Transcript 4.4 Nuclear

Nuclear Chemistry
•Nuclear chemistry is the study of the changes of the
nucleus of atoms.
•Nuclear Reactions involve changes within the nucleus
where as chemical reactions involve the loss, gain or
sharing of electrons.
The Nucleus
• Remember that the nucleus is made up of
protons and neutrons. The are collectively
called nucleons.
Radioactivity
• A stable nucleus holds together well.
An unstable nucleus will decay or break
down, releasing particles and/or energy
in order to become stable.
• An atom with an unstable nuclei is
considered “radioactive”.
Nuclear Transformations
Nuclear transformations can be
induced by accelerating a particle
and colliding it with the nuclide.
These particle accelerators are enormous, having
circular tracks with radii that are miles long.
There are several ways radioactive atoms
can decay into different atoms!
Transmutation:
• Type of nuclear reaction that will
change the number of protons and thus
will create a different element.
• Atoms with an atomic number larger
than 92 are created through this
process
Alpha Decay
Loss of an -particle (a helium nucleus)
4
2
He
OR
4
2
α
• Atomic number decreases by 2 and mass
number decreases by 4
• Penetrating Power: LOW: Can be
blocked by clothing or thin paper
• Example
238
234
4

92 U
90 Th+ 2 He
Alpha Decay
http://education.jlab.org/glossary/alphadecay.gif
Alpha Decay
Uranium
Thorium
Beta Decay
Loss of a -particle (a high energy electron)
0
−1

0
or −1
e
• Atomic number increases by 1 and mass
number stays the same. A neutron becomes
a proton and a high speed electron that is
discharged from the nucleus.
• Penetrating Power: Medium: Can be blocked
by thin metal or wood
131
131
0
• Example

53 I
54 Xe +−1 e
Beta Decay
Beta Decay
Thorium
Protactinium
Gamma Emission
Loss of a -ray (high-energy radiation that almost
always accompanies the loss of a nuclear
particle)
0
0

• Atomic number and mass number stays the
same
• Penetrating Power: High: Can only be blocked
by thick metal or thick concrete
131
131
• Example

I
I + e
53
53
Radioactivity
• Radioactive isotopes decay at a
characteristic rate measured in half life.
• A half life is the time required for half of
the amount of radioactive atoms to
decay. The time ranges from seconds to
millions of years
Examples
• Beta decay of zircomium-97
• Alpha decay of americium-241
• Alpha decay of uranium-238
• Complete this:
235
93
Np

239
94
Pu + ____
Common Radioactive
Isotopes
Isotope
Half-Life
Radiation
Emitted
Carbon-14
5,730 years
, 
Radon-222
3.8 days

Uranium-235
7.0 x 108 years
, 
Uranium-238
4.46 x 109 years

Radioactive Half-Life
• After one half life there is 1/2 of original
sample left.
• After two half-lives, there will be
1/2 of the 1/2 = 1/4 the original sample.
Graph of Amount of Remaining
Nuclei vs Time
A=Aoe-lt
A
Half Life Calculations
HOW TO’s
1. To calculate the number of half lives,
divide the half life (T1/2) into the total time
(T).
T/T1/2 = # of half lives
2. Use the equation to calculate remaining
amount left over after a certain number of
half lives have passed.
• Amt remaining = (initial amt) (.5)n (# of half lives)
Example
You have 100 g of radioactive C-14. The
half-life of C-14 is 5730 years.
• How many grams are left after one halflife?
• How many grams are left after two halflives?
Examples
• Suppose you have 20 grams of sodium-24.
Its half-life is 15 hours. How much is left
over after 60 hours.
Examples
• Uranium-238 has a half life of 4.46 x 109
years. How long will it take for 7/8th of the
sample to decay?
Examples
• The half life of radium-222 is 38 s. How
many grams of a 12.0 g sample are left after
114 s?
Examples
A sample of 3x107 Radon atoms are trapped
in a basement that is sealed. The half-life of
Radon is 3.83 days. How many radon atoms
are left after 31 days?
answer:1.2x105 atoms
Nuclear Fission: How does one tap all that energy?
• Large atoms split into smaller atoms that
generate huge amounts of energy.
• Carried out in nuclear reactors.
• Could result in a chain reaction of fission like
the atomic bomb
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.
• 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.
• 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 would be a superior
method of generating power.
 The good news is that the
products of the reaction are
not radioactive.
 The bad news is that in order to achieve fusion, the
material must be in the plasma state at several million
kelvins.
 Tokamak apparati like the one shown at the right show
promise for carrying out these reactions.
 They use magnetic fields to heat the material.
Nuclear Fusion
• Smaller atoms
are combine to
form a large
atom.
• Occurs in the sun
and stars
• Generates huge
amounts of
energy