Page 298 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 19] NUCLEAR REACTIONS 287

few elements that do not occur naturally are provided in parentheses in most periodic tables.) The atomic
masses that are given can give a clue to the mass number of the most abundant isotope in many cases, but
not all. For example, the atomic mass of Br is nearest 80, but the two isotopes that represent the naturally
occurring mixture have mass numbers 79 and 81.

4
4
19.4. What is the difference, if any, between α, α, and He ?
2+
2 2
Ans. There is no difference. All are representations of an alpha particle (also called a helium nucleus).
19.5. Complete the following nuclear equations:
4
(a) 211 Pb −→ 211 Bi + ? (c) 220 Rn −→? + α
82 83 86 2
(b) 223 Fr −→ 223 Ra + ? (d) 213 Bi −→ 213 Po + ?
87
83
88
84
4
Ans. (a) 211 Pb −→ 211 Bi + −1 0 β (c) 220 Rn −→ 216 Po + α
84
86
83
82
2
(b) 223 Fr −→ 223 Ra + −1 0 β (d) 213 Bi −→ 213 Po + −1 0 β
84
83
87
88
19.6. Complete the following nuclear equations:
(a) 225 Ac −→ 221 Fr + ? (c)? −→ 209 Bi + 0 β
89 87 83 −1
(b) 214 Bi −→ 214 Po + ? (d) 232 Th −→ 228 Ra + ?
83 84 90 88
4
Ans. (a) 225 Ac −→ 221 Fr + α (c) 209 Pb −→ 209 Bi + −1 0 β
89
87
82
83
2
4
(b) 214 Bi −→ 214 Po + −1 0 β (d) 232 Th −→ 228 Ra + α
84
88
90
2
83
19.7. Complete the following nuclear equations:
127 0 237 233 22 0
(a)? −→ Te + γ (b) Np −→ Pa + ? (c) Na −→ ? + β (positron)
52 +1
0
4
Ans. (a) 127 Te −→ 127 Te + γ (b) 237 Np −→ 233 Pa + α (c) 22 Na −→ 22 Ne + +1 0 β
11
2
93
52
52
0
91
10
19.8. Consider the equation
238 234 4
2
92 U −→ 90 Th + He
Does this equation refer to the nuclei of these elements or to atoms of the elements as a whole?
Ans. Nuclear equations are written to describe nuclear changes. However, in this equation, since the correct number
of electrons is available for the products, the equation can also be regarded as describing the reactions of
complete atoms as well as just their nuclei.
HALF-LIFE
19.9. A certain isotope has a half-life of 1.24 years. How much of a 1.12-g sample of this isotope will remain
after 3.72 years?
Ans. After the first 1.24 years, half the 1.12-g sample will still be the same isotope. After 1.24 years more, half
of the 0.560 g remaining will still be the original isotope. That is 0.280 g will remain. After the third 1.24
years, 0.140 g remain.
19.10. One-eighth of a certain sample of a radioactive isotope is present 30.0 min after its original weighing.
How much will be present after 10.0 min more?
Ans. The 30.0 min represents three half-lives, since the number of atoms is reduced to one-eighth the origi-
nal quantity. (One-half disintegrated in the first half-life, one-half of those left disintegrated in the second,
and half of that disintegrated in the third, leaving one-eighth of the original number at the end of
30.0 min.) The half-life is therefore 10.0 min, and the sample is reduced to one-sixteenth of its original
quantity after 10.0 min more. That is, half of the one-eighth number of atoms remains after one more
half-life.