2-7The Concept of the Mole and the Avogadro Constant55▲ FIGURE 2-16Distribution 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 oneprincipal 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).208Pb207Pb206Pb204Pb26Mg25Mg24Mg37Cl35Cl19F(a)6.02214 ±1023F atoms²18.9984 g(c)6.02214 ±1023Mg atoms²24.3050 g(b)6.02214 ±1023Cl atoms²35.4527 g(d)6.02214 ±1023Pb atoms²207.2 gMost elements are composed of mixtures of two or more isotopes so that theatoms in a sample of the element are not all of the same mass but are present intheir naturally occurring proportions. Thus, in one mole of carbon, most of theatoms are carbon-12, but some are carbon-13. In one mole of oxygen, most ofthe atoms are oxygen-16, but some are oxygen-17 and some are oxygen-18. Asa result,The Avogadro constant was purposely chosen so that the mass of one mole of
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