1. Calculating Average Atomic Mass
Unit 9 worksheet 2

2. Average atomic mass The decimal number on the periodic table
The weighted average of all the isotopes of an element
Depends on the percent (relative) abundance and the mass of each isotope
Measured in “atomic mass units” (amu)

3. Problem 1 Given: Element X has 2 isotopes
Mass = 6 amu and percent abundance = 7.5% (0.075)
Mass = 7 amu and percent abundance = 92.5% (0.925)

4. To solve
Average atomic mass = [(mass) x (abundance)] + [(mass) x (abundance)]
[(6) X (0.075)] + [(7) x (0.925)]
6.925 amu
On the periodic table, this is closest to Li, atomic number 3.