The distribution of isotopic masses, although nearly constant, does vary somewhat
depending on the source of the element. For example, the abundance of^1306 C in atmos-
pheric CO 2 is slightly different from that in seashells. The chemical history of a compound
can be inferred from small differences in isotope ratios.Figure 5-10 (a) A modern mass spectrometer. (b) The mass spectrum of Xeions. The
isotope^126054 Xe is at too low an abundance (0.090%) to appear in this experiment.19 0CHAPTER 5: The Structure of Atoms
(a) (b)
TABLE 5-3 Some Naturally Occurring Isotopic AbundancesElement Atomic Weight (amu) Isotope % Natural Abundance Mass (amu)boron 10.811^01005 B 19.91 10.01294
011
05 B 80.09 11.00931
oxygen 15.9994^01608 O 99.762 15.99492
017
08 O 0.038 16.99913(^01808) O 0.200 17.99916
chlorine 35.4527^03517 Cl 75.770 34.96885
(^03717) Cl 24.230 36.96590
uranium 238.0289^234092 U 0.0055 234.0409
(^235092) U 0.720 235.0439
238
092 U 99.2745 238.0508
The 20 elements that have only one naturally occurring isotope are^94 Be,^1909 F,^2311 Na,^2713 Al,^3115 P,^4521 Sc,^5525 Mn,^5927 Co,
(^7533) As, (^8939) Y, (^9341) Nb, (^103045) Rh, (^127053) I, (^133055) Cs, (^141059) Pr, (^159065) Tb, (^165067) Ho, (^169069) Tm, (^197079) Au,and (^209083) Bi.There are however other,
artificially produced isotopes of these elements.