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Chapter 2: Problem 91

Before \(1961,\) the standard for atomic masses was the isotope \(^{16} \mathrm{O},\) to which physicists assigned a value of exactly \(16 .\) At the same time, chemists assigned a value of exactly 16 to the naturally occurring mixture of the isotopes \(^{16} \mathrm{O},^{17} \mathrm{O},\) and \(^{18} \mathrm{O}\). Would you expect atomic masses listed in a 60 -year-old text to be the same, generally higher, or generally lower than in this text? Explain.

Short Answer

Expert verified
The atomic masses listed in a 60-year-old text should be generally lower than those listed in a more recent text. This is because an isotope with fewer neutrons, like \(^{16}\mathrm{O}\), is lighter than a mixture of isotopes including ones with more neutrons like \(^{17}\mathrm{O}\) and \(^{18}\mathrm{O}\). Therefore, when chemists considered the average mass of naturally occurring isotopes post-1961, the atomic masses would generally be higher compared to when physicists used only \(^{16}\mathrm{O}\) isotope for calibration.

Step by step solution

01

Analyzing the atomic masses

Before 1961, physicists considered the atomic mass of \(^{16}\mathrm{O}\) isotope to be exactly 16, whereas chemists used the average mass of the naturally occurring mixture of \(^{16} \mathrm{O},^{17} \mathrm{O},\) and \(^{18}\mathrm{O}\). This implies that the atomic masses provided by chemists were a weighted average of the isotopes, likely resulting in a value greater than 16 given that \(^{17}\mathrm{O}\) and \(^{18}\mathrm{O}\) are heavier than \(^{16}\mathrm{O}\).
02

Predicting the change in atomic masses

Since \(^{16}\mathrm{O}\) is lighter than the average mass of the naturally occurring isotopes, its atomic mass being exactly 16 would be lower than the average atomic mass calculated by the chemists. Therefore, if the atomic mass standard used by chemists was applied universally post-1961, the atomic masses in a 60-year-old text would be generally lower compared to a newer text.
03

Conclusion

Thus, atomic masses listed in a 60-year-old text (which used the physicists' standard of \(^{16}\mathrm{O}\) atomic mass) should generally be lower than those in a more recent text (which uses the average mass of the naturally occurring isotopes).

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Most popular questions from this chapter

Refer to the periodic table inside the front cover and identify (a) the element that is in group 11 and the sixth period (b) an element with atomic number greater than 50 that has properties similar to the element with atomic number 18 (c) the group number of an element \(\mathrm{E}\) that forms an ion \(\mathrm{E}^{2-}\) (d) an element \(M\) that you would expect to form the ion \(\mathrm{M}^{3+}\)

Identify the isotope \(X\) that has one more neutron than protons and a mass number equal to nine times the charge on the ion \(X^{3+}\)

Without doing detailed calculations, determine which of the following samples has the greatest number of atoms: (a) a cube of iron with a length of \(10.0 \mathrm{cm}\) \(\left(d=7.86 \mathrm{g} / \mathrm{cm}^{3}\right)\) (b) \(1.00 \mathrm{kg}\) of hydrogen contained in a \(10,000 \mathrm{L}\) balloon (c) a mound of sulfur weighing \(20.0 \mathrm{kg}\) (d) a 76 lb sample of liquid mercury \((d=13.5 \mathrm{g} / \mathrm{mL})\)

Assuming that the seventh period of the periodic table has 32 members, what should be the atomic number of (a) the noble gas following radon (Rn); (b) the alkali metal following francium (Fr)?

Deuterium, \(^{2} \mathrm{H}(2.0140 \mathrm{u}),\) is sometimes used to replace the principal hydrogen isotope \(^{1} \mathrm{H}\) in chemical studies. The percent natural abundance of deuterium is 0.015\%. If it can be done with 100\% efficiency, what mass of naturally occurring hydrogen gas would have to be processed to obtain a sample containing \(2.50 \times 10^{21}^{2} \mathrm{H}\) atoms?
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