In 1829 Johann Wolfgang Döbereiner (1780 to 1849) had noted that there are a number of triads of similar elements in which the atomic weight of the middle element is approximately the average of the other two elements' atomic weights. Among the triads are chlorine, bromine, and iodine; calcium strontium, and barium; sulfur, selenium and tellurium; and iron, cobalt and manganese.
Despite efforts to systematize the rapid accumulation of chemical facts, much confusion abounded. Friedrich Kekule for example, in 1860 discussed 19 different proposed formulae for acetic acid!
To find a solution to the confusion Kekule suggested, and a friend, Carl Weltzien organized and conveined on September 3, 1860 the first International Chemical Congress. The Congress did determine to base atomic weights on oxygen (=16 exact) which reacts with more elements than hydrogen. This would reduce experimental errors in atomic weights. But discussions at the conference did not lead to agreement how correct formulae should be determined. However a course description distributed by one of the participants, Stanislao Cannizzaro (1826 to 1910) from the University of Genoa, provided a revolutionary concept that eventually solved the confusion.
Cannizzaro described how Avogadro's hypothesis (that equal volumes of different gases contain the same numbers of molecules) actually leads to a consistent set of atomic weights. There was not immediate recognition at the Congress of the significance of Cannizzaro's proposal. But upon returning home, several chemists began using Avogadro's hypothesis to unravel the confusion about formulae and atomic weights and to organize the chemical elements.
| Atoms or Molecules | Symbols of the molecules | Numbers expressing their weights |
| Atom of Hydrogen Molecule of hydrogen Atom of Oxygen Molecule of ordinary Oxygen Molecule of electrised Oxygen (Ozone) Atom of Sulfur Molecule of Sulphur above 1000¡ (Bineau) Molecule of Sulphur below 1000¡ ... of Water ...of Sulphuretted Hydrogen |
H H2 O O2 O8 S S2 S6 H2O H2S |
1 2 16 32 128 32 64 196 18 34 |
| introduction | Greeks | alchemy | Lavoisier | Dalton | Berzelius | ↑ | spectra | electron | radiation | Bohr | isotopes | synthesis |
| to site menu | Discovery and Naming of Chemical Elements |
chemistry | physics | |||||||||
| created 23 March 2002; addition 30 March latest revision 4 November 2006 |
by D Trapp |  |
||||||||||