How to Find Average Atomic Mass
The average atomic mass of an element is calculated by summing the masses of each isotope multiplied by its natural abundance. This value, readily available on the periodic table, is crucial for various chemical calculations.
An element's isotopes are atoms with the same number of protons but different numbers of neutrons. This results in variations in their atomic mass. Each isotope exists in nature with a specific abundance, representing its percentage occurrence.
Calculating Average Atomic Mass: A Step-by-Step Guide
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Identify Isotopes and Abundances: Determine all the naturally occurring isotopes of the element and their respective percent abundances. These are usually given in a problem or can be found in chemistry reference materials.
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Convert Percentages to Decimals: Divide each percent abundance by 100 to express it as a decimal.
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Multiply and Sum: For each isotope, multiply its atomic mass (in atomic mass units or amu) by its decimal abundance. Add the results for all isotopes together.
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Result: The final sum represents the element's average atomic mass.
Example Calculation
Let's calculate the average atomic mass of boron (B), which has two naturally occurring isotopes:
- Boron-10 (¹⁰B): Atomic mass = 10.01 amu, Abundance = 19.9%
- Boron-11 (¹¹B): Atomic mass = 11.01 amu, Abundance = 80.1%
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Convert Percentages:
- ¹⁰B: 19.9% / 100 = 0.199
- ¹¹B: 80.1% / 100 = 0.801
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Multiply and Sum:
- ¹⁰B: 10.01 amu * 0.199 = 1.99199 amu
- ¹¹B: 11.01 amu * 0.801 = 8.81801 amu
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Average Atomic Mass: 1.99199 amu + 8.81801 amu = 10.81 amu (approximately)
Therefore, the average atomic mass of boron is approximately 10.81 amu. This value is the one you'll find listed for boron on the periodic table.
Where to Find Average Atomic Mass
The average atomic mass of any element is readily available on a standard periodic table. This table uses the weighted average of an element's isotopes, taking into account natural abundance, making it a convenient resource for all chemical calculations.
As stated in multiple sources, the average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element (LibreTexts, Biology LibreTexts, Chemistry LibreTexts). Always use the average atomic mass found on the periodic table for mass calculations.
