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Amedeo Avogadro

Lorenzo Romano Amedeo Carlo Avogadro de Quaregna e di Cerreto - better known as Amedeo Avogadro - was born in Turin, the capital city of Piedmont (now part of northern Italy) on June 9th, 1776. His family's business was the law, and Amedeo followed in his father's footsteps earning a doctorate of law in 1796 (He started college when he was only thirteen, graduated when he was sixteen and had his doctorate by the time he was twenty!).

While practicing his profession, he became interested in natural philosophy and mathematics, as a sideline or hobby. By the time he was thirty his hobby had become the major part of his life, so he gave up his ecclesiastical legal practice and took up the teaching mathematics and physics at a small college nearby.

He was apparently well liked by his students, who appreciated is impish sense of humor, and quickly settled down into a happy marriage blessed with six sons. In his free time he did a lot of reading and had a complete set of the current scientific journals in his library printed in four different languages.

law of combining volumes

In one of these journals he read in 1808 that Joseph Louis Gay-Lussac, writing in French, had found that when two gasses react together to form different products, the volumes of the reactants and the products (if they are all still gasses) are all whole numbers. This was called the Law of Combining Volumes

This set Avogadro thinking. Although the famous John Dalton was not a supporter of Gay-Lussac's work, Avogadro reasoned that equal volumes of gasses at the same temperature and pressure must contain the same number of molecules.

atoms, molecules and compounds

At this time the idea of "atoms", "molecules" and "compounds" were not very well formed, and Avogadro used the terms somewhat interchangeably (he never actually used the Daltonish word "atom" at all). He called his reacting particles "molecules" and "elementary molecules" (atoms), but he was able to use these ideas to explain the strange results obtained when hydrogen and oxygen gas reacted together to form water vapor.

When two volumes of hydrogen gas were reacted with one volume of oxygen gas it formed two volumes of water vapor (if the temperature was kept high enough). This needed some explaining - why two volumes of water vapor instead of just one volume?.

Avogadro's answer was pure genius; since the volume of water vapor formed was twice the volume of oxygen used to create it, the reacting "particles" of oxygen had to consist of two atoms of the element oxygen combined together into a single, compound structure, which he called a "molecule". It explained everything. Oxygen (and hydrogen) gas consisted of molecules that each in turn consisted of two atoms of the element!

From this effective reasoning Avogadro was able to see that the relative molecular weights of any set of gasses are the same as the ratio of the densities of those gasses under the same conditions of pressure and temperature. The fundamental particles of simple gasses were each composed of two atoms combined into a compound form!

back to the literature

Excited by his own ideas, Avogadro then went through all the literature published in his journals (particularly the experiments reported by Gay-Lussac) and explained all the measured gas results in terms of his new hypothesis. It all fitted. So he decided to write his own paper and have it published to. The power of his ideas comes in his conclusion; all gasses, simple or complex, contain the same number of molecules under the same conditions of temperature and pressure.

Today, this is called Avogadro's Principle and is clearly seen as one of the corner stones of chemistry.

no respect

But in his own time, Avogadro's principle was seriously neglected. Historians of science have several theories as to why this should be so, as Avogadro was a respected scientist during his life. One possibility was that a more famous scientist, J. J. Berzelius, was strongly advocating his "dualism" theory which explained compound substances (molecules?) on the basis that one half of the compound had to have a positive charge and the other half a negative charge (to hold the two halves together). It was hard to see how two atoms of oxygen in one of Avogadro's "molecules" could have different charges.

But the real reason is probably more prosaic. In the clannish world of scientific discovery, it pays to be at the center of the action. Avogadro was by this time a professor, and chairman, of physical chemistry at the University of Turin, but in Italy - far away from the major science centers of England, Germany, France or even Sweden. He never got to rub shoulders with the "great ones" of his day, so his ideas did not receive the credit they deserved.

He was a professor until his retirement at the age of 74. He died on July 9th, 1856.

Science@a Distance
© 2001, Professor John Blamire