1808 – Manchester, England
‘All matter is made up of atoms, which cannot be created, destroyed or divided. Atoms of one element are identical but different from those of other elements. All chemical change is the result of combination or separation of atoms’
Dalton struggled to accept the theory of GAY-LUSSAC because he believed, as a base case, that gases would seek to combine in a one atom to one atom ratio (hence he believed the formula of water to be HO not H2O). 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 that at the time the idea of the molecule was not understood.
Dalton believed that in nature all elementary gases consisted of indivisible atoms, which is true for example of the inert gases. The other gases, however, 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 as H2 and O2 respectively.
Gay-Lussac examined various substances in which two elements form more than one type of compound and concluded that if two elements A and B combine to form more than one compound, the different masses of A that combine with a fixed mass of B are in a simple whole number ratio. This is the law of multiple proportions.
AVOGADRO’s comprehension of molecules helped to reconcile Gay-Lussac’s ratios with Dalton’s theories on the atom.
Gay-Lussac’s ratio for water could be explained by two molecules of hydrogen (four ‘atoms’) combining with one molecule of oxygen (two ‘atoms’) to result in two molecules of water (2H2O).
2H2 + O2 ↔ 2H2O
When Dalton had considered water, he could not understand how one atom of hydrogen could divide itself (thereby undermining his indivisibility of the atom theory) to form two particles of water. The answer proposed by Avogadro was that oxygen existed in molecules of two and therefore the atom did not divide itself at all.