Average Atomic Mass

Measuring Atomic Mass Unit is the Atomic Mass Unit (amu) One twelfth the mass of a carbon-12 atom. Each isotope has its own atomic mass; therefore, use the average from % abundance. (1 amu) (1 amu) carbon atom (12 amu) The Mole - abbreviated mol Defined as the amount that contains the number of carbon atoms in exactly 12 g of isotopically pure carbon-12. The mass of carbon-12 contains 6.022 x 1023 atoms (Avogadro’s number)

Calculating averages You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks? Total mass = (4 x 50) + (1 x 60) = 260 g Average mass = (4 x 50) + (1 x 60) = 260 g 5 5 Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 5 California WEB

Isotopes Because of the existence of isotopes, the mass of a collection of atoms has an average value. Average mass = ATOMIC WEIGHT Boron is 20% B-10 and 80% B-11. That is, B-11 is 80 percent abundant on earth. For boron atomic weight = 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu

Average Atomic Mass Avg. (mass)(%) + (mass)(%) Atomic Mass 100 weighted average of all isotopes on the Periodic Table round to 2 decimal places Avg. Atomic Mass (mass)(%) + (mass)(%) = 100 Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Average Atomic Mass EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O. Avg. Atomic Mass (16)(99.76) + (17)(0.04) + (18)(0.20) 16.00 amu = = 100 Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Average Atomic Mass (35)(8) + (37)(2) 10 Avg. Atomic = = Mass EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37. Avg. Atomic Mass (35)(8) + (37)(2) = = 35.40 amu 10 Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Cl 35.4594 17 100 Mass spectrum of chlorine. Elemental chlorine (Cl2) contains only two isotopes: 34.97 amu (75.53%) and 36.97 (24.47%) 90 80 Cl-35 70 AAM = (34.97 amu)(0.7553) + (36.97 amu)(0.2447) 60 AAM = (26.412841 amu) + (9.046559 amu) AAM = 35.4594 amu 50 Abundance 40 30 Cl-37 Mass spectrum of chlorine. Elemental chlorine (Cl2) contains only two isotopes: 34.97 amu (75.53%) and 36.97 (24.47%) 20 10 34 35 36 37 Mass

Mass Spectrum for Mercury (The photographic record has been converted to a scale of relative number of atoms) The percent natural abundances for mercury isotopes are: Hg-196 0.146% Hg-198 10.02% Hg-199 16.84% Hg-200 23.13% Hg-201 13.22% Hg-202 29.80% Hg-204 6.85% 198 200 202 30 25 20 15 10 5 196 199 201 204 Mass spectrum of mercury vapor Relative number of atoms 196 197 198 199 200 201 202 203 204 Mass number

Hg 80 The percent natural abundances for mercury isotopes are: 200.59 80 The percent natural abundances for mercury isotopes are: Hg-196 0.146% Hg-198 10.02% Hg-199 16.84% Hg-200 23.13% Hg-201 13.22% Hg-202 29.80% Hg-204 6.85% A B C D E F G (% "A")(mass "A") + (% "B")(mass "B") + (% "C")(mass "C") + (% "D")(mass "D") + (% "E")(mass "E") + (% F)(mass F) + (% G)(mass G) = AAM (0.00146)(196) + (0.1002)(198) + (0.1684)(199) + (0.2313)(200) + (0.1322)(201) + (0.2980)(202) + (0.0685)(204) = x 0.28616 + 19.8396 + 33.5116 + 46.2600 + 26.5722 + 60.1960 + 13.974 = x x = 200.63956 amu

Mass spectrums reflect the abundance of naturally occurring isotopes. Natural Abundance of Common Elements Hydrogen 1H = 99.985% 2H = 0.015% Carbon 12C = 98.90% 13C = 1.10% Nitrogen 14N = 99.63% 15N = 0.37% Oxygen 16O = 99.762% 17O = 0.038% 18O = 0.200% Sulfur 32S = 95.02% 33S = 0.75% 34S = 4.21% 36S = 0.02% Chlorine 35Cl = 75.77% 37Cl = 24.23% Bromine 79Br = 50.69% 81Br = 49.31%

Atomic Mass Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of 64.93 amu. Isotope Percent Abundance Mass Cu-63 69.1 62.93 Cu-65 64.93 43.48463 30.9 20.06337 63.548 http://upload.wikimedia.org/wikipedia/commons/e/e2/United_States_penny,_obverse,_2002.jpg Cu 29 63.548