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#### Average v. Weighted Average

Let’s use chlorine isotopes as an example.

The atomic mass of chlorine-35 = 35
The atomic mass of chlorine-37 = 37

Calculating the average of the two masses will be $\frac{35+37}{2}=36$

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From the periodic table the atomic mass of chlorine = 35.3.

While 36 is a simple average, 35.5 is the weighted average.
The weighted average is more useful here because it accounts for the amount of the individual isotope.

Out of all the chlorine atoms that occur in nature, 75% are and the remaining 25% are ${}_{}{}^{37}C\mathrm{l}$

You can see that 35.5 is closer to 35 than 37. The value shows us that ${}_{}{}^{35}\mathrm{Cl}$ is more abundant in nature than ${}_{}{}^{37}\mathrm{Cl}$.