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AP Chemistry
Topic 5
Nuclear Chemistry
Recall:
A
Z
E
A – The mass number
Z – The atomic number
E – The chemical symbol of the element
Nucleons – protons and neutrons
Types of Radioactive Decay
Alpha (  )
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2
He emitted
Has a charge of 2+
Deflected by electric or magnetic fields
Stopped by a piece of paper
Example:
Beta (   )
Electron emitted
Has a charge of 1Created when a neutron is converted to a proton
Deflected by electric or magnetic fields
More penetrating than alpha particles
Example:
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Gamma (  )
Consists of photons, are not particles of matter
Unaffected by electric or magnetic fields
Example:
Positron (   )
Have the same mass as electrons but have a positive charge
Are formed when a proton converts to a neutron and a positron
Example:
Electron Capture
A process where the nucleus absorbs an electron from an inner shell
Once inside the nucleus, the electron combines with a proton to form a neutron
Example:
Radioactive Decay
Most lighter elements have stable nuclei.
Elements with an atomic number of greater than 83 (Bismuth) are radioactive.
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Half Life
A  N
ln
At
 t
A0
ln
Nt
  t
No
t1 / 2 
0.693

A = activity (decay rate)
N = the number of atoms present
 = the decay constant
t = time
t1 / 2 = half life
Example:
Sodium-24 has a half life of 14.659 hours.
a) What is the decay constant?
b) What is the activity of a freshly synthesized 1.00mg sample of Sodium-24?
c) What will be the rate of decay after 1 week?
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Transuranium Elements
These are elements with a Z > 92.
Nuclear Stability
A crucial factor in the stability of a nucleus is the ratio of neutron number to proton
number.
Nuclides with more the 20 protons require a larger number of neutrons than protons to
moderate the effect of increasing proton repulsions. (Nuclides with less that 20 protons
tend to have an equal number of protons and neutrons).
Nuclides with more than 83 protons are all radioactive because the proton repulsions are
too large to be overcome by neutron moderation.
There is a “belt of stability” for nuclides on a graph of Neutron Number versus Proton
Number (figure 19.5)
Note:
About 160 stable nuclides have an even number of both protons and neutrons.
About 50 stable nuclides have an even number of protons and odd number of neutrons.
About 50 stable nuclides have an odd number of protons and even number of neutrons.
Only 4 stable nuclides have an odd number of protons and odd number of neutrons.
Magic Numbers are: 2,8,20,28,50,82,126
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Energetics of Nuclear Reactions
In a typical spontaneous nuclear reaction, a small quantity of matter is transformed into a
small quantity of energy (so a small amount of mass is lost and some energy is released).
The mass lost is called the mass defect.
Nuclear energies are generally expressed in electron-volts. One electron-volt (eV) is the
energy an electron acquires as it moves through a potential difference of 1 volt. An
electron-volt is a very small quantity of energy. Thus, we usually use the unit Mega
electron-volt (MeV).
1 MeV = 1.6022  10 13 J
We can also relate electron-volts to atomic mass units (u).
Recall:
1u = 1/12 of the mass of 1 Carbon-12 atom
Using E  mc 2 , it is possible to show that:
1u = 931.5 MeV
So:
Energy ( MeV )  MassChange (u )  931.5( MeV / u )
Note:
Mass Change = Mass Products – Mass Reactants
Nuclear Mass = Atomic Mass – Mass of Extra-Nuclear Electrons
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Example:
Given the following nuclear masses, calculate the energy associated with the alpha decay
of Americium-241 in MeV.
241
95
Am  241.0046u
237
93
Np  236.9970u
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2
He  4.0015u
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Nuclear Binding Energy
The mass of a helium nucleus is slightly less than the mass of two protons and two
neutrons.
Why is this so?
As two protons and two neutrons come together to form the nucleus, there is a mass loss
of 0.0305u. This mass loss is a mass defect and is released as energy.
Mass Defect = Mass Protons + Neutrons – Mass Nucleus
The energy released when a nucleus is formed from protons and neutrons is called the
nuclear binding energy and is expressed as a positive quantity.
Nuclear Binding Energy(MeV) = Mass Defect(u)  931.5( MeV / u )
So the amount of energy released in the previous case can be calculated:
Nuclear Fission and Nuclear Fusion
Fission
- The breakup of heavier nuclei into two lighter fragments.
- Mass is converted into energy
- Can occur Spontaneously as the nuclei are unstable
- Used in Nuclear Reactors. Uranium-235 undergoes fission and releases energy. That
energy is used to heat water to generate steam, powering turbines that generate electricity
(see page 816).
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Fusion
- The combining of light nuclei into a heavier one.
- Mass is converted into energy
- Requires a large amount of energy to get started due to the repulsion between nuclei
- The sun is powered by fusion.
- In a hydrogen bomb, fusion of hydrogen atoms occurs to form Helium.
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