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.
Berzelius
Invented the 1-2 letter symbol for elements Published 43 elemental atomic masses in
1818 Determined the % composition of about
2000 compounds.
Method of Equivalent Mass
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 1 g O Compare PT mass of Ag
Valence (Franklin, 1852)
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)
Valence Method of Atomic Mass
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
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 Atomic mass = 6/0.2174 = 27.6 g Compare with PT and discuss why this
might be so close
The Problems
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).
The Karlsruhe Conference
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
Broke up in confusion But each got Cannizzaro's pamphlet
Cannizzaro's Method
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
Cannizzaro's Table of Calculations
Pubs.rsc.org No you don't have to explain it
Example
Substance
in gaseous
state
Density
(g/L)
Percentage
hydrogen by
mass
Product of
density &
percent (3)
Values in
(3) divided
by least
value
Water 0.589 11.2 0.0659 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
Next step
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
Acceptance
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?