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I am not a chemist, but I will be explaining chemistry to some third-graders. I want to give the example of how different things can be made of the same parts, but arranged differently. My "real world" example is a car and a motorcycle. The car has an engine and four wheels, but we can create a different vehicle with an engine and only two wheels: we get a motorcycle. Does there exist a molecule composed of the elements $\ce{X}$ and $\ce{Y}$, where $\ce{X}$ and $\ce{Y}$ can form a different molecule when the quantities of $\ce{X}$ and/or $\ce{Y}$ atoms are different?

I was hoping to use carbon and hydrogen because the children have already heard me use those words, and $\ce{CH4}$ is a gas that will make them laugh when I tell them where it comes from. But the only $\ce{CH2}$ molecule that I could find is this, which is too complicated and unfamiliar to explain. I would prefer molecules composed of the common elements hydrogen, oxygen, carbon. I would like to avoid molecules of elements that are commonly found in their elemental state such as gold, iron, or silver as at this stage I want to reinforce the idea that we cannot see molecules with the unaided eye. I might introduce the idea that a molecule (and macromaterial) can be composed of a single element later, if this lesson goes well.

For purposes of discussion, a molecule can be defined as "a discrete body composed of atoms". There is no need to address quantum chemistry, as the target audience is a third-grade classroom.

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    $\begingroup$ How about CH4 and C2H4? And C2H2 for that matter. $\endgroup$ – jerepierre Oct 14 '15 at 7:15
  • $\begingroup$ Sn and Cl or Pb and Cl. If you're limiting yourself to Period 2 elements nitrogen oxides are your best bet, although there is certainly no NO4 $\endgroup$ – orthocresol♦ Oct 14 '15 at 7:17
  • $\begingroup$ @jerepierre: Thanks, I might be able to work those in. I'll need to think of a different analogy, though. $\endgroup$ – dotancohen Oct 14 '15 at 8:11
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    $\begingroup$ You could also use formaldehyde ($\ce{CH2O}$) and methanol ($\ce{CH4O}$). You would just have to add something to your idea that gives the oxygen … a driver maybe? $\endgroup$ – Jan Oct 16 '15 at 9:48
  • $\begingroup$ You can do this with single elements and things that are very familiar (at least if you don't demand simple discrete molecules). Graphite and diamond are the same element built with different connectivity and radically different properties. Dioxygen and ozone also work, though ozone may be less familiar. If you must have compounds, ethanol and ether have the same formula but different properties (and are safe enough to show to children). Or oxides of nitrogen (less safe though except under careful conditions). $\endgroup$ – matt_black Jan 6 at 20:41
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  • $\ce{GeF2}$ and $\ce{GeF4}$
  • $\ce{GeCl2}$ and $\ce{GeCl4}$
  • $\ce{GeBr2}$ and $\ce{GeBr4}$
  • $\ce{GeI2}$ and $\ce{GeI4}$
  • $\ce{SnF2}$ and $\ce{SnF4}$
  • $\ce{SnCl2}$ and $\ce{SnCl4}$
  • $\ce{SnBr2}$ and $\ce{SnBr4}$
  • $\ce{SnI2}$ and $\ce{SnI4}$
  • $\ce{PbF2}$ and $\ce{PbF4}$
  • $\ce{PbCl2}$ and $\ce{PbCl4}$
  • $\ce{XeF2}$ and $\ce{XeF4}$
  • $\ce{XeO2}$ and $\ce{XeO4}$
  • Silicon dihalides ($\ce{F}$, $\ce{Cl}$, $\ce{Br}$, $\ce{I}$) have also all been prepared in the gas phase, but are much less stable than the tetrahalides.
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  • $\begingroup$ Thank you for this invested list. Unfortunately none of those are molecules that form materials which I would expect a third-grader to be familiar with. $\endgroup$ – dotancohen Oct 19 '15 at 9:40
  • $\begingroup$ @dotancohen I would modify your plan a little and do: CO is to CO2 as bicycle is to tricycle. $\endgroup$ – DavePhD Oct 21 '15 at 13:39
  • $\begingroup$ That is a terrific idea. I don't think that the third graders know what carbon monoxide is, but I can bring some H2O and H2O2 for a tricycle and a car. One issue: I don't think that modern children have ever seen a tricycle! At least in Israel, it has been decades since I've seen one myself. $\endgroup$ – dotancohen Oct 21 '15 at 13:45
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    $\begingroup$ @dotancohen My sons are both still in grade school and they both rode tricycles. I wanted to give away one of their old tricycles to charity since they ride bikes now, but they said they still want to keep it! Here (east coast USA) we have carbon monoxide detectors in many homes (too many deaths from generators and heaters) so some would know, but H2O2 would make a better visual demonstration. $\endgroup$ – DavePhD Oct 21 '15 at 14:27
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How about sulfur and oxygen?

$\ce{SO2}$ is an invisible gas that smells terrible. $\ce{SO4^2-}$ is an anion - it has extra two electrons that it must bond with something. That something can be hydrogen in the example of sulfuric acid, or with all kinds of metals. A favourite example of mine is celestine, $\ce{SrSO4}$:

Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0), via Wikimedia Commons
Source: Rob Lavinsky, iRocks.com – CC BY-SA 3.0, via Wikimedia Commons

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The closest I can come up with is $\ce{CH4}$ and $\ce{H2O}$, which obviously have different central elements, but cars and motorcycles have very different bodies with very similar wheels.

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  • $\begingroup$ I think the other one can be formaldehyde $HCOH$ $\endgroup$ – shaistha Oct 14 '15 at 8:06
  • $\begingroup$ @shaistha well hey, if X=CO and Y=H, then we have formaldehyde and H3COH alcohol, which are very different, especially their effect on the body. $\endgroup$ – Eubie Drew Oct 14 '15 at 9:10
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    $\begingroup$ @Aabaakawad he said that he wants to give the example of how different things can be made of the same parts, but arranged differently. $\endgroup$ – shaistha Oct 14 '15 at 11:10
  • $\begingroup$ @shaistha You are right. Some pair of molecules built out of H, C, & O. By the way, formaldehyde is CH2O with no H on the O. HCOH is the hydroxymethylene radical. $\endgroup$ – Eubie Drew Oct 15 '15 at 5:03

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