NUCLEAR CHEMISTRY
1. Balance each of the following nuclear equations and indicate the type of nuclear reaction (emission, -emission, fission, fusion, or “other”).
a)
239
94
Pu  01n 
c)
210
84
Po  24He  ?
U  01n 
e)
235
92
g)
238
92
U  ?
Sn  ?  301 n
130
50
72
30
234
?
?  ?  401n
Th
b) ?  36Li  2 24He
Ga  10e  ?
d)
66
31
f)
234
90
h)
60
29
Th  ? 
Cu 
234
91
?
Ni  ?
60
28
2. The isotope 137
55 Cs undergoes beta emission with a half-life of 30 years.
a) Write a balanced nuclear equation for this reaction.
b) What fraction of Cs-137 remains in a sample of the isotope after 60 years?
c) What mass of Cs will be left in a 24.0 g sample of 137
55 Cs after 90 years?
d) What fraction of Cs-137 has decayed after 120 years?
3. What is the half-life of an isotope that is 75% decayed after 16 days?
4. Explain what makes an isotope radioactive. Why do radioactive isotopes undergo radioactive
decay? How does the energy released by nuclear reactions compare to that released by
ordinary chemical reactions? Why?
5. Write balanced nuclear equations for:
a) positron emission by Sr-83
b) the fusion of two C-12 nuclei to give another nucleus and a neutron.
c) the fission of U-235 to give Ba-140, another nucleus and an excess of two neutrons.
6. What new element is formed when K-40 decays by -emission? Is the new element formed
likely to be stable? Why or why not?
7. Why is nuclear fission considered a “chain reaction”? What is “critical” about critical mass?
Why does nuclear fission produce radioactive waste?
ANSWERS
1. a)
c)
239
94
Pu  01n 
210
84
235
92
g)
238
92
2. a)
Zn 
b) n 
0
1
e
Sm  401n , fission
160
62
Th , -decay
U  He 
Cs 
206
82
72
30
4
2
137
55
Ru  301 n , fission
107
44
Pb , -decay
Po  24He 
U  01n 
e)
Sn 
130
50
234
90
H  36Li  2 24He , fusion
b)
2
1
d)
66
31
f)
234
90
h)
66
Ga  10e  30
Zn , other
Th 
Cu 
60
29
0
1
e
Pa , -decay
234
91
Ni  10e , other
60
28
137
56
Ba
n
90 years
3
c) n  # half  lives 
30 years
120 years
4
d) n 
30 years
2
1
 1
 1
fraction left         0.25 left
 2
 2
4
60 years
2
30 years
n
3
1
 1
 1
fraction left       
 2
 2
8
n
24.0g 
4
1
 1
 1
fraction left =   =   
 2
 2
16
fraction decayed =
1
 3.0g
8
15
= 0.94
16
3. 75% decayed = 25% left = 1/4 left
n
timepast
16days
1  1

    n = 2 # half-lives = n = 2 =
half  life half  life
4  2
16days
half life =
 8 days
2
4. Isotopes are radioactive due to an unstable nucleus. Nuclei can be unstable because they are
too large or if their neutron-to-proton ratio is “incorrect”. Radioactive decay enables a
nucleus to become more stable. Alpha-decay makes a nucleus smaller and beta-decay
reduces the neutron-to-proton ratio. The energy released in nuclear reactions is several orders
of magnitude greater than the energy released in ordinary chemical reactions. This enormous
amount of energy reflects the loss in mass that accompanies nuclear reactions.
5. a)
c)
6.
83
38
Sr  10e 
U  01n 
235
92
b) 126C  126C  01n 
83
37
Rb
Ba 
140
56
23
12
Mg
Kr  301 n
93
36
40
K  10e  20
Ca ; calcium
It is likely to be stable because 40 is close to the atomic mass of calcium.
40
19
7. Nuclear fission is a chain reaction because it uses up one neutron but produces two or more
neutrons, making it possible for the process to escalate rapidly. The critical mass is the mass
needed by a uranium sample for the reaction to be self-sustaining. Nuclear fission produces
radioactive waste because the two fission products have a neutron-to-proton ratio similar to
the uranium from which they were derived. Therefore, as smaller elements, these products
always have too many neutrons and are highly radioactive.