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Nuclear Reactions
A
X
Z
A
= number of protons + number of neutrons
Z
= number of protons
A – Z = number of neutrons
Number of neutrons = Mass Number – Atomic Number
There are many types of uranium:
235
238
U
92
U
92
A
235
A
238
Z
92
Z
92
Number of protons
92
Number of protons
92
Number of neutrons
143
Number of neutrons
146
Isotopes of any particular element contain the same
number of protons, but different numbers of neutrons.
Role of neutrons
• The purpose of
neutrons in an atom
is to balance out the
repulsion of protons
in the nucleus.
– It is like putting items
in between opposing
magnets.
Radioactive Decay
Radioactive decay is due to instability and results in
the emission of either:
• an alpha particle (a),
• a beta particle (b),
• or a gamma ray(g).
Alpha Decay
The element releases an alpha particle
which is identical to that of a helium nucleus.
(Two protons and two neutrons).
This can be stopped by a thin piece of paper.
The new element has 2 fewer neutrons
and 2 fewer protons.
Alpha Decay
A
X
Z
A-4
4
Y
He
+
Z-2
2
unstable atom
alpha particle
more stable atom
Alpha Decay
222
226
Ra
88
Rn
86
4
He
2
Alpha Decay
A
A-4
4
226
222
4
X
Z
Ra
88
Y
+
Z-2
Rn
+
86
He
2
He
2
Alpha Decay
222
Rn
86
222
Rn
86
A
4
Y
He
+
Z
2
218
Po
+
84
4
He
2
Alpha Decay
A
230
4
234
230
4
X
Z
U
92
Th
He
+
90
2
Th
He
+
90
2
Alpha Decay
230
Th
90
230
Th
90
A
4
226
4
Y
He
+
Z
2
Ra
He
+
88
2
Alpha Decay
A
214
4
218
214
4
X
Z
Po
84
Pb
He
+
82
2
Pb
He
+
82
2
Beta Decay
A beta particle is a fast moving electron which is
emitted from the nucleus of an atom undergoing
radioactive decay. Can be stopped by 3 mm of
aluminium foil or 10 mm of wood.
Beta decay occurs when a neutron changes into a
proton and an electron.
Beta Decay
As a result of beta decay, the nucleus has one less
neutron, but one extra proton.
The atomic number, Z, increases by 1 and the mass
number, A, stays the same.
Beta Decay
218
218
Po
84
At
85
b
-1
0
Beta Decay
A
X
Z
218
Po
84
A
b
-1
218
b
-1
Y
+
Z+1
Rn
+
85
0
0
Beta Decay
234
A
b
-1
234
234
b
-1
Th
90
Th
90
Y
+
Z
Pa
+
91
0
0
Beta Decay
A
210
b
-1
210
210
b
-1
X
Z
Tl
81
Pb
+
82
Pb
+
82
0
0
Beta Decay
210
A
b
-1
210
210
b
-1
Bi
83
Bi
83
Y
+
Z
Po
+
84
0
0
Beta Decay
A
214
b
-1
214
214
b
-1
X
Z
Pb
82
Bi
+
83
Bi
+
83
0
0
Gamma Decay
Gamma rays are not charged particles like a and b
particles.
Gamma rays are electromagnetic radiation with high
frequency which are stopped by several cm of lead.
When atoms decay by emitting a or b particles to form a
new atom, the nuclei of the new atom formed may still
have too much energy to be completely stable.
This excess energy is emitted as gamma rays (gamma ray
photons have energies of ~ 1 x 10-12 J).
Nuclear Fission
This occurs when a larger nucleus divides to create a
smaller nucleus. Alpha decay is a simple version of
this. When it divides, it creates a large amount of
energy, and often times releases neutrons, which if
surrounded by enough fissionable material can lead to a
chain reaction. By using control rods to slow the
process down, it powers nuclear power plants.
Nuclear Fission
Nuclear Fusion
This occurs when two smaller nuclei combine together
to form a single larger nuclei. This produces far more
energy than a fission reaction, and also does not have a
dangerous by-product. However we currently don’t
have the means to use it as a reliable energy source, as
we barely get more energy out, than we put in.
Nuclear Fusion