranium
Decay. Assuming No Loss or Gain from Leakage, if There Is
1024 g of Rn-222 in the House Today, How Much Will There be
in 5.4 Weeks? ( Rn-222 Half-Life Is 3.8 Days.), Continued
5.4 weeks x 7 days/wk = 37.8  38 days
Amount of
Rn-222
Number of
Half-lives
Time
(days)
Amount of
Rn-222
1024 g
0
0
512 g
1
256 g
Number of
Half-lives
Time
(days)
16 g
6
22.8
3.8
8g
7
26.6
2
7.6
4g
8
30.4
128 g
3
11.4
2g
9
34.2
64 g
4
15.2
1g
10
38
32 g26
5
19.0
Practice — How Much of a Radioactive
Isotope, Rn-222 (with Half-Life of 10
Minutes) Did You Start with if, After
One Hour if You Have 2 g?
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Practice—How Much of a Radioactive Isotope, Rn222(with Half-Life of 10 Minutes) Did You Start with if,
After One Hour if You Have 2 g?, Continued
Fill in the “Number of half-lives” and “Time…” columns first, then work
backwards up the “Amount…” column.
Amount Number of
of Rn-222 half-lives
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128 g
64 g
32 g
16 g
8g
4g
2g
0
1
2
3
4
5
6
Time
(min)
0
10
20
30
40
50
60
Nonradioactive Nuclear Changes
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A few nuclei are so unstable, that if their nuclei
are 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
smash together to make a larger nucleus. This is
called fusion.
Both fission and fusion release enormous
amounts of energy.
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Fusion releases more energy per gram than fission.
Fission
+ energy!!
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Fission Chain Reaction
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A Fission Chain Reaction is the process by which
neutrons from one reaction cause the fission process
to keep continuing
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Only small number of neutrons needed
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.
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Fission Chain Reaction, Con’t
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Fusion
+
2
1H
+
3
1H
deuterium + tritium
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4
2He
1
0n
helium-4 + neutron