1. Average Atomic Mass

2. carbon atom (12 amu) 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)

3. 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

4. Isotopes Atoms of the same element with different numbers of neutrons. Isotopes have the same number of protons, so the atomic number is the same. The atomic mass is different because the neutrons are different. Different isotopes of the same element are identified by their atomic mass. (IE: Carbon-12, Carbon-14)

5. Isotopes Because of the existence of isotopes, the mass of a collection of atoms has an average value. Average mass = ATOMIC WEIGHT

6. Example 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

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

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

9. Average Atomic Mass EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37. Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem Avg. Atomic Mass = (35)(8) + (37)(2) 100 = 35.40 amu

10. 100 90 80 70 60 50 40 30 20 10 0 34 35 3637 Abundance Mass Mass spectrum of chlorine. Elemental chlorine (Cl 2 ) contains only two isotopes: 34.97 amu (75.53%) and 36.97 (24.47%) AAM = (34.97 amu)(0.7553) + (36.97 amu)(0.2447) AAM = (26.412841 amu) + (9.046559 amu) AAM = 35.4594 amu Cl-35 Cl-37 Cl 35.4594 17

11. Negative grid Magnet Detector To vacuum pump Mass numbers Evacuated glass tube Heated filament Electron beam Neon gas inlet (+) (-) Image Copyrighted by Houghton Mifflin Company Mass Spectrophotometer

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

13. The percent natural abundances for mercury isotopes are: Hg-196 0.146% Hg-196 0.146% Hg-198 10.02% Hg-198 10.02% Hg-199 16.84% Hg-199 16.84% Hg-200 23.13% Hg-200 23.13% Hg-201 13.22% Hg-201 13.22% Hg-202 29.80% Hg-202 29.80% Hg-204 6.85% Hg-204 6.85% (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 Hg 200.59 80 (% "A")(mass "A") + (% "B")(mass "B") + (% "C")(mass "C") + (% "D")(mass "D") + (% "E")(mass "E") + (% F)(mass F) + (% G)(mass G) = AAM ABCDEFGABCDEFG

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

15. 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. Cu 29 63.548 IsotopePercentAbundanceMass Cu-6369.162.93 Cu-65 64.93 43.48463 20.0633730.9 63.548