Active Learning In Chemistry Education - Potomac School

3. Fission
In fission, a nucleus with a large mass splits into two nuclei with smaller masses. To cause fission,
man bombards certain nuclei with special particles. The fission process is used to generate heat in nuclear
power plants, and is the kind of reaction which occurs during the explosion of an atomic bomb. Let's see
where this energy comes from. Look at the equation below which represents the fission of uranium- 235.
Find the total of the mass numbers of the two particles on the left side of the equation: {18} __________.
235
U +
92
1
n
0
138
----> Ba +
56
95
Kr +
36
1
3 n + energy
0
Next, find the total of the mass numbers of the five particles on the right side: {19} __________. How do
these totals compare? {20} ______________________________ As a result, you would think that mass
the amount of matter) is conserved (neither created nor destroyed). However, this is a bit misleading.
Keep in mind that the mass number is the total number of the protons and neutrons in the nucleus, not
their exact total mass. Remember that masses of atoms and subatomic particles are expressed in very tiny
units called atomic mass units (amu). The mass of an atom of U-235 is actually a little greater than 235 amu,
and the masses of Ba-138 and Kr-95 are actually a little less than 138 and 95, respectively. Therefore, in
the equation above, there is a small loss of mass which appears as a great amount of energy. In other
words, some mass is converted into energy. An atomic bomb gives off a tremendous amount of heat
because some mass is converted into energy. A tiny amount of mass can produce a tremendous amount
of energy. When the uranium nucleus splits into smaller nuclei, the energy which was needed to hold the
whole thing together in the first place is no longer needed. This is the energy which is given off.
4. Fusion
When fusion occurs, the nuclei of two lower mass elements are combined to form a nucleus with a
greater mass representing a different element. Exceedingly high temperatures are needed to cause
fusion to occur, since the two nuclei repel each other due to their similar positive charges. Fusion
reactions are the source of the sun's energy where hydrogen nuclei combine to form helium nuclei. The
equation below shows the fusion of 2 deuterium nuclei to form one helium-4 nucleus (also called an alpha
particle).
2 2
4
H + H -------> He + energy
1 1
2
Fusion reactions were used in weapons such as the hydrogen bomb. Scientists are experimenting with
fusion reactions in devices known as breeder reactors which may someday replace fission reactors in
nuclear power plants. Fusion, like fission, results in a loss of mass which is converted into a great amount
of energy. However, fusion releases much more energy per gram of fuel than fission does.
Problem 1. Let's practice writing nuclear notation. Keep in mind that the superscript is the mass
number (sum of protons and neutrons) and the subscript is the atomic number (number of protons) if the
particle is a nucleus. If the particle is a subatomic particle (proton, electron, or neutron,) then the subscript
is the charge on the particle. Write the nuclear notation for each of the following:
a. an isotope of carbon (C) which contains 6 protons and 8 neutrons
b. an isotope of helium (He) which contains 2 protons and 4 neutrons
c. an isotope of uranium (U) which contains 92 protons and has a mass number of 233
d. an isotope of tin (Sn) which contains 50 protons and 60 neutrons
a.____________ b.____________ c.____________ d.____________
28-7 ©1997, A.J. Girondi