Atomic Weight Revisited

Lavoisier had provided the rule for building a table of elements. If chemists could decompose a substance, then that substance was a compound. If they could not decompose it, then it was to be regarded as an element. Its elementary status was, however, provisional and open to challenge, since what chemists could not decompose today, they might well be able to decompose tomorrow. In that case the substance would lose its status as an element and be revealed as a compound. 

Chemists, like other kinds of natural philosophers, were also attracted by ideas about the regularity and ultimate simplicity of nature. We have seen that Daltons atoms were individually simple, although there were a lot of different kinds. The list of elements stood at around thirty when Dalton published his atomic hypotheses or theories. It seemed to some chemists that simplicity and regularity might well result in a set of atomic weights that were all whole numbers. 

Keep It Tidy, Keep It Simple Prout, Gay-Lussac, and Avogadro 

This was the view of atomic weights advocated by the English chemist and physiologist William Prout (1785—1850), but he combined it with other ideas 

Dalton wrote down his ideas about atoms in notebooks of 1802. The first public mention of Daltons atomic theory and laws of combining proportions was by Thomas Thomson in 1807, and only in the following year, 1808, did Dalton publish his own account. That publication coincided with the publication in France of a different law of combining proportions. 

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Second, and contrary to many of his contemporaries, who believed strongly in the unity of all matter, Dalton believed that there were as many different kinds of atoms as there were elements. Finally, Dalton suggested that the weights of atoms would serve as a kind of bridge between the realm of microscopic unobservable atoms and the world of observable properties. But atomic weight is not necessarily the same as an equivalent weight, an issue raised in chapter 1 that will be revisited here. 

Here we revisit the VB resonance model in formamide by taking all six resonance structures into account. Such a study allows us to compare the individual contributions from resonance structures 1-6 to the resonance effect in formamide. For comparison, the isoelectronic systems vinylamine and formamidine are also investigated to gain insights into the trends of resonance stabilization. A 6-31G(d) basis set is employed in the calculations, and the orbitals in the VB functions are self-consistently determined for each resonance structure, but restricted to be atomic orbitals. The structural weights of the six resonance structures are listed in Table 2. 

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