How did the atom evolve?

The discovery of the atoms

The picture of the structure of matter from Democritus (460–371 BC) to Bohr (1885–1962)

The Greek philosopher Leukippus (around 450–370 BC) and his student Democritus (460–371 BC) were the first to imagine matter made up of indivisible basic building blocks (Greek: atomos). According to their ideas, these atoms should already have the properties of the matter made up of them. Which of these ideas come from Leukippus and which from Democritus can no longer be determined today, since Leukippus, unlike Democritus, left no writings behind.

Atomism was later taken up by the philosophical school of Epicurus (341–270 BC), but other philosophers such as Plato (428–348 BC) and Aristotle (384–322 BC) firmly rejected it . The main reason for this was the rejection of the vacuum, the empty space in which, according to Leukippus and Democritus, the atoms should move.

Graphic from Dalton's work "New System of Chemical Philosophy"

The first experimental evidence that matter is actually made up of small building blocks was not found until the beginning of the 19th century. It was then that chemists such as Joseph Louis Proust (1755–1826) and John Dalton (1766–1844) discovered that the chemical elements only combine to form molecules in certain integer ratios. John Dalton explained this phenomenon in 1808 with the fact that the elements consist of the smallest units that could no longer be divided and took up the old Greek concept of the atom for these parts.

The British physicist Joseph J. Thomson (1856–1940) proved in 1897 that atoms are by no means indivisible. In his experiments with a hot cathode, he was able to show that smaller, electrically charged particles can be knocked out of the atoms - the electrons. Thomson imagined atoms as tiny, elastic spheres in which mass and positive electrical charge were evenly distributed. Embedded in this mass, like raisins in cake batter, are the punctiform, electrically negative electrons.

But as early as 1911 Ernest Rutherford (1871–1937) destroyed this picture. To study the structure of the atoms, he shot radioactive alpha radiation on gold foil. The physicist discovered that the mass in the atoms is by no means as evenly distributed as Thomson assumed. Rather, the mass and also the positive charge of the atom are concentrated in a tiny area, the atomic nucleus.

Ernest Rutherford (1871-1937)

Rutherford and other physicists succeeded in showing that the atomic number - and thus also the number of electrons buzzing around the nucleus - is different for the individual chemical elements. Of course, this immediately created enormous difficulties for the atomic model. Because while chemistry had shown that all atoms of an element are completely alike, physics now gave atoms the freedom to adopt very different properties through different electron orbits.

And there was another serious argument against Rutherford's atomic model: According to the theory of electrodynamics, an electron rotating around the atomic nucleus would have to constantly emit energy - and consequently fall into the atomic nucleus. So the Rutherford atom is not stable.

It was the Danish physicist Niels Bohr (1885–1962) who in 1913 broke the Gordian knot of this contradiction between Rutherford's experiments on the one hand and mechanics and electrodynamics on the other. Bohr postulated that the electrons could only take certain orbits around the atomic nucleus and that transitions between these orbits were only possible in jumps. These postulates cannot be derived from classical physics - they were initially justified solely by the agreement of the Bohr model of the atom resulting from it with the experimental observations.

Only quantum mechanics later provided a reason for Bohr's postulates. And while Bohr imagined the electrons as small spheres that orbit the atomic nucleus like planets on circular or elliptical orbits, quantum mechanics provided a completely different picture. In the modern orbital model, the electrons are no longer localized particles, but only smeared probability clouds, the distribution of which results from the solution of the quantum mechanical Schrödinger equation.