What is the atomic theory of matter

The structure of matter

The beginnings in antiquity


Already 2500 years ago Democritus (460-371 BC, fig.) Claimed that all matter was composed of indivisible ("a tomos") basic building blocks. There should only be a small number of different types of these basic building blocks. Democritus explained the different properties of matter from the different arrangement and combination of these basic building blocks in empty space. However, this view was criticized by Aristotle because he rejected the idea of ​​empty space. Based on his teacher Plato, he explained the diversity of earthly phenomena instead from the interaction of the basic elements fire, water, earth and air. In the following 2000 years the West followed Aristotle in this regard, and atomism initially had no consequences. Ironically, however, we owe Aristotle to the fact that the atomistic ideas were passed on. Namely, while all of Democrit's original work was lost, Aristotle's criticism of atomism became the sole source.

2000 years later: Chemistry is rediscovering the atom

The renaissance of atomism began in the second half of the 18th century through chemical research. It turned out that the four Aristotelian elements (fire, water, earth, air) could by no means explain the diverse natural phenomena. The French chemist Antoine Lavoisier (1743-1794) distinguished 23 different substances, many of which already correspond to the chemical elements of our time. The mixture of these substances in characteristic volume ratios was also observed. For example, water is made up of one part by volume of oxygen and 2 parts by volume of hydrogen (at constant pressure and temperature). This observation is easily explained by the assumption that oxygen and hydrogen correspond to certain atoms that combine to form water in a 2: 1 ratio.

Order in the World of Atoms: The Periodic Table of the Elements

The periodic table of the elements. Click on it to get a larger picture.

With the discovery of ever new atoms, however, atomic theory moved away from its original goal of tracing the various properties of substances back to a few basic elements and thus explaining them. By the middle of the 19th century, around 60 different chemical elements were known. Nowadays we know well over 100. A justification of their different properties by assuming their structure from different atoms, however, meant nothing more than the statement that there are different forms of matter.

However, these numerous atoms were not unrelated to one another. In 1868/69 Meyer and Medelejew succeeded in inserting these elements independently of one another into an order scheme. The so-called periodic table of the elements (Fig.) Arranges the elements according to their properties and even allowed the prediction of elements that were still unknown at this point in time. In retrospect, this order structure among the atoms is the first indication that they are in turn composed of building blocks.

Atomic theory and physics

In addition to these chemical investigations, which looked at the transformation of substances, physical research also contributed to the development of atomic theory since the beginning of the 19th century. Above all, the theory of heat should be mentioned here. Through the work of Dalton and later Maxwell and Boltzmann, this led to the so-called "kinetic gas theory", according to which the properties of a gas are attributed to the movement of small particles of matter. These were eventually identified with the atoms. However, the atomic theory of matter did not gain acceptance until the 19th century, and the important physicist Ernst Mach (1838-1916) still held it at the end of the 19th century. for metaphysical nonsense. An influential dispute between supporters and opponents of this theory took place at the meeting of natural scientists on September 17, 1895 in Lübeck. Here the adherents of the atomic theory got the upper hand.

Inside the atoms

Schematic representation of Rutherford's scattering experiment.

At the same time new radiation phenomena were discovered by Röntgen (1895, X-rays, i.e. high-energy electromagnetic radiation), Becquerel (1896, "uranium radiation", i.e. radioactive alpha radiation) and J. J. Thompson ("cathode radiation", i.e. electron radiation). This discovery has a double meaning for what was later called atom and nuclear physics: On the one hand, it can only be explained within these theories; on the other hand, it opened up new experimental methods for studying atoms. The famous Rutherford experiment of 1911, shown in the picture, provides a good example of this. Rutherford examined gold foil with the help of alpha radiation. The characteristic shape of the deflection of the positively charged alpha particles (helium nuclei) on gold atoms showed that there is a very small positively charged nucleus inside atoms. This nucleus is surrounded by electrons. Only a short time later this nucleus was recognized as being composed of protons, and in 1932 Chadwick discovered another electrically neutral particle in the atomic nucleus, the neutron.

The search continues: leptons, quarks & Co.

Quite a few physicists were convinced in the 1930s that the "actual" atoms, i.e. the indivisible basic building blocks of matter, had been discovered in the form of electrons, protons and neutrons. At the same time, quantum theory developed, a successful theoretical description of atomic physics. This made it possible, for example, to derive the structure of the periodic table of the elements from the theory of the atomic structure.

The three generations of the building blocks of matter.

In the light of today's knowledge, the electron is actually elementary, i.e. structureless. However, the proton and neutron themselves turned out to be composed. Their building blocks are called quarks. There are 6 different types of these quarks. The electron also has relatives, i.e. particles that essentially differ from the electron only by their mass, they are called muons and tau. Together with the 3 neutrinos (uncharged and almost massless particles) these 6 particles form the group of leptons.

The new "Periodic Table of the Elements", that is, the list of the fundamental building blocks of matter, has the following form:

The distinction between quarks and leptons is only understandable if you not only consider the building blocks of matter, but also the forces that are effective between them.


Schematic representation of the structure of matter. Clicking on it starts an animation of the journey into matter!

This figure (by clicking on it starts a animation on the journey through matter) once again illustrates the current idea of ​​the structure of matter from the visible crystal to the elementary quarks and leptons. The myriad of molecules are made up of atoms, of which there are 114 different ones. These differ in the number of protons and neutrons in the nucleus and in the number of electrons in the shell. While these electrons are elementary according to today's understanding, protons and neutrons each consist of 3 quarks. Note the huge differences in size: the atomic nucleus is about 10,000 times smaller than the atom. Atoms are essentially made up of nothing.