by: Kerem ERYILMAZ
Adnan ÖZTÜREL
Nuclear Reactions
Chemical reactions all involve the exchange or sharing of electrons,
they never have an influence on the nucleus of the atom. Nuclear
reactions involve a change in the nucleus. There are forces in the
nucleus that oppose each other, the "Strong" force holding Protons
and Neutrons to each other and the electrostatic force of protons
repelling other protons. Under certain arrangements of protons and
neutrons the electrostatic force can cause instability in the nucleus
causing it to decay. It will continue to decay until it reaches a stable
combination.
This graph shows the stable nuclei in red. There are several things to
notice:
- There are no stable nuclei with an atomic number higher than 83 or
a neutron number higher than 126.
- The more protons in the nuclei, the more neutrons are needed for
stability. Notice how the stability band pulls away from the P=N line.
- Stability is favored by even numbers of protons and even numbers of
neutrons. 168 of the stable nuclei are even-even while only 4 of the
stable nuclei are odd-odd.
Unstable nuclei, called radioactive isotopes, will undergo nuclear decay
as it becomes more stable. There are only certain types of nuclear
decay which means that most isotopes can't jump directly from being
unstable to being stable. It often takes several decays to eventually
become a stable nuclei.
Types of Radioactive Decay
When unstable nuclei decay, the reactions generally involve the
emission of a particle and or energy. Below is a table describing the
types of nuclear decay. Notice that for each type of decay, the
equation is balanced with regard to atomic number and atomic
mass. In other words, the total atomic number before and after the
reaction are equal. And the total atomic mass before and after the
reaction are also equal.
Particle Name
alpha particles
What is it?
helium nuclei
or
Relative penetrating power
1
stopped by the skin
but very damaging due to ionization
Happens to nuclei with Z>83
The 2 p+ 2n loss brings the atom down
and to the left toward the belt of stable nuclei.
Particle Name
beta particles
What is it?
high speed electron
or
Relative penetrating power
100
penetrates human tissue to ~1cm
Happens to nuclei with high neutron:proton
ratio
A neutron becomes a proton causing a shift
down and to the right on the stability graph
Particle Name
gamma rays
What is it?
high energy photon
Relative penetrating power
10000
highly penetrating but not very ionizing
Generally accompanies other radioactive radiation because it is the energy
lost from settling within the nucleus after a change. Since gamma rays do not
affect the atomic number or mass number, it is generally not shown in the
nuclear equation.
Particle Name
positron emission
What is it?
positron
Relative penetrating power
100
Happens to nuclei with a low
neutron:proton ratio
A proton becomes a neutron causing
a shift up and to the left
Particle Name
electron capture
What is it?
inner shell electron
Relative penetrating power
no release of energy or particle
Happens to nuclei with a low neutron: proton ratio
A proton becomes a neutron causing a shift up and to the
left. Always results in gamma radiation
This graph shows all the trends of decay and the band of stable
nuclei. There are some exceptions to the trends but generally a nuclei will
decay following the trends (in multiple steps) until it becomes stable. For
example 92U238 will go through 8 alpha emissions and 6 beta emissions (not
all in order) before becoming 82Pb206
The steps a nuclei follows in becoming stable is called a radioactive
series. The series for 92U238 is shown below as an example.
Z > 83 -- alpha
92U
238
=> 90Th234 + 2He4
unpredicted Beta
234
90Th
=>
91Pa
234
unpredicted Beta
91Pa
234
=>
234
92U
+ -1eo
+ -1eo
Z > 83 -- alpha
234
92U
=> 90Th230 + 2He4
Z > 83 -- alpha
230
90Th
=> 88Ra226 + 2He4
Z > 83 -- alpha
88Ra
226
=> 86Rn222 + 2He4
Z > 83 -- alpha
222
86Rn
=> 84Po218 + 2He4
Z > 83 -- alpha
218
84Po
=> 82Pb214 + 2He4
Beta
214
82Pb
=> 83Bi214 + -1eo
Beta
83Bi
214
=> 84Po214 + -1eo
Z > 83 -- alpha
214
84Po
=> 82Pb210 + 2He4
Beta
210
82Pb
=> 83Bi210 + -1eo
Beta
210
83Po
=> 84Po210 + -1eo
Z > 83 -- alpha
210
84Po
=> 82Pb206 + 2He4
stable
206
82Pb
Differences Between Nuclear and Chemical
Reactions
Six Differences between nuclear reactions and chemical reactions.
Nuclear Reactions
1. Protons and neutrons react
inside nucleus.
Chemical Reactions
1. Electrons react outside nucleus.
2. Elements transmute into other
elements.
2. The same number of each kind of
atom appear in the reactants
and products.
3. Isotopes react differently.
3. Isotopes react the same.
4. Independent of chemical
combination.
5. Energy changes equal 10^8 kJ.
4. Depend on chemical combination.
6. Mass changes are detectable.
5. Energy changes equal 10 - 10^3 kJ/mol.
6. Mass reactants = mass products.