There are tons of teachers that do the average atomic mass lab – and that’s great! But in talking to teachers over the years, it appears to me that they aren’t considering mass number in their labs. What I mean is, they give the students a sample of a “mystery element” with three different isotopes. Usually its three different types of dry beans. But the teacher will use lentils, which will have a mass number of zero. I always try to get my isotopes’ mass numbers to be different. I feel like it adds to the quality of the lab, overall, and gives kids a better understanding of isotopes.
Average Atomic Mass Background:
The average atomic mass is the weighted average of all the naturally occurring isotopes of an element. It’s determined by taking a sample of an element and adding it to a mass spectrometer. It separates the isotopes by their mass, giving us a picture of which is most abundant. The weighted averages are taken and viola, we have the average atomic mass of the element.
Set Up:
For my average atomic mass lab, I like to simulate this as closely as possible. I will give my students a sample of a mystery element. It’s usually three types of the same food. Like three different sizes of marshmallows or three types of candies. These will represent three isotopes of the same element. Every year I do something a bit different, but the idea is that you want them to have different “mass numbers.” Really you want the mass of one “isotope” to be different from the mass of the others. I’ve been doing quite a bit of research here. I will continue to update this blog post with some “isotope” masses for your reference.
Then you have to set the “natural abundance” or the percentage of atoms in the particular sample. All of the students should have the same abundance of their isotopes. Or at least something close to each other. Unless you want each group to have a different mass.
My tips for the average atomic mass lab:
I like to make the heaviest isotope the least abundant. It helps the kids to realize that the abundance is truly what controls the average atomic mass.
I prefer to do this lab with stuff that is edible. It makes it fun and certainly increases buy-in. But you could very well do this lab with inedible “isotopes.” One of the most common choices is dry beans. I don’t eat a lot of dry beans so I haven’t found those masses. You could also use coins, different shapes of pasta (watch for them absorbing moisture) or even Lego blocks. You could even go to the school supply closet and use paperclips, erasers and pen caps.
I can’t say enough how important it is to have individual mass numbers for your three isotopes. It is truly the way that kids are going to see how the average atomic mass is calculated. As far as I’m concerned three different colors of M&Ms isn’t going to teach them as well.
Also beware of isotopes with masses that are too small and will round down to zero. I’m thinking of mini M&Ms, hot chocolate sized marshmallows and Lego studs. With a mass number of zero, the average atomic mass will be abnormally small.
If you’re doing this in a home school setting, I’ve found this kitchen balance to be effective as it goes out two decimal places, is accurate and doesn’t take up too much space.
Need more?
And if you need more information on teaching average atomic mass, you can check out this post.
If you need a chemistry curriculum, I have one for you! You can get a free copy of my chemistry curriculum outline. This outline explains how I teach chemistry and the order in which I teach the units. There’s also a link to a live document of my day-by-day outline of each and every lesson I teach.
