1 Atomic Mass The atomic mass of an element is listed below the symbol of each element on the...

1

Atomic Mass

The atomic mass of an element• is listed below the symbol of each element on

the periodic table.

• gives the mass of an “average” atom of each element compared to 12C.

• is not the same as the mass number.

Na22.99

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Calculating Atomic Mass

The calculation for atomic mass requires the

• percent(%) abundance of each isotope.

• atomic mass of each isotope of that element.

• sum of the weighted averages.

mass of isotope(1)x (%) + mass of isotope(2) x (%) +

100 100

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35Cl has atomic mass 34.97 amu (75.76%) and 37C has atomic mass 36.97 amu (24.24%). • Use atomic mass and percent of each isotope to

calculate the contribution of each isotope to the weighted average. 34.97 x 75.76 = 26.49 amu 10036.97 x 24.24 = 8.961 amu

100 • Sum is atomic mass of Cl 35.45 amu

Calculating Atomic Mass for Cl

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Calculating Atomic Mass Mg

Isotope Mass Abundance

24Mg = 23.99 amu x 78.70/100 = 18.88 amu25Mg = 24.99 amu x 10.13/100 = 2.531 amu26Mg = 25.98 amu x 11.17/100 = 2.902 amu

Atomic mass (average mass) Mg = 24.31 amu

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Atomic Mass of Magnesium

The atomic mass of Mg• is due to all the Mg

isotopes.• is a weighted average.• is not a whole number.

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Gallium is an element found in lasers used in compact disc players. In a sample of gallium, there is 60.10% of 69Ga (atomic mass 68.926) atoms and 39.90% of 71Ga (atomic mass 70.925) atoms.

What is the atomic mass of gallium?

Learning Check

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69Ga 68.926 amu x 60.10 = 41.42 amu (from 69Ga) 10071Ga70.925 amu x 39.90 = 28.30 amu (from 71Ga) 100 Atomic mass Ga = 69.72 amu

Solution

31

Ga

69.72