Ch 222 - M02_PETR4521_10_SE_C02.QXD 8:06 PM Page 55 2ND...

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2-7 The Concept of the Mole and the Avogadro Constant 55 FIGURE 2-16 Distribution of isotopes in four elements (a) There is only one type of fluorine atom, (shown in red). (b) In chlorine, 75.77% of the atoms are (red) and the remainder are (blue). (c) Magnesium has one principal isotope, (red), and two minor ones, (gray) and (blue). (d) Lead has four naturally occurring isotopes: 1.4% (yellow), 24.1% (blue), 22.1% (gray), and 52.4% (red). 208 Pb 207 Pb 206 Pb 204 Pb 26 Mg 25 Mg 24 Mg 37 Cl 35 Cl 19 F (a) 6.02214 ± 10 23 F atoms ² 18.9984 g (c) 6.02214 ± 10 23 Mg atoms ² 24.3050 g (b) 6.02214 ± 10 23 Cl atoms ² 35.4527 g (d) 6.02214 ± 10 23 Pb atoms ² 207.2 g Most elements are composed of mixtures of two or more isotopes so that the atoms in a sample of the element are not all of the same mass but are present in their naturally occurring proportions. Thus, in one mole of carbon, most of the atoms are carbon-12, but some are carbon-13. In one mole of oxygen, most of the atoms are oxygen-16, but some are oxygen-17 and some are oxygen-18. As a result, The Avogadro constant was purposely chosen so that the mass of one mole of
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