Big Ideas

IN CONTEXT

BRANCH

Chemistry

BEFORE

c.400 BCE The Greek thinker Democritus proposes that the world is ultimately made of tiny indivisible particles – atoms.

1759 English chemist Robert Dossie argues that substances combine when they are in the right proportion, which he calls the “saturation proportion”.

1787 Antoine Lavoisier and Claude Louis Berthollet devise the modern system of naming chemical compounds.

AFTER

1805 John Dalton shows that elements are made up of atoms of a particular mass, which combine to make compounds.

1811 Italian chemist Amedeo Avogadro makes a distinction between atoms and the molecules that are formed by atoms to make compounds.

The Law of Definite Proportions, published by French chemist Joseph Proust in 1794, shows that no matter how elements combine, the proportions of each element in a compound are always precisely the same. This theory was one of the fundamental ideas about elements that emerged at this period to form the basis of modern chemistry.

In making his discovery, Proust was following a trend in French chemistry, pioneered by Antoine Lavoisier, which advocated careful measurement of weights, ratios, and percentages. Proust studied the percentages in which metals combined with oxygen in metal oxides. He concluded that when metal oxides formed, the proportion of metal and oxygen was constant. If the same metal combined with oxygen in a different proportion, it formed a different compound with different properties.

Not everyone agreed with Proust, but in 1811, the Swedish chemist Jöns Jakob Berzelius realized that Proust’s theory fitted with John Dalton’s new atomic theory of elements – that elements are each made of their own unique atoms. If a compound is always made from the same combination of atoms, Proust’s argument that elements always combine in fixed proportions must be true. This is now accepted as one of the key laws of chemistry.

"Iron, like many other metals, is subject to the law of nature which presides at every true combination, that is to say, that it unites with two constant proportions of oxygen."

Joseph Proust