Complete the Following Nuclear Equations
Worked Solutions
Chapter 3
Question 16
Complete the following nuclear equations:
(a) 238 4
U -- + He
92 2
Answer: When a 238U isotope undergoes alpha decay, its atomic number decreases by 2, while its mass number decreases by 4:
238 234 4
U Th + He
92 90 2
(b) 3 0
H -- + e
1 -1
Answer: When a 3H isotope undergoes beta decay, its atomic number increases by 1, while its mass number remains the same:
3 3 0
H He + e
1 2 -1
(c) 239 4
Pu -- + He
93 2
Answer: When a 239Pu isotope undergoes alpha decay, its atomic number decreases by 2, while its mass number decreases by 4:
239 235 4
Pu Pa + He
93 91 2
(d) 32 0
P -- + e
15 -1
Answer: When a 32P isotope undergoes beta decay, its atomic number increases by 1, while its mass number remains the same:
32 32 0
P S + e
15 16 -1
(e) 212 4
Po -- + He
84 2
Answer: When a 212Po isotope undergoes alpha decay, its atomic number decreases by 2, while its mass number decreases by 4:
212 208 4
Po Pb + He
84 82 2
(f) 24 0
Na -- + e
11 -1
Answer: When a 24Na isotope undergoes beta decay, its atomic number increases by 1, while its mass number remains the same:
24 24 0
Na Mg + e
11 12 -1
(g) 226 4
Ra -- + He
88 2
Answer: When a 226Ra isotope undergoes alpha decay, its atomic number decreases by 2, while its mass number decreases by 4:
226 222 4
Ra Rn + He
88 86 2
(h) 131 0
I -- + e
53 -1
Answer: When an 131I isotope undergoes beta decay, its atomic number increases by 1, while its mass number remains the same:
131 131 0
I Xe + e
53 54 -1
Question 19
The radioisotope cobalt-60 has a half-life of 5.26 years. How many years would it take 2g of cobalt-60 to decay to 0.25g?
Answer:
Since the half-life of cobalt-60 is 5.26 years, the original 2g would have halved to 1g after 5.26 years, the 1g would have halved to 0.5g after a further 5.26 years, and the 0.5g would have halved to 0.25g after a further 5.26 years. Thus it would take 15.78 years for 2g of cobalt-60 to decay to 0.25g.
Question 31 (b)
Complete the following nuclear equations:
(i) 222 4
Rn -- + He
86 2
Answer: When a 222Rn isotope undergoes alpha decay, its atomic number decreases by 2, while its mass number decreases by 4:
222 218 4
Rn Po + He
86 84 2
(ii) 13 0
B -- + e
5 -1
Answer: When a 13B isotope undergoes beta decay, its atomic number increases by 1, while its mass number remains the same:
13 13 0
B C + e
5 6 -1
(iii) 196 4
Au -- + He
79 2
Answer: When a 196Au isotope undergoes alpha decay, its atomic number decreases by 2, while its mass number decreases by 4:
196 192 4
Au Ir + He
79 77 2
(iv)
42 0
K -- + e
19 -1
Answer: When a 42K isotope undergoes beta decay, its atomic number increases by 1, while its mass number remains the same:
42 42 0
K Ca + e
19 20 -1
(v)
185 4
W -- + He
74 2
Answer: When a 185W isotope undergoes alpha decay, its atomic number decreases by 2, while its mass number decreases by 4:
185 181 4
W Hf + He
75 72 2
(vi)
45 0
Ca -- + e
20 -1
Answer: When a 45Ca isotope undergoes beta decay, its atomic number increases by 1, while its mass number remains the same:
45 45 0
Ca Sc + e
20 21 -1
(vii)
223 4
Ra -- + He
88 2
Answer: When a 223Ra isotope undergoes alpha decay, its atomic number decreases by 2, while its mass number decreases by 4:
223 219 4
Ra Rn + He
88 86 2
(viii)
112 0
Ag -- + e
47 -1
Answer: When a 112Ag isotope undergoes beta decay, its atomic number increases by 1, while its mass number remains the same:
112 112 0
Ag Cd + e
47 48 -1
(ix)
239 239
U Np + --
92 93
Answer: The atomic number of 239U increases by 1, while its mass number remains the same. This means that it has undergone beta decay:
239 239 0
U Np + e
92 93 -1
(x)
232 228
Th Ra + --
90 88
Answer: The atomic number of 232Th decreases by 2, while its mass number decreases by 4. This means that it has undergone alpha decay:
232 228 4
Th Ra + He
90 88 2
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