Chemistry Reference
In-Depth Information
Table 21.4
Small Particles Involved in Artificial Radioactivity
Mass
Particle
Symbol
Charge
Number
Identity
Neutron
n
0
1
Uncharged nuclear particle
Proton
p
1
1
Hydrogen nucleus
2
H
Deuteron
d
1
2
Nucleus of
0
0
1
b
Positron
b
or
1
0
Positively charged electron
The positive particle produced along with the silicon isotope is a
positron,
a
particle identical to an electron except that it has a positive charge. When a
positron collides with an electron, both are annihilated and a relatively large
amount of energy is produced.
Just as we did for natural radioactive reactions we can balance nuclear equa-
tions for artificial reactions by making the superscripts on the left and right sides
add up to the same quantity, and also the subscripts.
Chain Reactions
One of the projectiles most often used in modern times to initiate nuclear reac-
tions is the neutron. To be effective, a neutron does not need as high an energy
as an alpha particle or a proton because it is uncharged and can penetrate a
nucleus more easily than a positively charged particle can. Some nuclear reac-
tions initiated with neutrons produce more neutrons. For example, the bom-
bardment of with a neutron produces two large nuclei plus two or three
new neutrons. The following are examples of the many possible reactions:
n
n
n
n
n
= neutron
= nucleus
235
U
n
n
n
n
235
1
0
n £
140
94
2
0
n
n
92
U
56
Ba
36
Kr
n
n
n
235
1
0
n £
9
38
Sr
143
3
0
n
n
n
92
U
54
Xe
n
n
235
When a neutron enters a sample of it may collide with the nucleus of
one of the atoms, producing a reaction in which two or three new neutrons
are produced. Each of these may react with another nucleus, producing more
reactions and an increased number of neutrons (Figure 21.5). Such reactions,
all started by a single neutron, can continue until the entire sample of
has reacted. The sequence of reactions is called a
chain reaction
and is the
source of energy by which nuclear power plants operate. The
atomic bomb,
which should more accurately be called the nuclear bomb, also uses a chain
reaction.
92
U,
n
n
n
n
n
n
n
n
235
92
U
n
n
n
n
n
n
n
Figure 21.5
Schematic
Representation of a Chain
Reaction
In this representation, each collision
of a neutron with a nucleus causes
a reaction that produces two new
neutrons (plus other products and
energy). The new neutrons can cause
further reactions.
EXAMPLE 21.16
How many neutrons are produced at the end if the following reaction is run
through five cycles and every neutron produced in the first four cycles causes
another such reaction? How many neutrons would the tenth cycle produce?
235
1
0
n £
9
38
Sr
143
3
0
n
92
U
54
Xe
