Atoms And Elements Worksheet Page 7
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18The following worked example illustrates the calculation of atomic mass from isotopic
mass and abundance data.
Worked Example 2-3
Calculate the atomic mass of magnesium using the three naturally occurring
isotopes below.
24
Mg (atomic mass 23.9850 amu, abundance 78.99%)
12
25
Mg (atomic mass 24.9858 amu, abundance 10.00%)
12
26
Mg (atomic mass 25.9826 amu, abundance 11.01%)
12
Solution:
The atomic mass of magnesium is called a weighted average of the atomic masses
of these three isotopes. To calculate the atomic mass, simply multiply the atomic
mass by the percent abundance in decimal form (the percent divided by 100), then
add the results.
78.99
Fraction Mg-24 =
= 0.7899
100
10.00
Fraction Mg-25 =
= 0.1000
100
11.01
Fraction Mg-26 =
= 0.1101
100
(23.9850 amu x 0.7899) + (24.9858 amu x 0.1000) + (25.9826 amu x 0.1101) = 24.31 amu
The “average” mass of a magnesium atom is 24.31 amu, although you should note
that no magnesium atom actually exists with a mass of 24.31 amu. In nature,
magnesium atoms weigh 23.9850 amu or 24.9858 amu or 25.9826 amu. A mass
of 24.31 amu represents the mass of a hypothetical average magnesium atom.
Mathematics tells that an average of data points is a more reliable representation
of the data than any individual value; we remain consistent with that principle
here.
The mass reported on the periodic table for magnesium is 24.31 amu. See the
periodic table.
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