The Avogadro Number

6.022 141 99 X 1023

This large number is approximately equal to the number of protons in a gram of pure protons. It is customary to introduce the term ‘gram molecule’ into explanations of the importance of this number:

a gram molecule is a mole of the molecules:
y grams of a molecule whose relative molecular mass is y, will contain 6.022 x 1023 molecules.


Historical Context

Lorenzo Romano Amedeo Carlo Avogadro, known as Amedeo Avogadro, one of the many creative Italians that have enriched our lives, was born in 1776 – a memorable date hitherto for other reasons.

Possibly of aristocratic descent, he was a lawyer until he decided to devote himself to one of his other interests: in his late twenties he was appointed to the first chair in mathematical physics in Italy, at L’ Universita` di Torino. Avogadro was the first to publish (in 1811) the idea that elements could exist as molecules – a latin word that he adopted for his hypothesis (which he wrote in French): explaining why mixing a volume of hydrogen gas with an equal volume of chlorine gas does not result in a doubling in the volume of gas. He was also responsable for the creation of this word; “molecule”, when he affirmed that matter is not continous (specially on fluids) but a great colection of “masses” or “molecules” (mole = mass; cule = little, small, thin, minute, petite, miniature, insignificant).

He died in 1856, this contribution to science was not recognised until 1860, when Stanislao Canizzaro , a co-national who for many years had been teaching a course of lectures on why Avogadro had been wrongfully ignored, was persuaded to propound his views at a conference in Karlsruhe, Germany – not far up from Heidelberg in the valley of the Rhine.

The basis for the number named after Avogadro is the number of atoms of 12Carbon in 12 grams of 12Carbon.


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