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Unformatted text preview: Atomic Mass Grams is not a very convenient unit for atomic masses, so a new unit called the atomic mass unit (amu) is defined. 1 amu = 1.660551 x 10 -24 g Reexpressing the subatomic particle masses in terms of atomic mass units we have Particle Mass(g) Mass(amu) proton 1.6727 x 10-24 1.007316 neutron 1.6750 x 10-24 1.008701 electron 9.110 x 10-28 0.000549 Using an instrument called a mass spectrometer we can very accurately measure the mass of atoms and molecules. Here are some measured isotope masses using a mass spectrometer . Isotope Mass(amu) 2 H 2.0140 4 He 4.00260 8 Be 8.005305 12 C 12.000000 16 O 15.994915 24 Mg 23.985042 That 12 C has a mass of exactly 12.000000 amu is not a coincidence. A mass spectrometer can only measure mass differences accurately. To solve this problem the 12 C isotope is defined to have a mass of exactly 12.000000 amu's. Then everything else is C isotope is defined to have a mass of exactly 12....
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