Transcript File

Nuclear
Chemistry
The Nucleus
• Remember that the nucleus is comprised
of protons and neutrons.
• The number of protons is the atomic
number.
• The number of protons and neutrons
together is effectively the mass of the
atom.
Isotopes
• Not all atoms of the same element
have the same mass due to different
numbers of neutrons in those atoms.
• There are three naturally occurring
isotopes of uranium:
– Uranium-234
– Uranium-235
– Uranium-238
Radioactivity
• It is not uncommon for some isotopes of an
element to be unstable, or radioactive.
– These unstable isotopes are called
radioisotopes
• Radioactive isotopes undergo changes in
the nucleus to gain a more stable
arrangement.
• When the nuclei undergo these changes
they emit rays and particles this process is
called radioactivity.
• The penetrating rays and particles emitted
by a radioactive source are called
radiation.
Nuclear Reactions
• Nuclear reactions, which account for
radioactivity, differ from chemical
reactions.
• Chemical Reactions
– Involve the gain, loss and sharing of electrons
– Involve relatively small changes in energy
• Nuclear Reactions
– Involve changes in the nucleus
– Give off very large amounts of energy
– Are not effected by temperature, pressure,
catalysts, presence of other elements
– Nuclear reactions cannot be sped up, slowed
down or turned off
Types of
Radiation
Types of Radiation
(Radioactive Decay)
• Alpha radiation
– Can penetrate up to 0.5mm
– Can be blocked by a piece of paper & clothing
• Beta radiation
– Can penetrate up to 4 mm
– Can be blocked by metal foil
• Gamma radiation
– Can penetrate paper, wood and the body
easily
– Can be blocked by several meters of concrete
or several centimeters of lead
• There are other types of radiation but we
will not be studying these in this chapter.
Types of Radiation
(Radioactive Decay)
• Radio waves, microwaves, visible
light, ultraviolet light, and X-rays are
all forms of radiation like gamma
radiation.
– How does a gamma ray differ from these
other types of electromagnetic
radiation?
• The other types of radiation are not
directly produced by the
Radioactive decay of a nucleus.
Alpha Radiation:
Loss of an -particle
(a helium nucleus)
4
2
238
92
U
Uranium -238
He
234
90
4
2
Th + He
Thorium -234
Alpha particle
Remember a helium nucleus has 2 protons and 2
neutrons and positive charge.
Beta Radiation:
Loss of a -particle
(a high energy electron)
0
−1
14
6
C
Carbon-14
radioactive

0
or −1
14
7
N
Nitrogen-14
stable
e
+
0
−1
e
Beta
Particle
In this nuclear reaction a neutron is converted
into a proton and an electron is released which
has a negative charge.
Gamma Emission:
Loss of a gamma-ray (): high-energy
electromagnetic radiation that almost
always accompanies the loss of a
nuclear particle) 0
0
230
90
Th
Thorium -230
226
88

4
2
Ra + He
Radon -226
Alpha
particle
+
0
0

Gamma
Ray
Gamma rays have no charge or mass –
this is why they can penetrate matter so easily
Positron Emission:
Loss of a positron (a particle that has
the same mass as but opposite charge
than an electron)
0
1
11
6
C
Carbon-11
e
11
5
B
Boron-11
+
0
1
e
positron
In this nuclear reaction a proton is converted
into a neutron and a positron is released which
has a positive charge.
Radiation Summary
• Alpha decay
– ↓ # of protons by 2
– ↓ # of neutrons by 2
• Beta decay
– ↑ # of protons and ↓ # of neutrons
• Gamma Rays
– Pure energy released no nuclear
particles gained/lost
• Positron emission
– ↑ # of neutrons and ↓ # of protons
QNA
• Q: How does an unstable nucleus
release energy?
A: An unstable nucleus releases energy by
emitting radiation during radioactive
decay
• Q: What are the 4 main types of
radiation?
A: Alpha, Beta, Gamma &
Positron Emission
QNA
• Q: What part of an atom undergoes
change during radioactive decay?
A: The nucleus
• Q: How is the atomic number of a
nucleus changed by alpha decay? By
beta decay? By gamma decay?
A: In alpha decay the atomic number
decreases by 2, in beta decay the
atomic number increases by 1, in
gamma radiation there is no change
in the atomic number.
QNA
• Q: Between alpha, beta & gamma radiation
which is the most penetrating?
A: Gamma radiation
Nuclear Stability & Decay
• A radioisotope undergoes changes as
it emits radiation
• Radioisotopes have unstable nuclei
• The stability of a nucleus depends on
the ratio of protons and neutrons
– Atomic Number < 20 the most stable
ratio is 1:1
– Atomic Number > 20 the most stable
arrangements have more neutrons than
protons.
• But too many or too few neutrons can
create an unstable nucleus
Neutron-Proton Ratios
• Any element with more
than one proton (i.e.,
anything but hydrogen)
will have repulsions
between the protons in
the nucleus.
• A strong nuclear force
helps keep the nucleus
from flying apart.
Neutron-Proton Ratios
• Neutrons play a key
role stabilizing the
nucleus.
• Therefore, the ratio of
neutrons to protons is
an important factor.
Neutron-Proton Ratios
For smaller nuclei
(Z  20) stable
nuclei have a
neutron-to-proton
ratio close to 1:1.
Neutron-Proton Ratios
As nuclei get
larger, it takes a
greater number of
neutrons to
stabilize the
nucleus.
Stable Nuclei
The shaded region
in the figure shows
what nuclides
would be stable,
the so-called belt of
stability.
Stable Nuclei
• Nuclei above this
belt have too many
neutrons.
• They tend to decay
by emitting beta
particles.
Stable Nuclei
• Nuclei below the
belt have too many
protons.
• They tend to
become more
stable by positron
emission or
electron capture.
Stable Nuclei
• There are no stable nuclei with an
atomic number greater than 83.
• These nuclei tend to decay by alpha
emission.
Half Life
• Half-life, t1/2, is the time required for
half the atoms of a radioactive nuclide
to decay.
– Each radioactive nuclide has its own halflife.
– More-stable nuclides decay slowly and
have longer
half-lives.
• Animation (hopefully these
animations work)
Half-Life
Half Life Calculations
Sample Problem
• Phosphorus-32 has a half-life of 14.3
days. How many milligrams of
phosphorus-32 remain after 57.2 days if
you start with 4.0 mg of the isotope?
Given: original mass of phosphorus-32 = 4.0 mg
half-life of phosphorus-32 = 14.3 days
time elapsed = 57.2 days
Unknown: mass of phosphorus-32 remaining after
57.2 days
Half Life Calculations
• Solution
number of half - lives  52.7 days 
1 half - life
 4 half - lives
14.3 days
amount of phosphorus - 32 remaining 
1
1
1
1
4.0 mg 



 0.25 mg
2
2
2
2
Radioactive Series
• Large radioactive
nuclei cannot
stabilize by
undergoing only
one nuclear
transformation.
• They undergo a
series of decays
until they form a
stable isotope
(often a isotope of
lead).
• Animation
Transmutation
• The conversion of an atom of one
element to an atom of another
element is called transmutation.
– Transmutation can occur by
radioactive decay. Transmutation
can also occur when particles
bombard the nucleus of an atom.
Transmutation
• The elements in the periodic table
with atomic numbers above 92, the
atomic number of uranium, are
called the transuranium elements.
• All transuranium elements undergo
transmutation.
• None of the transuranium elements occur in
nature, and all of them are radioactive.
Transmutation Reactions
• Transuranium elements are synthesized in
nuclear reactors and nuclear accelerators.
Nuclear Transformations
Nuclear
transformations
can be induced
by accelerating a
particle and
colliding it with
the nuclide.
Particle Accelerators
These particle accelerators are
enormous, having circular tracks with
radii that are miles long. Fermilab, CERN
Energy in Nuclear Reactions
• There is a tremendous amount of
energy stored in nuclei.
• Einstein’s famous equation, E = mc2,
relates directly to the calculation of
this energy.
QNA
• Q: What determines the type of
decay a radioisotope will undergo?
A: The neutron-to-proton ratio
• Q: How much of a sample of
radioisotope remains after 1 half
life? After 2 half lives?
A: After 1 half life 50% of the sample
remains. After 2 half lives 25%
remains.
QNA
• Q: What are 2 ways transmutation
can occur?
A: Radioactive decay & particle
bombardment of a nucleus
• Q: A radioisotope has a half life of 4
days. How much of a 20.0 gram
sample will be left at the end of 4
days? At the end of 8 days?
A: After 4 days: 10.0 grams remain;
after 8 days: 5 grams remain.
QNA
• Q: The mass of cobalt-60 in a sample is
found to have been decreased from 0.800
grams to 0.200 grams in a period of 10.5
years. Calculate the half-life of cobalt-60?
A: 5.25 years
Nuclear Fission
• How does one tap all that energy?
• Nuclear fission is the type of reaction
carried out in nuclear reactors.
• When the nuclei of certain isotopes are
bombarded with neutrons, they undergo
fission, the splitting of a nucleus into
smaller fragments.
• In a chain reaction, some of the neutrons
produced react with other fissionable
atoms, producing more neutrons which
react with still more fissionable atoms.
Nuclear Fission
• Nuclear Fission
Nuclear Fission
• Bombardment of the radioactive nuclide
with a neutron starts the process.
• Neutrons released in the transmutation
strike other nuclei, causing their decay
and the production of more neutrons.
Nuclear Fission
This process continues in what we call a
nuclear chain reaction.
Nuclear Fission
If there are not enough radioactive nuclides in
the path of the ejected neutrons, the chain
reaction will die out.
Nuclear Fission
Therefore, there must be a certain minimum
amount of fissionable material present for the
chain reaction to be sustained: Critical Mass.
Nuclear Reactors
In nuclear reactors the heat generated by
the reaction is used to produce steam that
turns a turbine connected to a generator.
Nuclear Reactors
• The reaction is kept in
check by the use of
control rods.
• These block the paths of
some neutrons, keeping
the system from
reaching a dangerous
supercritical mass.
Nuclear Fusion
• Fusion occurs when nuclei combine to
produce a nucleus of greater mass. In solar
fusion, hydrogen nuclei (protons) fuse to
make helium nuclei and two positrons.
Nuclear Fusion
• Fusion would be a superior method of
generating power.
– The starting materials are inexpensive and
readily available,
– The products of the reaction are not
radioactive or highly polluting,
– Such energy could eliminate dependence on
resources from other governments,
– The bad news is that in order to achieve
fusion, the material must be in the plasma
state at several million Kelvin‘s, and we just
can’t sustain that or control it safely (yet?).
We might have problems storing that kind of
energy as well.
Nuclear Fusion
• Tokamak apparati like the one below show
promise for carrying out these reactions.
• They use magnetic fields to heat the
material.
QNA
• Q: Explain what happens in a nuclear
chain reaction.
A: Neutrons produced by fissionable
atoms react with other fissionable atoms,
that produces more neutrons that react
with more fissionable atoms.
• Q: Why are spent fuel rods from a
nuclear reaction stored in water?
A: Water cools the spent fuel rods
and provides a radiation shield.
QNA
• Q: How are fusion reactions different
from fission reactions?
A: Fission reactions involve splitting
nuclei. In fusion reactions, small nuclei
combine and release much more energy.
• Q: What does nuclear moderation
accomplish in a nuclear reactor?
A: Slows down neutrons.
• Q: What is the source of the radioactive
nuclei present in the spent fuel rods?
A: Unused nuclear fuel and fission
products.
• Q: If we can overcome the technical
issues, what are the advantages of
using a fusion reactor to produce
electricity?
A: Potential fuels are inexpensive and
readily available. I.E. Hydrogen