1. Average Atomic Mass The weighted average of the masses of all the naturally occurring isotopes of an element.

2. 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

3. Steps for Calculating Average Atomic Mass 1 st Change the percent to a decimal (move the decimal two places to the left). 2 nd Multiply the mass number of the isotope by the percent. Do this for each isotope of the element. 3 rd Find the sum of (add) the products. 4 th Put units on your answer (amu).

4. 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 are titanium-46 (8%) titanium-47 (7.3%) titanium-48 (73.8%) titanium-49 (5.5%) titanium-50 (5.4%)