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