Dalton’s Dilemma
Dalton knew atoms were too small to measure the mass, but based on the Law of Definite
Composition (by mass) he deduced that the relative masses of atoms could be
determined. For example water was always 11.1% H by mass and 88.9% O by mass.
Dalton made the assumption that when atoms formed molecules it would be 1 atom of H
joining 1 atom of O to form HO known as water.
By defining H as having a relative atomic mass of 1.0 then oxygen would have a relative
mass of 88.9÷11.1 of 8.0. Since the composition of water was always the same then in
9.0 grams of water 1.0 gram of H would have the same number of particles as 8.0 grams
of O. Likewise in 1000 grams there would be 111 grams H and 889 grams of O, AND
there would be equal numbers of H and O atoms based on the assumption that the
formula for water was HO.
This idea of relative atomic mass was a major breakthrough. However, the notion that
molecules were formed by one of each atom type created a dilemma for Dalton.
Gay-Lussac determined by experiment that gases (at the same temperature and pressure)
combined in whole number ratios.
For example:
1 volume of H + 1 volume of Cl Æ 2 volumes of HCl
1 volume of N + 1 volume of O Æ 2 volumes of NO
2 volume of H + 1 volume of O Æ 2 volume of HO vapor
Avogadro interpreted the results of the Law of Combining Gases as equal volumes of
gases at the same temperature and pressure would have equal number of particles.
Dalton objected to this idea. He could see how it would work for HCl and NO but not
HO. Why? Recall that Dalton envisioned the atoms of all elements as monatomic.
Come up with a model that would fit the Dalton model
A) 2 Volumes of H + 1 Volume of O Æ 1 volume of HO + 1 volume H
B) 2 Volumes of H + 1 Volume of O Æ 2 volume of HO where O atoms split in half to
form HO0,5
Avogadro decided that some gases were diatomic, and the formation of water produced
H2O molecules.
Make a drawing…
Avogadro hypothesized that equal volumes of gases at the same temperature and pressure
have equal number of particles. This being so hydrogen and oxygen had to be diatomic.
This furthermore substantiated the idea the atoms were indivisible as their name implied.
The Avogadro hypothesis was not fully accepted for many years. It is no longer a
hypothesis though it still bears that name.