How to Find the Relative Abundance of Isotopes
Most elements come in multiple isotopes, atoms that have the same number of protons and electrons but different number of neutrons. Different isotopes react the same chemically, but their atomic weights are different; the atomic weights given in textbooks for the elements are the average of all the different isotopes. To figure out that average, scientists have to know the weights of the different isotopes and how abundant they are compared to each other.
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Instructions
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Spectrometry
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1
Set up a mass spectrometer. Retired chemistry teacher and author Jim Clark's website describes a spectrometer as a device that ionizes a stream of atoms and then deflects them as they pass through a vacuum chamber. Isotopes have different weights, so they'll each be deflected by a different amount, which the spectrometer measures.
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2
Feed samples of the element you're studying into the spectrometer. This will have to be done multiple times, with the spectrometer deflecting a different isotope on each run.
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3
Study the results. The spectrometer readings will show the atomic weights of the different isotopes, and their percentage in the sample. The relative size of the peaks on the graphs shows the relative percentages.
When You Already Know the Atomic Weights
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4
List the atomic weights of the different isotopes, if you already know them. Together they will add up to the element's atomic weight, which makes it possible to figure out the relative abundance using algebra.
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5
Determine the formula for element you're studying. St. Louis Community College's website lays out the calculations. If an element has two isotopes with weights of 13 and 16, and the average weight is 14, then the formula would be 13(1-x)+16x=14 and x would be the relative abundance of 16.
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6
Make the calculations. The equation in Step 2 would go as follows:
13-13x+16x=14
-13x+16x=1
3x=1.
x=0.333
So isotope 16 has a relative abundance of 33 percent.
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Tips & Warnings
Mass spectroscopy isn't always easy. Chlorine molecules tend to become ionized molecules rather than ionized atoms, and the molecular weights will give different readings. You'll have to make an extra effort to figure out the real isotopic weights.
References
Resources
- Photo Credit atom image by Oleg Verbitsky from Fotolia.com
