Some notes on Atomism, 1806-1860

The principal problem confronting the development of a suitable atomic weight scale was the concept that "molecules" of elements i.e. O₂, N₂, Cl₂, etc. could not form. This concept was principally due to John Dalton. Dalton referred to "simple" atoms and "compound atoms", compound atoms being our molecules. Great confusion reigned through most of the nineteenth century regarding such terminology, one man's atom being another man's molecule. The need for a common language developed slowly. Those present at the Karlsruhe Conference in 1860 were presented with the following question at the top of the agenda.

Even in 1860 no consensus was reached. Part of the original problem was Dalton's insistence that atoms of the same kind could not react together. This was reinforced by the electrochemical researches of Davy and Faraday which added the concept that metals were positive and non-metals negative e.g. non-metals would repel each other especially those of the same kind. On top of this, when atoms did combine there was no way of telling in what atomic ratio they combined.

A guiding principal was the "Rule of Greatest Simplicity" which said that the most stable compound between elements would be the 1:1 atomic ratio. Relative combining weights could be measured e.g. 8 g of oxygen combined with 1 g of hydrogen. Water being the most stable (the only) compound between these two elements the formula was HO. This meant the relative atomic weight for oxygen would be 8. There were several compounds between nitrogen and oxygen known to Dalton.

• N₂O, NO, and NO₂ for instance. The most stable of these would be NO. The other compounds between these two elements could form giving other formulas based on the weight ratios found and "Law of Multiple Proportions".

The solution to the problem presented by Cannizzaro at Karlsruhe and debated afterward was solved using two ideas from the early 1800s. These were:

• Gay Lussac's "Law of Combining Volumes" - In 1809 Gay Lussac presented his "Law of Combining Volumes of Gases", an experimental result which said "Whenever gases react or gases form under the same conditions of temperature and pressure they do so in the ratio of small whole numbers."

• Avogadro's Hypothesis - In 1811 A. Avogadro published what is now known as Avogadro's hypothesis, a theoretical result. This said that "equal volumes of gases under the same conditions of temperature and pressure contain equal numbers of molecules".(1809 Gay Lussac, 1811 Avogadro)

Dalton and many others would not accept the implications of these. The arguments using these two ideas are the following.

1) Suppose nitrogen reacts with oxygen to form NO, according to Dalton.  N + O -----> NO.  Gas volumes measure the following ratios:   1 volume N + 1 volume O ----> 2 Volumes NO. According to Avogadro's Hypothesis using one atom or molecule in each volume (equal numbers) (consider each box to be a volume with the same volume as every other box.)

This does NOT fit the hypothesis but

does fit because now there are the same number of atoms of each element on each side of the equation; i.e. the equation is balanced and the reaction obeys the Law of Conservation of Mass, one of Dalton's rerequirements. 

Therefore elemental nitrogen must be diatomic, elemental oxygen must be diatomic.

2) The same result is obtained for water.

H + O -----> HO (vapor)

from Gay-Lussac:

2 volumes H + 1 volume O -----> 2 volumes water vapor

from Avogadro:

works for H & H₂O but not for O but

works for all three. Therefore hydrogen must be H₂, oxygen O₂ and water H₂O.

The H₂O formula for water would mean that if the atomic weight for H were 1, then oxygen must be 16 and not 8. Other arguments can be made for the solution to other elements which we now take to be diatomic, Cl₂, F₂ etc.

The solution to this problem ultimately led to a correct scale of atomic weights.