Scan_Doc0085 - 3.2 Atomic Masses 79 atoms or molecules are...

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Most elements occur in nature as mixtures of isotopes; thus atomic masses are usually average values. It is much easier to weigh out 600 hex nuts than count them one by one. 3.2 Atomic Masses 79 atoms ormolecules are passed into abeam ofhigh-speed electrons, which knock elec- trons offthe atoms ormolecules being analyzed and change them into positive ions. An applied electric fieldthen accelerates these ions into amagnetic field. Because anaccel- erating ionproduces itsownmagnetic field,aninteraction withtheapplied magnetic field occurs, which tends tochange thepath oftheion.The amount ofpath deflection foreach iondepends onitsmass-the mostmassive ionsaredeflected thesmallest amount-which causes theionstoseparate, asshown inFig. 3.1.Acomparison ofthepositions where the ions hitthe detector plate gives very accurate values oftheir relative masses. For exam- ple, when 12Cand l3Care analyzed in amass spectrometer, the ratio oftheir masses is found tobe Mass l3C Mass 12C
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