Chapter 21 Nuclear Chemistry - slider-chemistry-12

The Isotopic Symbol
Remember that the nucleus is comprised of the two
nucleons, protons(p) and neutrons(n).
The number of protons is the atomic number (Z).
The number of protons and neutrons together is
effectively the mass of the atom- mass number (A).
Isotopes are atoms of the same element that
have different masses due to different numbers
of neutrons in those atoms.
There are three naturally occurring isotopes of
 Uranium-234
 Uranium-235
 Uranium-238
Uranium has 92 protons. How many neutrons do each of the
above isotopes have?
It is not uncommon for some nuclides of an element to
be unstable, or radioactive.
We refer to these as radioisotopes. When they
decay, they produce radiation and another element
called a daughter product.
There are several ways radioisotopes can decay into
a different nuclide. These are called nuclear
transformations. The 3 main types are:
Alpha radiation (helium nucleus)
Beta radiation (electron)
Gamma radiation (electromagnetic energy)
Types of Radioactive Decay
Alpha Decay
Loss of an -particle (a helium nucleus)
Th+ He
Types of Radioactive Decay
Beta Decay
Loss of a -particle (a high energy electron)
or −1
Types of Radioactive Decay
Gamma Emission
Loss of a -ray (high-energy radiation that almost
always accompanies the loss of a nuclear particle)
Notice that there is no mass indicated in the nuclear
symbol for this type of radiation
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Penetrating ability
Write the nuclear equation for the alpha decay of
Write the nuclear equation for the beta decay of
Neutron-Proton Ratios
Like charges repel - Any element
with more than one proton (i.e.,
anything but hydrogen) will have
electrostatic repulsions between
the protons in the nucleus.
A strong nuclear force helps keep
the nucleus from flying apart.
Neutrons play a key role
stabilizing the nucleus.
Therefore, the ratio of neutrons to
protons (n/Z) is an important
factor for stability.
Stable Nuclei
The shaded region in the
figure shows what
nuclides would be stable,
the so-called Belt of
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. Notice the
n/Z ratio has increased
in the “Belt of stability”
Beta Emitters
Nuclei above this belt
have too many
They tend to decay by
emitting beta particles.
Because a n0 has been
converted to a p+, n/Z
has decreased into the
“belt of stability”
Positron or Electron capture
Nuclei below the belt have
too many protons.
They tend to become more
stable by positron emission
or electron capture.
Because the number of
protons has been reduced,
the n/Z has increased.
Alpha Emitters
There are no stable nuclei with an atomic number
greater than 83.
These nuclei tend to decay by alpha emission.
Radioactive Decay Series
Large radioactive nuclei
cannot stabilize by
undergoing only one
nuclear transformation.
They undergo a series of
decays until they form a
stable nuclide (often a
nuclide of lead).
This shows the decay of
U-238. What are the
types of decay shown?
Half- life is the time it
takes for half of the
atoms in a sample to
Each isotope has a
particular half-life
associated with it.
Some common ones
are to the right.
Decay of 20.0 mg of 15O. What remains
after 3 half-lives? After 5 half-lives?
Nuclear Transformations
Nuclear transformations can be induced by
accelerating a particle and colliding it with the
nuclide. Transuranic (Z>92)elements are made
this way.
These particle accelerators are enormous, having
circular tracks with radii that are miles long.
Measuring Radioactivity
One can use a device like this Geiger counter to
measure the amount of activity present in a radioactive
The ionizing radiation creates ions, which conduct a
current that is detected by the instrument.
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.
In chemical reactions the amount of mass
converted to energy is minimal.
However, these energies are many thousands of
times greater in nuclear reactions.
Energy in Nuclear Reactions
For example, the mass change for the decay of 1 mol of
uranium-238 is 0.0046 g.
The change in energy, E, is then
E = (m) c2
E = (4.6  10−6 kg)(3.00  108 m/s)2
E = 4.1  1011 J
Uses of Radioactivity
There are a number of uses for radioactivity. They
 Carbon dating – determine age of dead material
 Medical uses – radiation therapy, nuclear imaging
techniques (Tc-99), sterilising equipment
 Industrial uses – fire detectors (Am-241), thickness
gauges, detecting cracks in pipes
 Food irradiation – to kill bacteria and fungi (Co-60
or Cs-137)
Nuclear Fission
Fission is the splitting of a radionuclide releasing
Nuclear fission is the type of reaction carried out in
nuclear reactors.
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. This is known as the 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. The
process is similar to a coal fired power plant, but Uranium has a
greater energy content than coal.
A 1000MW power plant will use 8,500,00kg of coal or 74 kg of uranium
Nuclear Reactors –Are they safe?
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
which can generate too much
heat and cause a meltdown.
Nuclear Fission and Power
Currently over 100
nuclear power plants
in the U.S. and over
400 worldwide.
Approximately 17%
of the world’s energy
comes from nuclear.
Nuclear Fusion
small nuclei combine
+ 2H
+ 1n + Energy
Occurs in the sun and other
Nuclear Fusion
Fusion (combining nuclei together)
would be a far 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.