* Isotopes have different abundances: The average atomic mass accounts for the fact that isotopes exist in different proportions in nature. For example, chlorine has two major isotopes: chlorine-35 and chlorine-37. Chlorine-35 is much more abundant than chlorine-37. Simply averaging their masses would give you a number that doesn't reflect the true distribution.
* Weighted average: The average atomic mass is actually a *weighted average*. It's calculated by considering the mass of each isotope *and* its relative abundance. The more abundant an isotope is, the more its mass contributes to the overall average.
Here's how it works:
1. Identify the isotopes: Determine the isotopes of the element and their respective masses.
2. Find the relative abundance: Determine the percentage of each isotope present in nature (usually given in a periodic table or a textbook).
3. Calculate the weighted average: Multiply the mass of each isotope by its abundance (expressed as a decimal). Then add up the results.
Example: Chlorine
* Chlorine-35: Mass = 34.9689 amu, Abundance = 75.77%
* Chlorine-37: Mass = 36.9659 amu, Abundance = 24.23%
Average Atomic Mass of Chlorine:
(34.9689 amu * 0.7577) + (36.9659 amu * 0.2423) = 35.453 amu
Key Takeaway: You need to factor in the abundance of each isotope to calculate the average atomic mass, not just the average of their individual masses.
