Atomic theory

About 400 B.C. the Greek philosopher Democritus suggested that all matter was formed of different types of tiny discrete particles and that the properties of these particles also determined the properties of matter. This theory had some support from other philosophers, such as Lucretius, but the methods to verify it did not exist in that era, so it was not widely adopted for many centuries.

It re-emerged in the early 19th century in order to explain many laws that had been established in the previous century, when chemists had begun to measure the mass of reactants and products.

One of those laws was the law of conservation of mass, first stated by the French scientist Pierre Lavoisier, who noted that the total mass does not change with a chemical reaction.

Another was the law of constant composition, which came from the observation that compounds always contain the same elements in the same proportions. Today it is known that some compounds, particularly metal oxides and sulfides, exist in ratios that vary slightly from simple whole number and they are known as nonstoichiometric compounds.

Yet another was the law of multiple proportions which stated that given masses of different elements always combine in small whole number ratios.

The English scientist John Dalton revived the atomic theory in order to explain these observations. In 1808 he proposed that a chemical element consisted of tiny particles (atoms), all with the same chemical properties. Also, the atoms of different elements have different properties and these atoms are not changed during ordinary chemical reactions. Compounds are formed by combining atoms of different elements in certain simple whole number ratios.

It took many years for the idea to become widely accepted, but nowadays the atomic theory is fundamental to the physical sciences. As the eminient 20th Century scientist Richard Feynman succintly put it - "Everything is made of atoms!"