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Learn about the concept of atomic mass and how it is calculated using weighted averages of different isotopes. Discover why atomic masses on the periodic table are decimal numbers.
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TAKE A …. • GUIDED NOTES PAGE • ATOMIC MASS W.S. ….OFF MY DESK
Average atomic mass How we get those weird decimal numbers on the periodic table
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.
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% = ________
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
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
