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238 U
92 92 U
β α
α Pa
91
β
90 Th Th
α
89 Ra

Atomic number 88 α

87

86
Rn
α
85

84 Po Po Po
β α β
83 α Bi Bi α

β β
82 Pb Pb Pb

238 234 230 226 222 218 214 210 206
Mass number

FIGURE 13.7 The radioactive decay series for uranium-238. This is one of three naturally occurring series.

The rate of radioactive decay is usually described in terms measured in seconds, minutes, hours, days, months, years, or
of its half-life. The half-life is the time required for one-half of billions of years. Table 13.3 lists half-lives of some of the iso-
the unstable nuclei to decay. Since each isotope has a charac- topes, and the process is illustrated in Figure 13.8.
teristic decay constant, each isotope has its own characteristic
half-life. Half-lives of some highly unstable isotopes are mea-
sured in fractions of seconds, and other isotopes have half-lives 1,000
900 1,000
grams
800
700
500
TABLE 13.3 grams
600
Mass (g) 500
Half-lives of some radioactive isotopes
Isotope Half-Life Mode of Decay
400 250
3
H (tritium) 12.26 years Beta grams

1
14 5,730 years Beta 300
C
6
90
Sr 28 years Beta 200 125

38
131
I 8 days Beta 100 grams
53
133
Xe 5.27 days Beta

54
238 9
U 4.51 × 10 years Alpha 0 1 2 3 4 5 6 7 8

92
242 5

Pu 3.79 × 10 years Alpha Time (days)
94
240

Pu 6,760 years Alpha
94
239 FIGURE 13.8 Radioactive decay of a hypothetical isotope with

Pu 24,360 years Alpha a half-life of one day. The sample decays each day by one-half to
94
40 9
K 1.3 × 10 years Alpha some other element. Actual half-lives may be in seconds, minutes,

19
or any time unit up to billions of years.
330 CHAPTER 13 Nuclear Reactions 13-8