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Chemistry (including Biochemistry)/atomic mass vs. density

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Okay, so I'm teaching my 5th graders that the chemical properties of a given element are determined by the arrangement of its electrons, while its mass is (mostly)determined by the contents of its nucleus.  I typically contrast the mass & density of gold vs. the mass and density of aluminum to make my point.  Then, I notice that this is not really true:  By my understanding, the mass of gold (197 vs. lead #207 would imply that lead have a higher density than gold when, in fact, lead's density 11.3 g/cc is far lower than that of gold 19.3 g/cc).  Egads!  Before I bright 5th grade student catches me with my pants down, what am I missing here?   Thanks a lot!

Answer
Edit - you might try different size marbles since they will be closer in size and weight. But you have the right idea.

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You simplified it too much.

chemical properties - it is number AND arrangement of electrons. Show them the columns of the periodic chart. Columns have similar, rows do not, but there are patterns in rows.

mass - you have mass correct, but density is mass and packing arrangement

The best demo for mass and density is to take a tall container (even a glass and marbles) and two types of balls, one slightly bigger than the other. Weigh out the same mass of each kind of ball, then place them in the container. It takes some trial and error to get the correct balls and container. With the small balls, it will take little volume and of course more space for the bigger. So the bigger balls have a lower density. If the container is bigger and wider, the volumes will be nearly the same.

One thing to always teach kids is that science rules follow patterns but there can always be exceptions. Give them real life examples. A tall person is not always a good basketball player for example.

And always offer to get them answers. That is what this website does (except for things that burn or explode). Hopefully, you will never get THE question. "Why is mercury a liquid?" Even if they have the explanation, most PhDs can not understand it.

Keep up the good work with the kids. It is much easier today since all the dangerous demos we would like to do for them are on Youtube. If you are looking for things they can do, google the term, "Kitchen chemistry". Lots of demos they can do with things from home, but only at school. Mom's object to making goop at home. Let me know if I can help more.

Chemistry (including Biochemistry)

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Chemistry (non-biochemistry), environmental science, occupational health and safety, environmental regulation and management, environmental engineering, and wastewater engineering. I'm the Director of Environmental, Health, and Safety and the Director of Research at the Institute of Textile Technology.

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Chemistry (non-biochemistry), environmental science, occupational health and safety, environmental regulation and management, environmental engineering, and wastewater engineering. I'm the Director of Environmental, Health, and Safety and the Director of Research at the Institute of Textile Technology.

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PhD, MS, BS in Chemistry

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