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.

Average Atomic Mass

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) + 0.80 (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 www.nisd.net/communicationsarts/pages/chem

Avg.AtomicMass

= (mass)(%) + (mass)(%)

100

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.

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Avg.AtomicMass

= (16)(99.76) + (17)(0.04) + (18)(0.20)

100= 16.00

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 www.nisd.net/communicationsarts/pages/chem

Avg.AtomicMass

= (35)(8) + (37)(2)

100= 35.40 amu

100

90

80

70

60

50

40

30

20

10

034 35 36 37

Ab

un

dan

ce

Mass

Mass spectrum of chlorine. Elemental chlorine (Cl2) 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

Cl35.4594

17

Mass Spectrum for Mercury

196 197 198 199 200 201 202 203 204

Mass numberMass number

Rel

ativ

e n

umb

er o

f at

oms

Rel

ativ

e n

umb

er o

f at

oms

30

25

20

15

10

5

196 199 201 204

198 200 202

Mass spectrum of mercury vaporMass spectrum of mercury vapor

The percent natural abundances The percent natural abundances for mercury isotopes are: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)

The percent natural abundances The percent natural abundances for mercury isotopes are: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

Hg200.59

80

(% "A")(mass "A") + (% "B")(mass "B") + (% "C")(mass "C") + (% "D")(mass "D") + (% "E")(mass "E") + (% F)(mass F) + (% G)(mass G) = AAM

ABCDEFG

Mass spectrums reflect the abundance of naturally occurring isotopes.

Hydrogen

Carbon

Nitrogen

Oxygen

Sulfur

Chlorine

Bromine

1H = 99.985% 2H = 0.015%

12C = 98.90% 13C = 1.10%

14N = 99.63% 15N = 0.37%

16O = 99.762% 17O = 0.038% 18O = 0.200%

32S = 95.02% 33S = 0.75%

34S = 4.21% 36S = 0.02%

35Cl = 75.77% 37Cl = 24.23%

79Br = 50.69% 81Br = 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 62.93 amu and the rest has a mass of 64.93 amu.

... )B"" )(massB"" (% )A"" )(massA"" (% (AAM) massatomic Average

Copper for amu 63.548 A.A.M.

amu 20.06337 amu 43.48463 A.A.M.

amu) .93(0.309)(64 amu) .93(0.691)(62 A.A.M.

Cu29

63.548

IsotopeIsotopePercentPercent

AbundanceAbundanceMassMass

Cu-63 69.1 62.93

Cu-65 64.93

43.48463

20.0633730.9

63.548