1811

Avogadro’s Hypothesis

John Dalton (1766–1844), Joseph-Louis Gay-Lussac (1778–1850), (Lorenzo Romano) Amedeo (Carlo) Avogadro (conte di Quaregna e Cerreto) (1776–1856)

In 1811, Italian scientist Amedeo Avogadro published his hypothesis about molecular weights, but not enough people noticed. It would have saved a great deal of trouble if they had. Avogadro proposed that, if you took the exact same volumes of several different gases and compared their weights, these would correspond to the individual molecular weights of the gases themselves. This implies that identical volumes of different gases have the same number of molecules in them, so any differences in weight must come from the heavier or lighter molecules themselves. Avogadro concluded this after studying the work of John Dalton and Joseph-Louis Gay-Lussac, the latter of whom discovered in 1805 that when gaseous reagents reacted, the volumes of the reactants and their products were always in whole-number ratios to each other (first by showing that two volumes of hydrogen and one volume of oxygen were needed to produce water). Despite the fact that Gay-Lussac’s results supported his own ideas, Dalton seems to have rejected them. Instead, it was Avogadro who cleared up a lot of the confusion about atoms and molecules.

Dalton always favored the simplest explanations, but Avogadro had the key insight that things were a bit more complicated: many common gases were actually composed of two identical atoms bonded together. Hydrogen gas, we now know, is H₂, oxygen gas is O₂, and nitrogen is N₂, but there was no reason to assume this in the early 1800s. It was known (from experiments by Gay-Lussac and others) that if you burned hydrogen gas with oxygen at a constant temperature and pressure, the volume of the water vapor was twice that of the oxygen you started with. This was a great problem for many theories, but Avogadro boldly proposed that this made sense if the oxygen started out as O₂ and then split to become part of two new water molecules. Despite Avogadro’s sound logic, chemists of his day were stumped by their belief that chemical bonding resulted from positive and negative charges attracting each other. Why, then, should two identical atoms bond together? Decades later, the Italian chemist Stanislao Cannizzaro finally resolved this conundrum in a paper that gave Avogadro his due credit.

This photograph shows a long match igniting hydrogen soap bubbles, which results in an exothermic reaction between the hydrogen and the oxygen in the air.