A presentation showing the historical sequence from Avogadro to Mendeleev, including the all-important role of Cannizzaro.

1. The Atomic Mass Table
Students will be able to explain how
Avogadro's Hypothesis, the Law of Dulong
and Petit, and Cannizzaro's method put
order into chemistry by fabricating a useful
and consistent table of atomic masses.

2. Berzelius
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Invented the 1-2 letter symbol for elements
Published 43 elemental atomic masses in
1818
Determined the % composition of about
2000 compounds.

3. Method of Equivalent Mass
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The equivalent mass of an element is
defined as the amount of that element that
will react with 8.0 grams of oxygen or its
equivalent.
Example: in silver oxide, 13.5 g Ag reacts
with 1.0 g oxygen
EqMass = 13.5 g Ag x 8 g O = 108.0 g Ag
1gO
Compare PT mass of Ag

4. Valence (Franklin, 1852)
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How many atoms does atom X combine
with?
Defined by number of hydrogens it
combines with or replaces.
Valence negative if it combines with
hydrogen (e.g. Cl, O, N)
Valence positive if it replaces hydrogen
(e.g. Zn, Na, Fe)

5. Valence Method of Atomic Mass
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Atomic mass of an
element is the product of
its valence and its
equivalent mass
Example, zinc: 32.6 and
valence +2

Product 65.2

Compare with PT mass

6. Dulong & Petit (1819)

For a solid, the product of specific heat (in
cal/g) and atomic mass is about 6

Example, copper, SH = 0.0932 cal/g

Atomic mass = 6/0.0932 = 64.4 g

Example, aluminum, SH = 0.2174 cal/g
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Atomic mass = 6/0.2174 = 27.6 g
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Compare with PT and discuss why this
might be so close

7. The Problems
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The methods were close to each other but
did not converge on one or the other.
There were some big errors. Sulfur, for
instance, by Dulong-Petit has 10% error.
The methods did not work for all elements
and compounds (e.g. liquids, gases don't
work for Dulong-Petit, and some elements
have multiple valences, like Fe (+2, +3).

8. The Karlsruhe Conference
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Purpose: come to a consensus on atomic
masses
Every important chemist in the world
Chaired by Jean Dumas, who invented the
Dumas method of molecular mass of
liquids you just studied
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Broke up in confusion
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But each got Cannizzaro's pamphlet

9. Cannizzaro's Method
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Sketch of a Course of Chemical Philosophy
Impressed Lothar Meyer, who used it
extensively. Others followed.
Used Avogadro's Hypothesis to get relative
molecular masses of gases
Found smallest mass of element in several
compounds
This was the relative atomic mass

10. Cannizzaro's Table of Calculations
Pubs.rsc.org
No you don't have to explain it

11. Example
Substance
Density
in gaseous
(g/L)
Percentage Product of
hydrogen by
mass
state
Values in
density &
(3) divided
percent (3)
by least
Water
0.589
11.2
0.0659
value
2
Ammonia
0.557
17.7
0.0986
3
Methane
0.524
25.1
0.132
4
You see that the numbers in the last column are
the number of hydrogen atoms in each molecule of
the substance

12. Next step
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Once Cannizzaro had the number of atoms
of an element in a compound, he could use
the several tables to converge on an
atomic mass
Our example: fixed air, a compound of C
and O, had a MM of 44. Once we knew
that there are two oxygen atoms in each
molecule, we subtract 32 from 44 and get
12, the atomic mass of carbon

13. Acceptance
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Lothar Meyer said “the scales fell from my
eyes.”
With a reliable set of atomic masses, the
science of stoichiometry and prediction of
amounts of products and reactants moved
forward rapidly.
This gave Meyer and Mendeleev the data
they needed to produce periodic tables.
Why did Mendeleev prevail?