Calculating the weighted atomic mass, often called the average atomic mass, is straightforward. It reflects the average mass of an element's atoms, considering the different isotopes and their relative abundances.
Understanding Isotopes
An element can exist as different isotopes. Isotopes are atoms of the same element with the same number of protons but a different number of neutrons. This means they have the same atomic number but different mass numbers (protons + neutrons). Each isotope has its own specific mass.
Calculating Weighted Atomic Mass: A Step-by-Step Guide
The weighted average accounts for the different masses and abundances of each isotope. Here's how to calculate it:
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Identify Isotopes and Abundances: First, determine all the naturally occurring isotopes of the element. For each isotope, you need its mass (in atomic mass units, amu) and its natural abundance (usually expressed as a percentage).
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Convert Percentages to Decimals: Change each isotope's percentage abundance to a decimal by dividing by 100.
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Multiply and Sum: For each isotope, multiply its mass (in amu) by its decimal abundance. Then, add up all these products.
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Result: The sum from Step 3 is the weighted average atomic mass of the element. This value is typically found on the periodic table.
Example: Calculating the Average Atomic Mass of Copper
Copper (Cu) has two naturally occurring isotopes:
- ⁶³Cu: Mass = 62.93 amu, Abundance = 69.17%
- ⁶⁵Cu: Mass = 64.93 amu, Abundance = 30.83%
Calculation:
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Convert percentages to decimals: 69.17% = 0.6917; 30.83% = 0.3083
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Multiply and sum: (62.93 amu * 0.6917) + (64.93 amu * 0.3083) = 63.55 amu (approximately)
Therefore, the average atomic mass of copper is approximately 63.55 amu. This value is consistent with the value shown on most periodic tables.
Key Considerations:
- The average atomic mass is a weighted average, meaning isotopes with higher abundances contribute more significantly to the final value.
- The accuracy of the calculation depends on the accuracy of the isotopic masses and abundance percentages. These values are typically determined experimentally.
