1. Average Atomic Mass

2. 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 x 1023 atoms (Avogadro’s number)

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
Average mass = 4 x x 60 = 260 g
California WEB

4. 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 That is, B-11 is 80 percent abundant on earth.
For boron atomic weight
= (10 amu) (11 amu) = amu

5. 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

6. 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

7. 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

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

9. 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 %
Hg %
Hg %
Hg %
Hg %
Hg %
Hg % 30 25 20 15 10 5
Mass spectrum of mercury vapor
Relative number of atoms
Mass number

10. Hg 80 The percent natural abundances for mercury isotopes are:
200.59 80
The percent natural abundances
for mercury isotopes are:
Hg %
Hg %
Hg %
Hg %
Hg %
Hg %
Hg % 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
( )(196) + (0.1002)(198) + (0.1684)(199) + (0.2313)(200) + (0.1322)(201) + (0.2980)(202) + (0.0685)(204) = x
= x
x = amu

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

12. Atomic Mass
Calculate the atomic mass of copper if copper has two isotopes % has a mass of amu and the rest has a mass of amu.
Isotope
Percent
Abundance
Mass
Cu-63
69.1
62.93
Cu-65
64.93
30.9
63.548 Cu 29
63.548