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Published byDina Snow Modified over 5 years ago

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Isotopic Abundance SCH 3U

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Atomic Mass The mass of an atom (protons, neutrons, electrons) Relative Atomic Mass: An element’s atomic mass relative to the mass of a carbon-12 atom Atomic Mass Units (amu or u) A unit of mass defined as 1/12 the mass of one C-12 atom OR 1 proton or neutron E.g. C-12 has a mass of 12 X 1 u = 12 u

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E.g. The mass of a sodium atom is 1.92 times that of C-12. Calculate the atomic mass of sodium in atomic mass units. m c-12 = 12u m Na = 1.92 X 12 u = 23.04 u Compare this to the average atomic mass. 22.99 u What accounts for the difference? It’s an average of an element’s isotopes

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Isotopic Abundance For most elements, the isotopic composition remains constant E.g. any sample of naturally occurring carbon has the same percentage of C-12 and C-13 Because this is constant, we can use an average value for the atomic mass of carbon The percentage of an isotope in a sample is called its isotopic abundance

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E.g. Given Isotopic Abundance of C-12 = 98.89% Isotopic Abundance of C-13 = 1.110% Calculate the average atomic mass of carbon. m avg = [m C-12 X % C-12 ] + [m C-13 X % C-13 ] = [12 u X 0.9889] + [13 u X 0.0110] = 11.8668 u + 0.1443 u = 12.01 u

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E.g. Chlorine has 2 isotopes, Isotopic Abundance of Cl-35 = 75.53% Isotopic Abundance of Cl-37 = 24.47% Calculate the average atomic mass of chlorine. m avg = [m Cl-35 X % Cl-35 ] + [m Cl-37 X % Cl-37 ] = [35 u X 0.7553] + [37 u X 0.2447] = 26.4355 u + 9.0539 u = 35.49 u

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