Average Atomic Mass

The weighted average of the masses of all the naturally

occurring isotopes of an element.

Calculating Average Atomic Mass

Copper-63 accounts for 69% of the copper found naturally.

Copper-65 accounts for 31% of the copper found naturally.

What is the average atomic mass of copper?

63 x 0.69=43.47 amu

65 x 0.31=20.15 amu

43.47 + 20.15= 63.62 amu

Steps for Calculating Average Atomic Mass

1st Change the percent to a decimal (move the decimal two places to the left).

2nd Multiply the mass number of the isotope by the percent. Do this for each isotope of the element.

3rd Find the sum of (add) the products.

4th Put units on your answer (amu).

You Try It

1. Chlorine-35 makes up 76% of all the chlorine in nature, and Chlorine-37 makes up the other 24%. What is the average atomic mass of chlorine?

2. Calculate the atomic mass of boron, which occurs naturally as 20% boron-10 and 80% boron-11.

3. Titanium has 5 isotopes they aretitanium-46 (8%)titanium-47 (7.3%)titanium-48 (73.8%)titanium-49 (5.5%)titanium-50 (5.4%)