1. Chapter 4 AVERAGE ATOMIC MASS

2. Atomic Mass… n The weighted average of the masses of all the naturally occurring isotopes of that element. n Is not a whole number because it is an average. n are the decimal numbers on the periodic table. n Is based on ABUNDANCE (percentage) of each element in nature. n Also based on the MASS NUMBER of the isotope

3. Difference between weighted average and arithmetic averages n Arithmetic averages are the averages you are familiar with. –Find the average of the following grades: 86 45 79 = ( 86 + 45 + 79) / 3 = 70

4. n Weighted average takes in to account the abundance or percentage. n n Avg. Atomic Mass = Mass # x Percent (as a decimal) n Calculate the weighted average for the following scores: o86 (20 % of grade) o69 (20 % of grade) o45 (60 % of grade) 86 x.20 = 69 x.20 = 45 x.60 = 17.2 13.8 27.0 58.0 % To convert percents to decimals – move decimal two places to the left ADD THE NUMBERS TOGETHER

5. Calculating Atomic Mass n Chlorine-35 makes up 76% of all chlorine in nature, Chlorine-37 makes up the other 24%. What is the atomic mass of chlorine? n 35 x.76 = n 37 x.24 = 26.60 8.88 ADD THE NUMBERS TOGETHER 35.48 amu

6. Calculating Atomic Mass n Calculate the atomic mass of Gallium if gallium-69 occurs at 60% and gallium-71 occurs at 40% n 69 x.60 = n 71 x.40 = 41.4 28.4 69.8 amu