1. 1 Atomic Mass The atomic mass of an element is listed below the symbol of each element on the periodic table. gives the mass of an “average” atom of each element compared to 12 C. is not the same as the mass number. Na 22.99

2. 2 Calculating Atomic Mass The calculation for atomic mass requires the percent(%) abundance of each isotope. atomic mass of each isotope of that element. sum of the weighted averages. mass of isotope(1)x (%) + mass of isotope(2) x (%) + 100 100

3. 3 35 Cl has atomic mass 34.97 amu (75.76%) and 37 C has atomic mass 36.97 amu (24.24%). Use atomic mass and percent of each isotope to calculate the contribution of each isotope to the weighted average. 34.97 x 75.76 = 26.49 amu 100 36.97 x 24.24 = 8.961 amu 100 Sum is atomic mass of Cl 35.45 amu Calculating Atomic Mass for Cl

4. 4 Calculating Atomic Mass Mg Isotope Mass Abundance 24 Mg = 23.99 amu x 78.70/100 = 18.88 amu 25 Mg = 24.99 amu x 10.13/100 = 2.531 amu 26 Mg = 25.98 amu x 11.17/100 = 2.902 amu Atomic mass (average mass) Mg = 24.31 amu

5. 5 Atomic Mass of Magnesium The atomic mass of Mg is due to all the Mg isotopes. is a weighted average. is not a whole number.

6. 6 Gallium is an element found in lasers used in compact disc players. In a sample of gallium, there is 60.10% of 69 Ga (atomic mass 68.926) atoms and 39.90% of 71 Ga (atomic mass 70.925) atoms. What is the atomic mass of gallium? Learning Check

7. 7 69 Ga 68.926 amu x 60.10 = 41.42 amu (from 69 Ga) 100 71 Ga 70.925 amu x 39.90 = 28.30 amu (from 71 Ga) 100 Atomic mass Ga = 69.72 amu Solution 31 Ga 69.72