Amedeo Avogadro

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May 25th, 2012
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Imagine working enthusiastically in scientific research for most of your life and your contribution to the advancement of the subject being a single achievement or theory. Indeed, many scientists invest years of effort to find themselves in this position and are rightly recognized for their one addition. Now imagine, however, your only theory of note being almost completely ignored in your lifetime and for half a century after you originally proposed it, even though it was the one idea scientists had needed for fifty-years to advance their field! Amedeo Avogadro, who found himself in exactly this position, would have had every right to die a frustrated man.

COMBINATION OF ATOMS

The theory the Italian scientist formulated in 1811 involved a way of integrating the apparently irreconcilable hypotheses of Joseph Louis Gay-Lussac (1778—1850) and John Dalton (1766—1844). Indeed, the latter had actively sought to discredit Gay-Lussac’s law of combining volumes. Gay-Lussac had observed that gases always combined in simple, consistent ratios of whole numbers such as 2:1 or 2:3 (and never in fractions), under the same temperature and pressure conditions. Dalton struggled to accept this because he believed, as a base case, that gases would seek to combine in a one atom to one atom ratio (hence believing the formula of water to be HO, not H20). Anything else would contradict Dalton’s theory on the indivisibility of the atom, which he was not prepared to accept.The reason for the confusion was because at that time the idea of the molecule was not understood. Dalton believed that in nature all elementary gases consisted of individual atoms, which is true, for example, of the inert gases. This is not the case, however, for other gases which naturally exist, in their simplest form, in combinations of atoms called molecules. In the case of hydrogen and oxygen, for example, their molecules are made up of two atoms, described in chemical notation as H2 and 02 respectively. Avogadro realised a comprehension of molecules would explain Gay-Lussac’s ratios while at the same time not contradicting Dalton’s theories on the atom. For example, by this method GayLussac’s ratio for water could be explained by two molecules of hydrogen (making four ‘atoms’) combining with one molecule of oxygen (or two ‘atoms’) to result in two molecules of water (2H20). When Dalton had considered water previously, he could not understand how one ‘atom’ of oxygen could divide itself (thereby undermining his indivisibility of the atom theory) to form two particles of water. The answer, that oxygen existed in molecules of two and therefore the atom did not divide itself at all, was exactly what Avogadro proposed.

AVOGADRO’S LAW

He built on this principle to famously suggest that at the same temperature and pressure, equal volumes of all gases have the same number of molecules. This became known as ‘Avogadro’s Law: The principle in turn allowed a very simple calculation for the combining ratios of all gases, merely by measuring their percentages by volume in any compound (which in itself facilitated simple calculation of the relative atomic masses of the elements of which it was made up).

REDISCOVERY OF AVOGADRO

Few scientists (André-Marie Ampere (1775—1836) was an exception) accepted Avogadro’s speculation, partly due to lack of experimental evidence, until the Italian Stanislao Cannizzaro ‘rediscovered’ it and vehemently backed its suggestions at a large conference of chemical scientists in 1860. The law was consequently accepted by many present, immediately clearing up the confusion of the previous fifty years concerning atoms and molecules, and the calculation of the relative atomic and molecular masses of elements.

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