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Nuclear Chemistry
CHAPTER 25
Key Terms
 Radioactivity- the process by which nuclei emit
particles and rays
 Radiation- the penetrating rays and particles emitted
by a radioactive source
 Radioisotope- an isotope that has an unstable
nucleus and undergoes radioactive decay
History
 1896- French chemists Antoine Henri Becquerel;
Marie and Pierre Curie
Chemical
 Atoms tend to
attain stable
electron
configurations by
losing or
sharing
electrons
Nuclear
 The nuclei of unstable
isotopes gain stability by
undergoing changeswhich emit large
amounts of energy
 Not affected by: temp,
pressure, or catalysts
 Cannot be sped up,
slowed down or turned
off
Discovery of nuclear reactions
 Disproved Dalton’s assumption
that atoms are indivisible.
Why does this happen?
 Unstable nuclei
 Bad proton: neutron ratio
 An unstable nucleus releases
energy by emitting radiation during
the process of radioactive decay.
When this happens
 Unstable radioisotopes of one element
transferred into stable isotopes of another
element.
 TRANSMUTATION
 Radioactive decay is spontaneous
 does not require any input of energy.
Types of Radiation
 Alpha (α)
 Beta (β)
 Gamma (γ)
Alpha Radiation
helium nuclei emitted from
a radioactive source
+
o
2 p and 2 n ; double
positive charge
42He OR α
Alpha Radiation- Equations
Atomic
number decreases by 2
Mass number decreases by 4
Alpha Particles
 Do not travel far/not very penetrating
because they are so large
 Stopped by a piece of paper or skin
 Dangerous when ingested
Beta Radiation
 A neutron breaks apart into a
proton, which remains in the
nucleus, and a fast moving
electron, which is released.
Beta Radiation- Equations
Atomic
number increases by
1
Mass number remains the
same
Beta Particles (β)
 More penetrating- can pass through paper
but are stopped by aluminum foil or thin
pieces of wood.
Gamma Radiation (γ)
 A high energy photon
 Electromagnetic radiation (wave-like)
 Nuclei often emit gamma rays along
with α or β particles during radioactive
decay
 Does not change mass or atomic
numbers
Gamma Radiation
Gamma Rays
 Very penetrating
 Stopped by lead shields
Electron Capture
 “inverse beta decay”
 electron in an atom's inner shell drawn into the
nucleus
 it combines with a proton, forming a
neutron and a neutrino.
 The neutrino is ejected
 Atomic # , mass # doesn’t change
A POSITRON
 Is a particle with the mass of an
electron but a positive charge
 During positron emission, a proton
changes to a neutron and positron
(which is emitted)
 Atomic # , mass # doesn’t change
Radiation Comparison
Modes of radioactive decay
 Alpha Decay (α)
2
protons, 2 neutrons 42He nucleus
 Beta Particle (β-)
 Electron emitted from the nucleus 0-1e
 Positron Particle (β+)
 Mass of an electron but positive charge 0+1e
 Gamma Radiation (γ)
 High energy radiation (higher than x-ray)
 No mass and no charge
The symbols used in
nuclear chemistry can be
found on Reference
Table O
Stability and Decay
 More than 1500 nuclei are known
 Only 264 are stable
All nuclei that have
 An atomic number >83 are
radioactive- too many p+ and no
NOTE:
 If all the masses in a nuclear reaction were
measured accurately enough, you would find
that mass is not exactly conserved.
 An extremely small quantity of mass is
converted into energy released in
radioactive decay
Half-life (t1/2)
 The time required for one-half of the nuclei
of a radioisotope sample to decay to
products
 The half-life of a radioactive nuclide cannot
be changed
Uranium
 U-238 decays through a complex series of
radioactive isotopes to the stable isotope Pb206
 t1/2= 4.5 x 109 years- possible to date rocks
as old as the solar system
Carbon Dating
 C-14: t1/2= 5,715 years
Exactly how much time must elapse before 16
grams of potassium-42 decays, leaving 2 grams
of the original isotope?
8 x 12.4 hours
2. 2 x 12.4 hours
3. 3 x 12.4 hours
4. 4 x 12.4 hours
1.
Transmutation
 The conversion of an atom of one element to an atom
of another element
 can occur by radioactive decay (natural) or when
particles bombard the nucleus of an atom
(artificial)
Elements with atomic numbers
 Greater than 92- transuranium elements
 None occur in nature
 Synthesized in nuclear reactors and nuclear
accelerators
Nuclear Fission
 Fission- the splitting of a nucleus into
smaller fragments
 U-235 and Po-239 are the only fissionable
isotopes
 chain reaction
 Can release enormous amounts of energy:
 1 kg U-235 --> explosion of 20,000 tons of
dynamite
Nuclear Fission
Nuclear Reactors
 controlled fission=useful energy
 Neutron moderation- slows down neutrons;
reactor fuel captures them to continue the
chain reaction
 Neutron absorption- decreases the number
of slow-moving neutrons
 Control rods- used to absorb neutrons
Nuclear Reactors
Nuclear Fusion
 nuclei combine to produce a nucleus of
greater mass
 In solar fusion, hydrogen nuclei (protons)
fuse to make helium nuclei
 Fusion reactions release much more energy
than fission reactions
 Problems with achieving the high
temperature necessary for reactions
Nuclear Fusion
There are benefits and risks associated
with fission and fusion reactions
 Benefits to making electricity with nuclear
fission
 A small amount of fuel makes a large
amount of electricity
 Not dependent on foreign oil
 Using fission instead of burning fossil
fuels does not pollute the air
 Cheap electricity
There are benefits and risks associated with fission
and fusion reactions
 Risks to making electricity using nuclear
fission
 Exposure to radioactive material can
cause cancer, mutations or death
 Transportation and storage of fissionable
material is dangerous
 Nuclear accidents
 Disposal of nuclear waste
 Thermal pollution
Nuclear Waste
 Water cools the spent rods, and also acts as
a radiation shield to reduce the radiation
levels
Radiation in your life
Ionizing Radiation
 Is radiation with enough energy to
knock electrons off some atoms of the
bombarded substance to produce ions
 Radiation cannot be seen, heard, felt of
smelled
Devices such as
 Geiger counters, scintillation counters and
film badges are commonly used to detect
radiation
 Geiger Counters: gas-filled metal tube used
to detect the presence of beta radiation
 Scintillation Counter: device that uses a
coated phosphor surface to detect ionizing
radiation
Radioisotopes can be used
 To diagnose medical problems, and in some
cases, treat diseases
 I-131: thyroid
 Co-60: cancer
 Irradiated food
 Gamma rays