I am trying to understand some of the historical context behind the discovery of atoms (pre-Avogadro). In particular, I am getting stuck in reading the following documents:

http://physics.unl.edu/~klee/phys261/readings/class09.pdf http://physics.unl.edu/~klee/phys261/readings/class11.pdf

My understanding is that "atoms" (as they understood them at the time) were discovered to react in whole-number ratios by Proust/Dalton/Berzelius (who measured by mass) and by Gay-Lussac (who measured by volume). While this was highly suggestive of some sort of "atom" concept, there was a discrepancy between the two sets of measurements. But I'm having trouble following the specific discrepancy pointed out by the above PDFs.

The first PDF gives the following mass data:

                         Mass of Nitrogen    Mass of Oxygen
Nitrous Oxide (N₂O)             100 g             58 g
Nitric Oxide (NO)               100 g            127 g
Nitrogen Dioxide (NO₂)          100 g            239 g

The text doesn't explicitly mention how this data was interpreted by Proust/Dalton/Berzelius. Looking at the data, I don't see that much can really be said:

? N + 1 O = ? Nitrous Oxide (N₂O)
? N + 2 O = ? Nitric Oxide (NO) 
? N + 4 O = ? Nitrogen Dioxide (NO₂)

The question marks are there since they didn't know what was "in" 100 g of nitrogen, which would mean that they didn't really know what was in the product, either, right?

So that's my first question: Am I correct in thinking that the above is about as far as Proust/Dalton/Berzelius could go with that data? Or were they pretty sure they knew how to fill in the question marks?

Now, continuing on to the second PDF, when Gay-Lussac measured by volume, he had:

                        Volume Nitrogen  Volume Oxygen  Volume Product 
Nitrous Oxide (N₂O)             100           49.5             100
Nitric Oxide (NO)               100          108.9             200
Nitrogen Dioxide (NO₂)          100          204.7             200

Now, the document goes on to say that the above data might suggest the following formulas:

2 N + 1 O = Nitrous Oxide (N₂O)
1 N + 1 O = Nitric Oxide (NO)
1 N + 2 O = Nitrogen Dioxide (NO₂)

The text then says that the equation for nitrous oxide represents a contradiction.

But I don't see how that contradiction follows from the data. As the text suggests, if we say that a volume of "50" represents one "atom" (so that volume of "100" nitrogen represents two atoms), wouldn't we say:

2 N + 1 O = 2 Nitrous Oxide (N₂O)
2 N + 2 O = 4 Nitric Oxide (NO)
2 N + 4 O = 4 Nitrogen Dioxide (NO₂)

Wouldn't that mean really it's the other two equations that don't make sense based on the number of atoms not working out? I feel like I'm missing a basic idea in the text.

(FYI, I am not in school and so I'm not a student, nor is this homework.)

  • $\begingroup$ maybe the contradiction would be that nitrogen and oxygen should have theoretically equal volumes $\endgroup$
    – amish dua
    May 12, 2018 at 5:59
  • 1
    $\begingroup$ Today the PDFs have disappeared from what I assume to be a supplemental reading material for a class. $\endgroup$
    – MaxW
    Jul 17, 2018 at 15:54

1 Answer 1


In regards to your first question, I would think that Dalton would have a sense of the molecules' formulas based on the information at hand. Not only did Dalton have the mass ratios for these compounds, but he had also developed a measure of the relative mass of the atoms, as shown in your first document. Given this information, it would not have been difficult to compute that there were $\sim2$ atoms of nitrogen for every one of oxygen in nitrous oxide, nitric oxide was one to one, etc.

In regards to your second question, I think you are misinterpreting the contradiction. The authors proposal for the reactions was that if the compound formed was $\ce{N2O}$, it should only form one volume, since it should only form as much volume as there was volume of $\ce{O}$ originally. This is because there can only be as many molecules formed as there were atoms of the limiting reactant. The two volumes of $\ce{N2O}$ that are actually produced runs counter to this, as it would seem to require having $2$ volumes of $\ce{O}$ to start with.

For all the reactions, the products and reactants were related by whole number ratios, but not the same ratios suggested by their predicted molecular formulas.

  • $\begingroup$ Thanks for the clarification on the known ratios. So going back to the first part to fill in those question marks, I would think that since Proust etc didn't know about diatomic molecules, they would say something like 2N + 1O = 1N₂O, 2N + 2O = 2NO, and 2N + 4O = 2NO₂. As for the second part, however, I'm not totally sure I follow. If Guy-Lussac thought that we should "only form as much volume as there was volume of O originally," then aren't both the N₂O and NO forumulas inconsistent? Because in both of those cases, the volume of the product does not match the volume of oxygen? $\endgroup$
    – anon
    May 18, 2018 at 4:42
  • $\begingroup$ @anon I phrased that confusingly, focusing too much on the one example. I will fix it, but to clarify I believe they had a concept of a limiting reactant at the time. So Gay-Lussac would predict that all of those would form the same volume as the limiting reactant, so in fact all of the reaction schemes he proposed are inconsistent, they give twice the volume we would expect based on their proposed ratios. I think the author might have only highlighted N2O as an example. $\endgroup$
    – Tyberius
    May 18, 2018 at 13:06
  • $\begingroup$ @anon it's also worth noting that the author doesn't say that Gay-Lussac proposed these reactions, but just that someone might, so it isn't clear if he interpreted the data this way. $\endgroup$
    – Tyberius
    May 18, 2018 at 13:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.