1. Average Atomic Mass What is average atomic mass? Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element

2. Calculating a Weighted Average Example A box contains two size of marbles. If 25.0% have masses of 2.00 g and 75.0% have masses of 3.00 g what is the weighted average? (.250) (2.00) + (.750) (3.00) =.500 + 2.25 = 2.75g

3. Calculating Average Atomic Mass EXAMPLE Boron has two isotopes: B-10 (mass 10.013 amu) 19.8% abundance B-11 (mass 11.009 amu) 80.2% abundance Calculate the average atomic mass. (.198) (10.013) + (.802) ( 11.009) = 1.98 amu + 8.83 amu = 10.81 amu

4. Average Atomic Mass Helium has two naturally occurring isotopes, He-3 and He-4. The atomic mass of helium is 4.003 amu. Which isotope is more abundant in nature? He-4 is more abundant in nature because the atomic mass is closer to the mass of He-4 than to the mass of He-3.