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?