Download CHAPTER 11 – NUCLEAR CHEMISTRY

CHAPTER 11 – NUCLEAR CHEMISTRY
Stable and Unstable Nuclei:
- stable nuclei do not change
- unstable nuclei breaks down spontaneously to give off rays
and particles
- Stability depends on ratio of neutrons to protons
For elements of small atomic number (1 – 20):
- when the ratio of neutrons to protons is about 1, nucleus of
the isotope is stable
(Remember, an isotope has same atomic number but different
mass number – this means a different number of neutrons!!)
Ex: Determine if the isotope
16
Ex: Determine if the isotope
12
O is stable:
C is stable:
1
For elements with higher atomic numbers (21 or higher):
- nucleus is generally stable when the ratio of neutrons to
protons is greater than 1
Ex:
56
Fe
The ratio for stability of the heaviest stable isotopes is
1.5:
Ex: 206Pb
Radioactivity:
- the nucleus of certain atoms breaks down
spontaneously and gives off rays and particles (decay).
- See Reference Table O
1. Alpha Decay:
- nucleus decays and gives off alpha particles
- alpha particle is a helium nucleus ( 4He)
- symbol is 
2
Ex:
226
Ra
222
Rn +
4
He
- Atomic number on the left side of the arrow (88) must
equal all atomic numbers on right side of arrow (86 +
2).
- Mass number on the left side of the arrow (226) must
equal all the mass numbers on the right side of the
arrow (226 and 4).
- A new element is produced
2. Beta Decay:
- nucleus decays and gives off a beta particle
- beta particle is a high speed electron (0e)
- symbol is 
Ex:
234
Th
234
Pa +
0
e
- Atomic number on the left side of arrow (90) is equal
to atomic number on right side of arrow (91 and –1 =
90)
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- Mass number on the left side of the arrow (234) is
equal to all the mass numbers on the right side of the
arrow (234 and 0).
- New element is produced.
3. Positron Decay:
- nucleus decays and gives off a positron
- no mass, positive charge (0e)
- symbol is +
(In case you were wondering…
Transmutation is when the nucleus of an atom decays and one
element changes into another element)
4.Gamma Rays:
- nucleus decays and gives off gamma rays
- gamma rays are similar to high energy x-rays
- gamma rays are not particles (no mass and no charge!)
- symbol is 0
Decay Mode:
- each radioactive isotope decays and gives off either alpha,
beta, gamma, or positron
- decay mode of an isotope is dependent on particle/ray given
off
Ex:
4
Alpha decay mode means that the isotope decays and gives off
alpha particles
Ex:
Determine the decay mode of the following isotope and finish
the equation:
234
Th
234
Pa +
0
e
Separating alpha particles, beta particles, and gamma rays:
- use an electric field
- positive alpha particle will be deflected toward the
negative electrode
- negative beta particle will be deflected toward the
positive electrode
- neutral gamma ray will not be deflected
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+ electrode
Beta
Lead
Block
Gamma
Alpha
Radioactive
Source
- electrode
Half-Life (a.k.a. Rate of Decay):
- the time required for half of a sample to decay
- SEE TABLE N!
Ex:
It takes 2.69 days for half of a
198
Au sample to decay.
If you have 10 grams of Au, in 2.69 days, 5 grams will
decay and you will only have 5 grams left (1/2 x 10
grams).
In another 2.69 days, half of what you have left will
decay, and you will only have 2.5 grams left (1/2 x 5
grams).
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In another 2.69 days, half of what you have left will
decay, and you will only have 1.25 grams left (1/2 x 2.5
grams).
In 8.07 days, your sample has decayed from 10 grams to
1.25 grams.
To determine the fraction of
following formula:
198
Au remaining, use the
Fraction Remaining = Mass Remaining
Original Mass
1 ¼ grams /10 grams = 1/8 remaining
You can use the following formulas to find:
1. Fraction remaining
2. Half life
3. Initial Amount
Formula 1:
Fraction Remaining = (1/2)t/T
Number of half life periods = t/T
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Where
t = total time elapsed
T = half life
Example:
To find fraction remaining:
131
I has a half life of 8.07 days. A 10 gram sample was allowed
to decay for 32 days. What fraction will remain?
To find Initial Amount:
A radioisotope has a half life of 10 days. 1 gram remains
after 40 days. What was the initial amount of the
radioisotope?
Fraction Remaining = (1/2)t/T
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To find half life:
100 grams of isotope decayed to 12 ½ grams after 90.7 years.
What was the half life?
Transmutation:
1. Natural Transmutation:
- nucleus spontaneously decays and changes into a new
element
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2. Artificial Transmutation:
- transformation of one element into another element by
bombarding the nucleus with high energy particles (i.e.,
protons, neutrons, and alpha particles
Ex:
27
27
Al is bombarded with alpha particles
Al +
4
He
30
P + 1n
Left side atomic numbers (13 + 2) must equal right side atomic
numbers (15 + 0)
Left side atomic masses (27 + 4) must equal right side atomic
masses (30 + 1)
Ex:
9
Be + 1H
What is X?
X +
4
He
10
Ex:
Given the nuclear reaction:
27
30
Al + X
P + 1n
What is the identity of X?
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Chapter 11 continued….
Nuclear Reactions:
- mass is converted into tremendous amounts of energy
(nuclear energy)
- two types: fission and fusion
1. Fission Reactions:
- one atom splits into two or more pieces
- gives off a lot of energy
- one atom absorbs a neutron, splits into pieces, and gives off
energy
Ex:
235
U + 1n
[236U]
141
Ba +
92
Kr
+
3 1n + energy
a. Uranium is bombarded with a neutron.
b. Neutron is captured by uranium nucleus.
c. An unstable intermediate nucleus is formed.
d. Nucleus splits into several pieces (141Ba + 92Kr + 3
neutrons).
e. Some of the mass is changed to energy.
f. The three neutrons that are given off bombard other
uranium nuclei.
g. More nuclei split to form more neutrons that bombard more
uranium nuclei.
h. If fission continues, it is an uncontrolled chain reaction.
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- Ex: atomic bomb
- the sum of the masses of the products formed is less than
the mass of the original, because some mass is converted
into energy
2. Fusion Reactions:
- two nuclei unite to form a heavier nucleus
Ex:
2
4
H + 2H
He
- produces energy
- high temperature and high pressure is necessary to give
nuclei enough energy to unite (this is because nuclei are
positive and tend to repel each other)
- Ex: hydrogen bomb
- mass of the nucleus formed is less than the sum of the
masses of the reactant nuclei, because some of the mass is
converted into energy
- fusion reaction energy is much more than that of an fission
reaction
Comparison of Nuclear (Fission and Fusion) and Chemical
Reactions:
- energy released in nuclear reaction is much more than that
released in chemical reactions
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Benefits and Risks of Fission and Fusion:
Benefits:
- provide energy
Ex: Energy from fission reactions is used for electric
power
Ex: Fusion reactions take place in the sun and produce
solar energy
Risks:
- wastes from nuclear reactors are very radioactive
- waste must be stored for more than 100 thousand years
without leaking into the environment
- accidents in reactors can release dangerous levels of
radioactivity
Benefits of Radioactive Isotopes (Radioisotopes):
(Remember: radioisotopes are radioactive….which means they
give off rays and particles!!!)
1. Tracers:
- used to follow the course of a chemical or biological
reaction
Ex: carbon-14
2. Medical:
- radioisotopes can be used in medical diagnosis and
treatment of diseases
- radioisotope must have a very short half-life
- radioisotope must be eliminated quickly from the body
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Ex:
Technetium-99 (Tc-99): pinpointing of brain tumors
Iodine-131 (I-131): diagnosis and treatment of thyroid
disorders
Radium (Ra) and cobolt-60 (Co-60): treatment of cancer
3. Food can be stored longer.
- radiation kills bacteria, yeast and molds in food
4. Radioactive Dating:
a. Geologic Dating:
- based on half-life
Ex:
The ratio of Uranium-238 to Lead-206 is used to
find the age of the rock or age of a geological
formation
b. Dating Living Materials (organisms that were
previously alive):
- ratio of carbon-14 to carbon-12 can determine the
age of wood, bone, animal skin, fabric
5. Nuclear Power:
- reactors are used to produce electricity
6. Industrial Measurement:
- use a beam of subatomic particles (ex: alpha, beta,
gamma) for measurements
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- beam is blocked at a certain thickness of metal
- by measuring fraction of the beam that is blocked, a
precise measurement of the metal thickness can be
made
Risks of Radioactivity or Radioactive Isotopes:
1. Biological Damage:
- exposure can damage/destroy cells of organisms (burns,
cataract, cancer)
2. Long Term Storage:
- Nuclear reactors produce highly radioactive waste
- Gaseous waste must be stored until it is no longer
radioactive
- Solid and liquid waste must be stored for > 100 thousand
years
- still unknown if these substances can be stored for so
long without escaping and harming people
3. Accidents:
- can cause fuel and wastes to escape from a nuclear
reactor
- Ex: Chernobyl
4. Pollution:
- traces of radioactive materials are in air, water, soil
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