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Isotopes = atoms with the same number of protons but different number of neutrons OR different versions of the same element but with different masses.
Formula for Weighted Averages (Same formula two different ways)
Isotope Equation Version #1
Use this if you want to plug in percents the way you see them in the problem
Isotope Equation Version #2
(%abundance1 X mass1) + (%abundance2 X mass2) ...
↑
Information for Isotope 1
↑
Information for Isotope 2
Use this if you change your % into a decimal ahead of time (move the decimal to the left two spaces).
Example 3% abundance = .03 Why do it this way? you don't need to divide by 100 = fewer numbers to work with.
Video above changes the percent to a decimal.
To do this simply move the decimal two spaces to the left. 2% = .02 in decimal form.
Below: #3 From Abundance of Isotopes Worksheet:
❶ Read the Problem
Calculate the atomic mass of silicon. The three silicon isotopes have atomic masses and relative abundances of 27.9769 amu (92.2297%), 28.9765 amu (4.6832%) and 29.9738 amu (3.0872%).
❷ Dissect the problem:
note that there are three different isotopes in this example:
Calculate the atomic mass of silicon. The three silicon isotopes have atomic masses and relative abundances of 27.9769 amu (92.2297%), 28.9765 amu (4.6832%) and 29.9738 amu (3.0872%).
❸Plug into the formula and calculate - see how close you get!
Each isotope has two pieces of information to plug into each parenthesis. Your answer should reflect that of the atomic mass (bottom number) on the periodic table.
Wrong answers??
Double check your formula:
҉ If you are not dividing by 100 check that your %'s must have the decimal moved two places to the left:
eg 2% = .02
҉ Make sure you closed your parenthesis in the right locations. (% X amu ) + (% X amu )
Go ҉ here ҉ for More challenging % abundance & Atomic Weight Problems
to navigate the page, look at the last sentence to see if it is similar to the problem you are stuck on.
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