13/05/2020

In Folha, Marcelo Viana explains what the world is made of.

Foto: Pxfuel

Reproduction of Marcelo Viana's column in Folha de S.Paulo.

The thinkers of Greece had all sorts of ideas about the composition of the things that surround us. One of the most exotic, proposed by the philosophers Leucippus (5th century BC) and Democritus (460 BC–370 BC), was that matter consists of small, indivisible particles, atoms. There was no experimental basis for this theory, and it was forgotten.

Until the beginning of the 19th century, when the English scientist John Dalton (1766 – 1844) was trying to explain a curious mathematical property of chemical reactions. For example, there are two types of tin oxide, and the quantities of the ingredients are mysteriously related: in one type, 100g of tin combine with 13.5g of oxygen; in the other, the same 100g of tin react with 27g of oxygen. Is it a coincidence that 13.5 is half of 27?

Dalton suggested that the two reactants would be formed by atoms, which would join together to form the new substance. One atom of Sn (tin) with one atom of O (oxygen) would give the first type of oxide, with the formula SnO. But each Sn could also combine with two O, generating the oxide SnO2. Dalton's theory explained many other chemical reactions well, but it did not prove that atoms actually exist.

The first experimental evidence in favor of atomic theory came from the discovery, in 1827, of Brownian motion. The Scottish botanist Robert Brown (1773–1858) observed under a microscope that pollen particles in liquid suspension are constantly agitated, as if they were alive. He suggested that this was because they are constantly bombarded by the atoms of the liquid.

The mathematical explanation of Brownian motion was given by Albert Einstein (1879–1955) in 1905 and by the Polish physicist Marian Smoluchowski (1872–1917) in 1906. The French physicist Jean Perrin (1870–1942) confirmed the theory experimentally and used it to calculate the size and mass of different atoms. For these achievements, he won the Nobel Prize in Physics in 1926.

For the first time in history, humanity had a scientific answer to the question "what is the world made of?". But it couldn't be that simple, of course.

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