1. TAKE A …. GUIDED NOTES PAGE ATOMIC MASS W.S. ….OFF MY DESK

2. Average atomic mass How we get those weird decimal numbers on the periodic table

3. Whole numbers If protons and neutrons weigh 1 amu each, why is the atomic mass on the periodic table a decimal number? Answer: The atomic weights given on your table are “weighted averages” of the different isotopes of that element.

4. Quick review of percentages A percentage is equal to that number divided by 100. So, 25% = 25/100 When you divide a number by 100, just move the decimal point two places to the left. Ex. 25% =.25 30% =.30 2% = _____ 1.25% = ______.05% = _______ 125% = ________

5. Example of how to find the average atomic mass Let’s say that Rb has two isotopes, Rb-85 and Rb-87. 72% of all Rubidium is Rb-85. 28% of all Rb is Rb-87. What is the atomic mass of Rb? Answer: 0.72 x 85 amu= 61.2 0.28 x 87 amu = 24.32 Next add the two numbers you get together: (61.2 + 24.32 = 85.56) **This is slightly different than what you will see on the periodic table because I rounded the percentages a bit

6. Example 2: 92.5 % of all Lithium atoms are Li-7. 7.5% are Li-6. What is the average atomic mass?.925 x 7 amu = 6.475.075 x 6 amu= 0.45 6.925 amu