Dalton, Gay-Lussac,
and Avogadro
Contributions to Atomic Mass
Connecting Mass to Moles
 One of the greatest challenges early
chemists faced was trying to find a way
to connect the mass of a substance to
the number of particles in the sample.
 It was determined that “elementary
particles” combined in fixed ratios by
weight.
Connecting Mass to Moles
 This led Dalton to the “atomic model” of
matter
 Example: The mass ratio of oxygen to
hydrogen in water is 8:1
 This does not tell us how many atoms of each
element are involved
 It could tell us this if we knew the relative mass of
each kind of atom
Relative Mass
 To assign relative masses to elements it
is necessary to know that the samples
being compared have the same number
of particles
 If particles are all the same size, the same
size sample would have the same number of
particles
 If particles are different sizes, the same size
sample would have different numbers of
particles
Relative Mass
 At the time, chemists did not know which
was true and tended to think the latter
was more likely
 Example: A bucket of baseballs has fewer
balls than an identical bucket of golf balls
 If this is true in the macroscopic world, why
wouldn’t it be true in the sub-microscopic
one?
Relative Mass
 Consider earlier this year when we
studied density: was iron more dense
than aluminum because iron had more
particles per given volume than
aluminum or because iron’s individual
particles were more massive than
aluminum’s? Could it be some
combination of both?
Relative Mass
 The truth is, based on the experiments
we conducted earlier in the year, we
couldn’t say which was true.
 Dalton did not know what was true during
his time either.
 Since the mass of individual atoms could not
be determined, a system of atomic masses
had to be determined by comparison.
Relative Mass
 To determine a system of masses by
comparison, one element would have to
be chosen as the basis of comparison for
all others
 Dalton chose hydrogen and assigned it a
mass of 1.
Relative Mass
 To find the mass of another element like
oxygen:
 Compare the masses of equal number of
oxygen and hydrogen atoms OR
 Find the combining masses of oxygen and
hydrogen in water
Relative Mass
 Dalton thought that the former approach
was invalid because he thought identical
volumes of hydrogen and oxygen gases
would have different numbers of particles
 He thought the latter was valid but did not
take into account that it is valid ONLY if
the ratio of atomic combination is known
Reactions of Gases
 Research conducted by Gay-Lussac
suggested that equal volumes of gases,
at the same temperature and pressure,
contain equal numbers of particles
Reactions of Gases
 Gay-Lussac noted that gases appear to
react in simple integer ratios
 Example: Two volumes of hydrogen reacted
with one volume of oxygen to produce two
volumes of water
 These findings appeared to contradict the
idea that equal volumes of gases have
equal numbers of particles
Reactions of Gases
 Why? Well, if water is was H2O, then two
volumes of hydrogen and one volume of
oxygen should make one volume of
water
+
+
=
Reactions of Gases
 Gay-Lussac’s law that equal volumes of
gases have equal numbers of particles
and Dalton’s atomic theory did not seem
to be compatible
 In 1811, Avogadro wrote a paper that
reconciled the two
Avogadro’s Hypothesis
 Avogadro assumed
 Equal volumes of gases have equal
numbers of molecules
 These molecules can be split into halfmolecules during chemical reactions
 That molecules of elemental gases could
contain more than a single atom
Avogadro’s Hypothesis
 Two volumes of hydrogen react with one
volume of oxygen to produce two
volumes of water when hydrogen and
oxygen can be split into half-molecules!
+
+
=
+
Avogadro’s Hypothesis
 If we accept Avogadro’s Hypothesis, we
can compare the mass of various gases
and deduce the relative mass of the
molecules
 To do this, we pick a weighable amount
of the lightest element (how about 1.0 g?)
then use mass ratios to assign atomic
masses to the other elements
Implications
 If two volumes of hydrogen combine with
one volume of oxygen gas, it is
reasonable to assume that two molecules
of hydrogen are reacting with each
molecule of oxygen
 The word chosen to represent the
standard weighable amount of stuff, the
mole, comes from the Latin “mole cula”
or little lump