Average Atomic Mass. 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.

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Presentation transcript:

Average Atomic Mass

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)

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 x 60 = 260 g California WEB

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)

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

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) (11 amu) = 10.8 amu

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

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 Avg. Atomic Mass = (16)(99.76) + (17)(0.04) + (18)(0.20) 100 = amu

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 Avg. Atomic Mass = (35)(8) + (37)(2) 100 = amu

Abundance Mass Mass spectrum of chlorine. Elemental chlorine (Cl 2 ) contains only two isotopes: amu (75.53%) and (24.47%) AAM = (34.97 amu)(0.7553) + (36.97 amu)(0.2447) AAM = ( amu) + ( amu) AAM = amu Cl-35 Cl-37 Cl

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

Mass Spectrum for Mercury Mass number Relative number of atoms Mass spectrum of mercury vapor The percent natural abundances for mercury isotopes are: Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % (The photographic record has been converted to a scale of relative number of atoms)

The percent natural abundances for mercury isotopes are: Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % ( )(196) + (0.1002)(198) + (0.1684)(199) + (0.2313)(200) + (0.1322)(201) + (0.2980)(202) + (0.0685)(204) = x = x x = amu Hg (% "A")(mass "A") + (% "B")(mass "B") + (% "C")(mass "C") + (% "D")(mass "D") + (% "E")(mass "E") + (% F)(mass F) + (% G)(mass G) = AAM ABCDEFGABCDEFG

Mass spectrums reflect the abundance of naturally occurring isotopes. Hydrogen Carbon Nitrogen Oxygen Sulfur Chlorine Bromine 1 H = % 2 H = 0.015% 12 C = 98.90% 13 C = 1.10% 14 N = 99.63% 15 N = 0.37% 16 O = % 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

Atomic Mass Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of amu and the rest has a mass of amu. Cu IsotopePercentAbundanceMass Cu Cu