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Cannizzaro, Stanislao

atomic mass hydrogen molecular

[kanee dzah roh] (1826–1910) Italian chemist: resolved confusions on atomic and molecular mass.

Cannizzaro began his university life as a medical student, but attended a variety of courses and became attracted to chemistry, partly because he saw it as the basis of physiology. In 1847 he joined the rebel artillery in one of the frequent rebellions in his native Sicily, where his magistrate father was at the time chief of police. The rebellion failed, and Cannizzaro wisely continued his chemistry in Paris, with . He returned to Italy 2 years later, teaching chemistry in three universities, all poorly equipped. In 1853 he discovered the Cannizzaro reaction, in which an aldehyde (aromatic, or having no alpha-hydrogen) is treated with a strong base to give an acid and an alcohol:

However, his main work was done in 1858, when he cleared the way to a single system of relative atomic and molecular mass. He did this by seeing the value of the theory due to (then dead) and using it to deduce that common gaseous elements existed as molecules (H2 , N2 , O2 ) rather than as single atoms. With this in mind, Avogadro’s Law enables relative atomic and molecular mass to be deduced from the densities of gases and vapours. (Initially, as hydrogen gas was the lightest known, the hydrogen atom was assigned atomic mass = 1; we now use as a basis the common isotope of carbon = 12, which gives a very similar scale.) Cannizzaro’s fervour as a speaker at a chemical conference in 1860, and a pamphlet he distributed there, convinced most chemists and removed basic ambiguities in chemical ideas during the 1860s. The half-century of confusion on atomic mass which had followed atomic theory had ended.

He became a Senator in 1871 and afterwards worked mainly on public health.

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