(A) The masses and percent isotopic abundances of the three naturally occurring isotopes of silicon are ²⁸Si, 27.9769265325 u, 92.223%; ²⁹Si, 28.976494700 u, 4.685%; ³⁰Si, 29.973377017 u, 3.092%. Calculate the weighted-average atomic mass of silicon.

(B) Use data from Example 2-5 and the conventional atomic mass of Li (Table 2.2) to estimate the percent isotopic abundances of lithium-6 and lithium-7.

Example 2-5

(A) The two naturally occurring isotopes of boron, boron-10 and boron-11, have masses of 10.0129370 u and 11.0093054 u, respectively. Which of these two occurs in greater abundance?

(B) Indium has two naturally occurring isotopes and a weighted atomic mass of 114.818 u. One of the isotopes has a mass of 112.904058 u. Which of the following must be the second isotope: ¹¹¹In, ¹¹²In, ¹¹⁴In, or ¹¹⁵In? Which of the two naturally occurring isotopes must be the more abundant?

Table 2.2

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TABLE 2.2 Conventional Atomic Masses and Atomic Mass Intervals for Selected Elements Atomic Number 1 3 5 6 7 8 12 14 16 17 35 81 Atomic Symbol Н LFDSSHONDENH Li B 60 Conventional 1.008 6.94 10.81 12.011 14.007 15.999 24.305 28.085 32.06 35.45 Atomic Mass, u 79.904 204.38 Interval [1.00784, 1.00811] [6.938, 6.997] [10.806, 10.821] [12.0096, 12.0116] [14.00643, 14.00728] [15.99903, 15.9997] [24.304, 24.307] [28.084, 28.086] [32.059, 32.076] [35.446, 35.457] [79.901, 79.907] [204.382,204.385]