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Nuclear Chemistry
DHS Chemistry
Chapters 4 and 25
Little House on the Prarie
Review:
Structure of an Atom
Structure of an Atom
 An atom consists of three types
of subatomic particles, protons,
neutrons, and electrons.
Structure of an Atom
• Protons and neutrons are located
in the nucleus
• Electrons are in a cloud
surrounding the nucleus.
• The number of protons is equal
to the atomic number.
Structure of an Atom
 If the atom is neutral, the
number of electrons is equal to
the number of protons.
 The mass number is equal to
number of protons + number of
neutrons.
Structure of an Atom
• Elements with the same number
of protons, but have different
numbers of neutrons is called an
isotope.
Structure of an Atom
Protons
Neutrons
Electrons
Symbol
P+
no
e-
Charge
+
0
-
Location
Nucleus
Nucleus
Electron
cloud
Relative
mass
1 amu
1 amu
negligible
What’s a
chemical
reaction?
Chemical Reactions
Why Call it Nuclear Chemistry?
See unitedstreaming.com
Chemical VS. Nuclear
• Nuclear reactions are caused from
unstable nuclei becoming stable through
radioactive decay.
• Releasing particles and high-energy waves
• Alters the number of nuclear particles (neutrons
and protons).
• Nuclear reactions are very different from chemical
reactions.
Chemical Changes
1. Occur when bonds are broken and formed.
2. Atoms remain unchanged, though they may
be rearranged.
3. Involve only valence electrons.
4. Associated with small energy changes.
5. Reaction rate is influenced by temperature,
pressure, concentration, and catalyst.
Nuclear Changes
1. Occur when nuclei emit particles and/or
rays.
2. Atoms are often converted into atoms of
another element. Thus their identity
changes.
3. May involve protons, neutrons, and
electrons.
4. Associated with large energy changes.
5. Reaction rate is not normally affected by
temperature, pressure, or catalysts.
What is an isotope?
Isotope Examples
6
Mass Numbers
Li
3
Atomic # = protons
Lithium - 6
7
Li
3
Lithium - 7
Isotopes
• Isotopes are atoms of the same
element that vary in their number of
neutrons, thus they have different
mass numbers.
• The convention for writing isotope
names is first the element name
dash and then the mass number.
For example:
Carbon-14, carbon-12 carbon-13
How many protons and neutrons does each of the
isotopes of carbon have?
• Carbon – 14
• Carbon – 12
• Carbon – 13
8 neutrons, 6 protons
6 neutrons, 6 protons
7 neutrons, 6 protons
Mass Number
(protons + neutrons)
More on Isotopes
• When an isotope is unstable it is
called a radioisotope.
• To gain a more stable
configuration, the nuclei emit
radiation.
• The resulting stable atom is called
the daughter product.
• This is called radioactive
decay.
Strong Nuclear Force
• The dense nucleus has two different
kinds of nuclear particles (protons +
neutrons) closed packed together.
• The protons are positively charged
nucleons.
• The neutrons are neutral nucleons.
• All of the protons in the nucleus repel
each other and cause an
electrostatic force that pushes
the nucleus apart.
• However, there is a force holding the
nucleus together.
• The strong nuclear force is a force that
acts only on subatomic particles that are
extremely close together.
• If the strong nuclear force overcomes
the electrostatic force, the
nucleus stays together.
Neutron to Proton Ratio
• The strong nuclear force is not always
strong enough to overcome the
electrostatic force. When this happens
the nucleus breaks apart. The stability
of the nucleus can be determined by
the ratio of neutrons to protons.(n/p)
Neutron to Proton Ratio
• Elements with low atomic numbers (< 20)
are most stable when the neutron to
proton ratio is 1:1.
• As the atomic number increases, more
and more neutrons are needed to
overcome the electrostatic force.
• Thus, the stable ratio of neutron to proton
increases as the atomic number
increases.
• 1.5:1 is the largest ratio for a stable
nucleus.
Example: Determine the neutron to
proton ratio for Lead-206. Is it stable?
Lead – 206
82 protons
124 neutrons (206 – 82)
124 = 1.51  stable
82
1
ratio
Nuclear Stability
Neutron : Protons
Stable if:
• Smaller than Fe
~1
1
• Between Fe & Pb
1-1.5
1
• Bigger than Pb
1.5
1
Extra Practice
• Calculate the neutron to proton
ratio, and determine if the
isotope is stable.
1.
2:2  1:1
stable
201:92  2.18:1
unstable
3. Lead-206
stable
124:82  1.5:1
2. U- 293
The Band of Stability
• The graph on the right
plots the neutrons versus
protons. The band
created is called the band
of stability. Anything that
falls outside of that band
is radioactive.
• All elements with an
atomic number greater
than 83 are radioactive.
III. Radioactivity
Radioactive Substances
• Bananas
• Atomic fire ball candies
• Colored gemstones (blue
topaz)
• Fiesta Ware
• Table ware that contained
unsafe amounts of radioactive
Uranium
• Uranium for color glaze. Up to
14% can be Uranium
Geiger Counter
Types of Radiation
The experiment
Types of Decay
• Nuclear reactions change an atom of
one element to an atom of another
element. This process is called
transmutation. In a nuclear reaction
there are three common types of
radiation that are emitted: alpha, beta,
and gamma. The first two are involved
in transmutation, changing the identity
of the atom.
Properties of Alpha, Beta, and Gamma Radiation
Property
Alpha(α)
Beta (β)
Gamma (γ)
Alpha
particles
Beta particles
High-energy
electromagnetic
radiation
Composition
Helium Nuclei
Electrons
Photons
Charge
2+
1-
0
Relative Mass
heaviest
lightest
0
Relative
Penetrating
Power
Blocked by
paper
Blocked by
metal foil
Not completely
blocked by lead
or concrete
Description of
Radiation
• Emit means to give off or release
Alpha Particles
• Alpha particles contain the same
composition as a helium nucleus. Out
of all of the radiation particles, alpha
particles move the slowest and are the
least penetrating. As a result of alpha
decay, the mass number decreases by 4
and the atomic number decreases by 2.
4
2
He
or

4
2
Alpha decay
Beta Particles
• Beta particles are similar to an electron
except they come from the unstable nucleus
of the atom. Beta particles are formed and
ejected when a neutron decays to a proton
and an electron. The proton stays in the
nucleus and the electron is the beta particle.
Beta emission is a constant flow of quick
moving electrons that can be stopped by a
metal foil. As a result of beta decay the
atomic number increases by one. The mass
number does not change.
Gamma Rays
• Gamma rays are short wavelengths
(photons) that move the quickest of all
the types of decay. They are very highenergy electromagnetic radiation.
These rays often are released at the
same time as an alpha or beta particle.
The ray is the energy lost in the
reaction. Gamma emission does not
affect the atomic number or the
mass number of the isotope.
Gamma Rays
Are very dangerous!!
(write that down)
C. Gamma ( ) Emission
• usually occurs along with other
forms of radiation.
• Gamma particle is emitted
• No change in mass number
• No change in atomic number
Practice Problems
1. Why is radiation given off?
2. What is the most penetrating particle?
3. What is a main difference between a
nuclear reaction and a chemical reaction?
4. Is Carbon-14 radioactive?
Why or Why not?
III. Balancing Nuclear
Reactions
Balancing Nuclear Reactions
• In nuclear reaction equations we account
for all of the changes in the mass number
and atomic mass that occur through the
decay of the nucleus.
• To verify this, we include the mass number
and atomic mass of every particle or atom
involved in the reaction.
Mass number
Atomic
number
Mass number
4
2
He
Atomic
number
0
1

Solving Problems
• When solving/balancing a nuclear reaction,
• Look to find the difference of the mass
numbers and atomic numbers between the
reactants and the products.
• This will indicate the particle that was
released or the atom that was formed.
• Make sure you have the same total mass
number and atomic number on
both sides of the equation.
Nuclear Equations
Nuclear Reactions
Ex 1: Write a balanced equation
for the alpha decay of
polonium-210
210
210
Po
Po
84
84
206
206
4
Pb
+
He
Pb
82
82
2
Ex 2: Write a balanced equation
for the beta decay of carbon-14
Practice
Fill in the blank with the proper
radiation particle or isotope
120
120
1)
Cs
Ba + ____
55
2)
150
64
3)
241
95
4)
Gd
Am
_____
56
146
62
Sm + _____
4
2
_____ + He
210
81
Tl +
4
2
He
Examples
• Ex 1. Write an equation for the alpha
decay of Protactinium-231
• Ex. 2
Bi  ____ Tl
211
83
207
81
III. Radioactive Decay Rates
Radioactive Decay Rates
• A. Half-Lives
• We measure radioactive decay in terms of
half- lives.
• A half life is the time it takes for half of a
radioactive sample to decay.
Candy Bar Bandits
• There is a candy bar left in the
teacher’s lounge. Every 5
minutes a teacher walks in, looks
at the candy bar, breaks the
candy bar in half and eats it. If
the candy bar originally had a
mass of 20 grams, how much is
left after 4 teachers have a
taste?
Keeping Track
0
1
2
3
4
5
6
7
8
A
T
20g
10g
5g
2.5g
1.25g
0
5 min
10 min
15 min
20 min
Candy Bar
Keeping Track
0
1
2
3
4
5
6
7
8
A
T
20g
10g
5g
2.5g
1.25g
0
5 min
10 min
15 min
20 min
Example
• The half life of Carbon-14 is approximately
5730 years. If you had 12g of Carbon-14
today, in 5730 years you would only have 6g.
The missing 6g decayed and turned into
Nitrogen 14. And, in another 5730 years you
will have 3g of Carbon-14 left and then in
5730 more years you would only have 1.5g of
Carbon-14 left. Every 5730 years your
sample is cut in half.
Practice
• What is the half-life of the sample in
the graph?
~11 billion years
Half-Lives
# of HLs
Remaining mass
Remaining mass
Ratio of
remaining mass
to original mass
1
Original Mass x (1/2)
Org. Mass x
(.5)1
0.5
2
Original Mass x (1/2) x (1/2)
Org. Mass x
(.5)2
0.25
3
Original Mass x
(1/2) x (1/2) x (1/2)
Org. Mass x
(.5)3
0.125
4
Original Mass x
(1/2) x (1/2) x (1/2) x (1/2)
Org. Mass x
(.5)4
0.0625
5
Original Mass x
Org. Mass x
(.5)5
0.03125
Original Mass x
Org. Mass x
(.5)6
0.015626
(1/2) x (1/2) x (1/2) x (1/2) x (1/2)
6
(1/2) x (1/2) x (1/2) (1/2) x (1/2) x (1/2)
In the box
Amount Remaining
= initial amount (1/2)HL
= initial amount (0.5HL)
HL = # of half lives
Total Time Passed = # of Half Lives
Time of one HL
Ex. 1 If gallium-68 has a half-life of 68.3
minutes, how much of a 10.0 mg sample is left
after 342 minutes?
HL
A
T
0
1
2
3
4
5
6
10mg
5mg
2.5mg
1.25mg
.625mg
0.3125
0
68.3
136.6
204.9
273.2
341.5
0.3125 mg
Ex. 1 If gallium-68 has a half-life of 68.3
minutes, how much of a 10.0 mg sample is left
after 342 minutes?
Amount Remaining = (Initial amount) (0.5) n
n = # of half lives that have passed
342 total minutes / 68.3 minutes =
Ans = 0.3125 mg
half lives
More Examples
Ex. 2 If the passing of
116 years leaves
25.00 mg of an
original 400 mg
sample of Strontium90, what is the half
life of Strontium-90?
116/4 = 29 years
HL
0
1
2
3
4
5
6
7
8
A
T
400
0
200
100
50
25mg 116y
Don’t get it? Here it is in words.
• If you are trying to solve for the amount of element
left, divide the time passed from the half life and that
will give you the amount of half lives your sample
has had. You can then divide your original mass of
sample by 2 as many times as you have half lives.
• If you are trying to solve for the half life of your
sample, take the remaining mass and count how
many times you have to multiple it by 2 to get your
original mass. That will tell you how many half lives
have elapsed. Take that number and divide it by the
total time that has elapsed to get the length
of just one half life.
More on Half-lives
• Because the half-life is constant,
radioisotopes can be used to date
objects.
• Radiochemical dating is the
process of determining the age of
an object based on the amount
of a particular radioisotope
is remaining in the object.
Carbon Dating
• Carbon dating is a specific type of
radiochemical dating
• All living organisms have the same
ratio of carbon-14, carbon-13, and
carbon-12 as the in atmosphere.
• However, once an organism dies and
there is no new carbon intake, the
unstable carbon-14 starts to
break down.
Practice
HL
A
0
1
2
3
4
5
6
7
8
2000
1000
500
250
1. Iron-59 is used in
T
medicine to diagnose
blood circulation
0
disorders. The half44.5
life of iron-59 is 44.5
89
days. How much of a
2000 mg sample will
133.5
remain after 133.5
days?
250 mg of
the sample
Practice
• After 2 years, 1.99 g of a radioisotope remains
from the sample that had an original mass of
2.00 g. What is the half-life of this isotope?
200 years
More on Carbon dating
• Scientists calculate the ratio of carbon12 and carbon-13 to carbon-14 in dead
organisms.
• Then they compare the ratio to the
atmospheres ratio and determine how
many half-lives have passed.
• With that information they can
calculate how long the
organism has been dead.
More Practice
Solving Half-Life Problems
1. Strontium’s half life of the
radioisotope strontium-90 is 29 years.
If you had 10.0g of strontium-90
today, how much would you have left
after 87 years? 1.25g
2. If you start off with 64 grams of some
substance, how much will you have
after 3 half lives?
8g
3. Iodine-131 has a half life of 8 days.
What fraction of the original
sample would remain at the
end of 32 days? 1/16
1) Strontium’s half life of the radioisotope strontium-90 is
29 years. If you had 10.0g of strontium-90 today,
how much would you have left after 87 years?
Sr-90 half-life = 29 years
How many half-lives is 87 years?
87years ÷ 29 years = 3 half-lives
1 half-life
How much is left after 3 half-lives?
10.0g  5.0g  2.5g  1.25g
start
after 1 half-life
after 2 half-lives
after 3 half-lives
1) Strontium’s half life of the radioisotope strontium-90 is
29 years. If you had 10.0g of strontium-90 today,
how much would you have left after 87 years?
HL
A
T
0
1
2
3
4
5
6
10g
5g
2.5g
1.25g
0
29
58
87
2) If you start off with 64 grams of some
substance, how much will you have after
3 half lives?
HL
A
0
1
2
3
4
5
6
64g
32g
16g
8g
T
3) Iodine-131 has a half life of 8 days. What
fraction of the original sample would
remain at the end of 32 days?
HL
A
T
0
1
2
3
4
5
6
1/1
1/2
1/4
1/8
1/16
0
8
16
24
32
IV. Nuclear Energy
A. Fission
Fission
• Fission means to break apart. Nuclear
fission occurs when a nucleus splits apart
into different fragments.
• This generally occurs with atoms that
have a mass number heavier than 60.
• The nuclei do not always split the same
way. Scientists have found 200 different
products from the fission reaction of
Uranium-235.
More on Fission
• Another important factor of fission reactions
is that they cause a chain of reactions.
• The products of the initial reaction can
collide with other molecules and cause a new
fission reaction to occur.
• This domino affect could go on for a long
time. This is how an atomic bomb works.
Fission Reaction
Nuclear Power
• Nuclear power plants harness the energy
released in fission reactions and turn it into
electricity.
• One of the main issues the power plant has
to deal with is keeping the chain reactions
going, but not letting them speed out of
control. To this date there have been two
large nuclear accidents.
Nuclear Power
• Nuclear power plants have to be very precise
in their regulations of the reactions. Some of
the products of the fission reaction are
extremely radioactive.
• To ensure safety of all living things, the
waste must be properly stored.
• It can take up to twenty half-lives for such
radioactivity to reach levels safe enough for
exposure.
• For some waste products this can be
thousands of years.
Fusion
Fusion
• Fusion means to come together.
• Nuclear fusion is the combining of
atomic nuclei.
• In fusion reactions, scientists bring
together nuclei of atoms that have
mass numbers less than 60.
Fusion
• This kind of a reaction also releases
large amounts of energy.
• It useful to know, that the sun is
powered by fusion reactions.