Nuclear Chemistry
Structure and Stability of Nuclei,
Fission, Fusion,
and
Radiation
Standards
11. Nuclear processes are those in which an atomic nucleus changes, including radioactive decay of
naturally occurring and human-made isotopes, nuclear fission, and nuclear fusion. As a basis for
understanding this concept:
a. Students know protons and neutrons in the nucleus are held together by nuclear forces that
overcome the electromagnetic repulsion between the protons.
11. b. Students know the energy release per gram of material is much larger in nuclear fusion or fission
reactions than in chemical reactions. The change in mass (calculated by E = mc2 ) is small but
significant in nuclear reactions.
11. c. Students know some naturally occurring isotopes of elements are radioactive, as are isotopes
formed in nuclear reactions.
11. d. Students know the three most common forms of radioactive decay (alpha, beta, and gamma) and
know how the nucleus changes in each type of decay.
11. e. Students know alpha, beta, and gamma radiation produce different amounts and kinds of damage
in matter and have different penetrations.
11. f.* Students know how to calculate the amount of a radioactive substance remaining after an
integral number of half-lives have passed.
11. g.* Students know protons and neutrons have substructures and consist of particles called quarks.
Nuclear Chemistry
has to do with an
plural of
atom’s nucleus
nucleus = nuclei
What’s a nucleus? Let’s review
An atom’s nucleus contains
almost all of an atom’s mass,
but takes up very little of it’s volume.
Subatomic Particles
the particles that make up an atom
• Protons – high mass, positive charge.
Found in nucleus.
• Neutrons – high mass, no charge.
Found in nucleus.
• Electrons – low mass, negative charge.
Found orbiting around nucleus.
(abbreviated e– )
Comparison of Masses
Electron
Proton
Neutron
An Atom
Size of
atom
Size of nucleus
2 protons = He = helium
Basic Electrical Charge Laws
+ and – : Attract
(pull together)
– and – : Repel
(push away)
+ and + : Repel
(push away)
Like charges repel and Opposites attract
So why don’t the protons in the nucleus
fly apart?
nuclear
is stronger
force
over short
distances
electrostatic
force is stronger
over long
distances
Key
11
Na
Sodium
22.99
Atomic Number
•Number of Protons
•Number of Electrons (when
atom is neutrally charged)
•Property unique to each
element
Key
11
Na
Sodium
22.99
Average atomic mass*
•Weighted Average number of
Protons and Neutrons (approximately)
Isotopes
When atoms have the same number of
protons, but different numbers of neutrons,
they are called isotopes.
Examples:
Carbon-14 has 6 protons and 8 neutrons.
Carbon-12 has 6 protons and 6 neutrons.
Carbon-14 is found more often in living
organisms than in non-living matter. It also
undergoes radioactive decay – which is why it
is used for fossil dating.
Isotopes
More Examples:
Uranium-238 has 92 protons and 146 neutrons.
Uranium-235 has 92 protons and 143 neutrons.
Uranium-235 is more rare in nature than
Uranium-238, but it also undergoes nuclear
chain reactions more easily, which is why small
amounts of Uranium-235 are used in nuclear
weapons and nuclear power plants.
Some isotopes are more stable than others.
Which isotope is more abundant?
Neon-20
Neon-22
How many neutrons in each?
Neon-20 20 – 10 = 10 neutrons
Neon-22 22 – 10 = 12 neutrons
Which isotope is more abundant?
Bromine-79
Bromine-80
How many neutrons in each?
Bromine-79
Bromine-80
Lead has two isotopes with the following
relative amounts:
80% Lead-207
20% Lead-208
What would the average atomic mass
calculate to be?
0.8(207) + 0.2(208) = 207.2
Chlorine has two stable isotopes:
??% Chlorine-35 x
100 –??% Chlorine-37 1 – x
What are the relative abundances (in %) of
each isotope?
-2x = -1.55
x(35) + (1-x)(37) = 35.45
-2
-2
35x – 37x + 37 = 35.45
x = 0.775
-2x + 37 = 35.45
–37 –37
-2x = -1.55
Chlorine has two stable isotopes:
77.5% Chlorine-35 0.775
22.5% Chlorine-37 0.225
What are the relative abundances (in %) of
each isotope?
-2x = -1.55
x(35) + (1-x)(37) = 35.45
-2
-2
35x – 37x + 37 = 35.45
x = 0.775
-2x + 37 = 35.45
–37 –37
-2x = -1.55
So why don’t the protons in the nucleus
fly apart?
nuclear
is stronger
force
over short
distances
electrostatic
force is stronger
over long
distances
Sometimes these forces are overcome.
Fission – a nucleus breaks apart.
(ex. atomic bomb, nuclear power plants,
radioactive decay)
Fusion – a nucleus merges with another.
(ex. the sun, hydrogen bomb,
experimental fusion reactors)
Both fission and fusion release radiation.
It is called radiation
because it radiates out.
harder to block
Symbol
Common Types of Radiation
4
2
He
α (alpha): helium nucleus at high speed.
0
-1
e
β (beta): electron at high speed.
0
0
γ
γ (gamma): high energy photon.
Symbol
1
0
1
1
0
1
Other Types of Radiation
n
neutron at high speed.
p
Proton (or hydrogen nucleus) at
high speed.
β+
β+ positron at high speed (the antimatter version of an electron).
atomic
mass
number
of protons
23
11
Na
22
11
Na
We can use conservation of mass and
charge to figure out nuclear reactions
just like chemical reactions.
238
92
234
90
U
4
2
He + ???
Th
0
-1
e +
???
We can use conservation of mass and
charge to figure out nuclear reactions
just like chemical reactions.
238
92
234
90
U
4
2
He +
Th
0
-1
e +
234
90
Th
???
We can use conservation of mass and
charge to figure out nuclear reactions
just like chemical reactions.
238
92
234
90
U
4
2
Th
0
-1
He +
234
90
Th
e +
234
91
Pa
Half-Life
• Half-life – how much time has passed
when half of the original amount
1
remains.
or 50 %
2
• How much of the original remains after
1
two half-lives?
or 25 %
4
• How much after three half-lives?
1
or 12.5 %
8
• How much after four half-lives?
1
or 6.25 %
16
Table of Half-Lives
Isotope Name
Uranium-238
Uranium-235
Carbon-14
Strontium-90
Cobalt-60
Symbol
238 U
92
235 U
92
14 C
6
90 Sr
38
60 Co
27
Half-Life
4.51 × 109 years
7.1 × 108 years
5,730 years
28 years
5.27 years
Table of Half-Lives
Isotope Name
Symbol
234 Th
Thorium-234
90
Protactinium-234 23491Pa
218 Po
Polonium-218
84
219 Rn
Radon-219
86
214 Po
Polonium-214
84
Half-Life
24.1 days
6.75 hours
3.08 minutes
4.00 seconds
1.6 × 10-4 seconds
As a archaeologist you find a dead rat that
contains 0.0009 grams of Carbon-14. A rat
that died a year ago has 0.01 grams of
Carbon-14. How long ago did the rat die?
t½ = 5,730 years
1
0.0009 g
=
11.1
0.01 g
1
1
1
1
1
1
2
4
16
32
64
8
1 H.L. 2 H.L. 3 H.L. 4 H.L. 5 H.L. 6 H.L.
As a archaeologist you find a dead rat that
contains 0.0009 grams of Carbon-14. A rat
that died a year ago has 0.01 grams of
Carbon-14. How long ago did the rat die?
t½ = 5,730 years
3(5,730) = 17,190 years
1
4(5,730) = 22,920 years
11.1
1
1
17,190 – 22,920 years ago
16
8
3 H.L. 4 H.L.
Strontium-90 is one of the “fallout” products
from atmospheric testing of nuclear weapons. If
there were 50,000 grams of Strontium-90 in the
U.S. southwest region when the 1963 testing ban
began, how much Strontium-90 currently
remains?
t½ = 28 years
2013 – 1963 = 50 years 50 yr 1.8 half=
28 yr lives
1
1
1
1
1
1
2
4
16
32
64
8
1 H.L. 2 H.L. 3 H.L. 4 H.L. 5 H.L. 6 H.L.
Radioactive Decay Formulas
-(
1
2
A = A0 e
t½ =
ln (2)
A0
A
ln (2)
t½
)t
t
A0 = original amount
A = current amount
t = current time
t½ = half-life
ln ( )
A
ln
(
)
A
3 t =
t½
0
ln (2)
Energy-Mass Equivalence
Sometimes in nuclear change a small amount
of the mass disappears. It has been converted
to a large amount of energy according to the
formula:
2
E = mc
energy = (mass) × (speed of light)2
J = kg × (m/s)2
speed of light: c = 3 × 108 m/s
Fission Nuclear Reaction
4 e– in valence shell
Which isotope is more abundant?
Lead-207
Lead-208
How many neutrons in each?
Lead-207
Lead-208