Q. How to calculate atomic mass of an element?
Copper survive as a mixture of two isotopes. The lighter isotope (Cu-63) amid 29 protons and 34 neutrons makes up 69.17% of copper atoms. The heavier isotope (Cu-65) with 29 protons and 36 neutrons constitutes the remaining 30.83% of copper atoms. The atomic mass of Cu-63 is 62.930 amu, as well as the atomic mass of Cu-65 is 64.928 amu. Use the data above to calculate the atomic mass of copper.
For a trouble that has several numerical values it is wise to organize the data into a table.
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Isotope
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Copper-63
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Copper-65
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Number of protons
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29
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29
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Number of neutrons
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34
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36
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Atomic mass
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62.930 amu
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64.928 amu
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Abundance
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69.17%
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30.83%
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First compute the contribution of each isotope to the average atomic mass being sure to convert each percent to a fractional abundance.
Mass contribution = mass of isotope * abundance of isotope
For Cu-63
Mass contribution = 62.930 amu * 0.6917 = 43.53 amu
For Cu-65
Mass contribution = 64.928 amu * 0.3083 = 20.02 amu
The average atomic mass of the element is the total of the mass contributions of each isotope.
Atomic mass Cu = mass contribution Cu-63+ mass contribution Cu-65
Atomic mass Cu = 43.529 amu + 20.017 amu = 63.546 amu