Why were protons discovered earlier than electrons?

History of atomic physics

How to trace the whole material world back to 92 basic building blocks, the elements, and, above all, how to prove the existence of atoms and molecules and determine their properties, is one of the most exciting chapters in the history of science and the development of the human mind . It encompasses knowledge that not only determined our physical view of the world and revolutionized our conception of the universe, but also made the whole wealth of modern technology - including its dark sides - possible in the first place.

The fundamental building blocks of matter

All matter in the universe, on earth or in the stars, in living beings or rocks, is composed of fundamental building blocks, molecules and atoms. For example, each drop of water consists of an unimaginably large number of water molecules (about 30 quadrillion, mathematically represented \ (3 \ cdot 10 ^ {19} \)), each of which in turn consists of two atoms of hydrogen (H) and one atom of oxygen (O ), abbreviated \ ({\ rm H_2O} \), is formed. Our entire material world consists of no more than 92 types of atoms, which are called elements. They are summarized in the periodic table of the elements, an indispensable tool for all physicists and chemists.

How to trace the entire material world back to these 92 building blocks and, above all, how to prove the existence of atoms and molecules and determine their properties is one of the most exciting chapters in the history of science and the development of the human mind. It encompasses insights that have not only determined our physical view of the world and revolutionized our conception of the universe, but also made the full range of modern technology - including its dark sides - possible in the first place.

The path of natural science to the atoms

Ever since people started thinking about nature and the world as a whole, there have been questions about which opinions have divided. What is ultimately the basis of the diverse manifestations of the material world? Are there one or many primary substances? Is matter always further divisible or is there something tiny in the end that cannot be further divisible? Such an indivisible, Greek “atomon”, was introduced into Western natural philosophy by Democritus around four hundred BC. He said that all of nature is composed of atoms and emptiness. However, this postulate was soon forgotten, instead the Aristotelian conception of the four primary substances fire, water, air and earth dominated Western thought for many hundreds of years.

It was only towards the end of the 18th century, when natural scientists began to systematically break down and reassemble various substances such as water, that Democritus' idea of ​​the atoms was revived - and this time for good. Dalton discovered the law of "constant proportions": If a substance is synthesized from two or more different components, they always have the same mass and volume ratio. For example, to produce 18 grams of water from the elements hydrogen and oxygen, you need two parts by volume of hydrogen to one part by volume of oxygen or exactly 2 grams of hydrogen and 16 grams of oxygen. Excess amounts of either element are not synthesized. These findings, confirmed in many analog experiments, could only be explained as follows without artificial assumptions: There are certain basic substances (elements such as hydrogen, oxygen ...) whose smallest units (atoms) have masses that are an integral multiple of the mass of the lightest atom, the hydrogen atom , are. These atoms combine with each other in integer proportions to form molecules. The somewhat imprecise term “atomic weight” was later introduced for the atomic mass of an element, based on the mass of the hydrogen atom.

Spectral lines: alphabet of the universe

After Dalton's revolutionary experiments, it would be more than a hundred years before the reasons for the constant proportions could be explained and the size and structure of the atoms could be imagined. But long before it had been recognized that heated gases emit radiation whose wavelengths are characteristic of the respective type of atom or molecule. The significance of this discovery, that every element and every complex compound can be identified by means of radiation, which is therefore an unmistakable fingerprint of the atoms and molecules it contains, was not yet recognized at the time. In truth, it opened the door to today's physics and technology.

The first astrophysical application of spectral analysis succeeded Kirchhoff and Bunsen in Heidelberg in the fifties of the 19th century. They examined the light coming from the sun and the stars and found that it must have come from the same elements as those known on earth. This was the first time that the unity of the universe had been experimentally proven and it had been shown that its secrets could be spelled using the alphabet of spectral lines.

Atoms are not indivisible and almost "empty"

Ernest Rutherford (1871-1937)

In the first decade of the 20th century it was possible, experimentally and theoretically, to prove the existence of atoms directly and to determine their structure and size. In London, Rutherford bombarded gold atoms with fast alpha particles (positively charged helium atoms) and was able to deduce from the scattering distribution of these alpha particles that a gold atom consists of a heavy, multiply positively charged atomic nucleus and negative charges far away, which are later combined with the electrons discovered by Thomson identified. Although such a gold atom is tiny (its diameter is roughly ten billionths of a meter, \ (10 ​​^ {- 10} \) meters), it is almost empty. If you mentally enlarge it to a diameter of ten centimeters, then the diameter of its core is still no larger than that of a human hair.

In the following years it was learned that the atomic nucleus is also divisible and consists of so-called nucleons, positively charged protons and uncharged neutrons. And even the nucleons can be further divided into three even smaller building blocks, the “quarks”. Although the term “atom” in the original sense of “indivisible” is no longer appropriate, it still has a certain justification. In practically all natural occurrences on earth, atoms still play the role of the smallest fundamental building blocks. The modification of atomic nuclei, i.e. radioactive decay, nuclear fission or fusion, usually only takes place under extreme pressure and temperature conditions, such as those found in the hot and dense interior of stars or in modern particle accelerators.

Niels Bohr, the hydrogen atom and quantum mechanics

Rutherford's pioneering experiments paved the way for a detailed idea of ​​the structure of the atoms and the chemical elements that make up all matter in the universe: Each element has a number of positively charged protons in the nucleus and corresponding to its atomic number (Z) an equal number of negatively charged electrons in the shell (\ (Z = 1 \) for hydrogen, \ (Z = 2 \) for helium to \ (Z = 92 \) for the heaviest element, uranium). The electrical attraction between protons and electrons holds the atom together; the atom appears electrically neutral to the outside world. In addition to the fixed number \ (Z \) of protons, the atomic nucleus of an element with the atomic number \ (Z \) can contain a different number of neutrons, so that its mass can be variable (isotopy). But what about the electrons?

Niels Bohr

This question preoccupied Niels Bohr in Copenhagen. In 1912 he developed a revolutionary model of the possible energy states of the electrons in the hydrogen, based on the "elementary quantum of action \ (h \)" postulated by Max Planck in 1900 and on the observations of the Basel drawing teacher Balmer of the strange regularities in the spectral lines of hydrogen Atom, the "Bohr model of the atom". It was to lead to the fundamental theory of modern physics, "quantum mechanics", within just ten years.

According to Bohr's atomic model, only certain discrete energy states are allowed for the electrons, namely those for which the orbital angular momentum is an integral multiple of the quantum of action \ (h \). With this hypothesis, the energies of the hydrogen spectral lines, which have now been measured with great precision, could be interpreted (almost) perfectly as transitions of the electrons between the permitted energy shells. Bohr's model had only one, but serious, disadvantage: it was not based on established physical theories, but on assumptions made on an ad hoc basis.

The deeper truth, which apparently had to underlie Bohr's atomic model, now led directly to the fundamental theory of modern physics, quantum mechanics. Based on the most astonishing observation that the elementary particles have the properties of particles in some experiments but those of waves in others, Werner Heisenberg, Louis de Broglie, Max Born and Erwin Schrödinger introduced the concept of "matter waves". The quantized Bohr energies resulted from Schrödinger's wave equation for the electron in the hydrogen atom - but this time as exact solutions to a mathematical differential equation.

The "Pauli principle", the shape of the world and the periodic table of the elements

Erwin Schrödinger

In contrast to the exactly soluble hydrogen atom with its two particles, the energy states in atoms with several electrons can in principle only be determined approximately. Nevertheless, in all cases there was also an amazing correspondence between the quantum mechanical theory and the observations if one also considered the "Pauli principle" derived from quantum mechanics: In each of the possible discrete energy shells of the electrons there may only be a certain number of electrons. If a shell is occupied, then a subsequent electron has to move into the next shell with a smaller binding energy.

This apparently harmless Pauli principle has a significance for the shape of the world that cannot be overestimated. If this were not the case, then all electrons in all atoms would, according to a basic principle of nature, gather in the deepest energy shell with the greatest binding energy. With the exception of hydrogen, there could never be any formation of molecules and thus the diversity and complexity of our reality. There would be neither water nor carbon dioxide, not even the simplest form of life. The reason for this is that in molecules the outer electrons with a small binding energy belong together to several or all atomic nuclei. But if all electrons were in the innermost energy shell of their atoms, any use of electrons across all atoms would be excluded.

Bohr's atomic model

The Pauli principle, together with quantum mechanics, made it possible to understand how the simplest molecule can be formed from two hydrogen atoms (\ ({\ rm H_2} \)). However, quantum mechanics and the Pauli principle showed their real potential in the quantitative explanation of the periodic table of the elements and thus of chemistry as a whole. Dmitri Mendeleev and Lothar Meyer set up this system empirically and independently of one another in the 1870s, without using any model of the atom, by combining all elements with similar chemical properties.

But only quantum mechanics and the Pauli principle were able to establish these chemical similarities through the respective electronic structure. In the series of noble gases, for example, the outermost electron shell is closed, so these elements are not ready for any kind of molecular formation. In the alkali series, on the other hand, there is one electron in the outermost shell, which can very easily be transferred to an atom of the halogen series with one electron missing in the outermost shell. Therefore, alkali atoms, for example sodium (Na), and halogen atoms, for example chlorine (Cl), form a molecule the fastest and most permanently, in this case NaCl (table salt). The same applies to all the other ranks established by Mendeleev and Meyer.

Atomic and Molecular Physics: the “mothers” of many sciences

Atomic and molecular physics are indispensable for many areas of science. Astronomy was and is to a large extent atomic or molecular spectroscopy. The source of electromagnetic radiation that reaches us over the entire wavelength range from hot star plasmas, the interstellar gas or from the photo and chromosphere of the sun is determined by comparing it with the spectral lines of neutral or (highly) ionized atoms and molecules measured in the laboratory can often only be generated using powerful particle accelerators. In addition to chemical identification, the speed of the radiation can be determined from the wavelength shift and the temperature of the source can be determined from the line width.

The periodic table of the elements according to Mendeleev

Trace elements in rocks or water, pollutants in the air are detected by stimulating their characteristic radiation in the optical, microwave or X-ray range. Every climate or atmosphere model requires precise knowledge of the possible absorption and emission lines of the molecules present there (for example ozone). This can become infinitely complex because of the huge number of different molecules and the range of their electromagnetic spectra corresponding to the higher number of "degrees of freedom" compared to atoms (the atoms of a molecule can also rotate, vibrate against or with one another). Research into the chemical reaction that is central to us, photosynthesis, is largely based on atomic spectroscopy in ultra-short time ranges.

The penetrating X-rays discovered by Wilhelm Röntgen in 1896 could also be included in the context of atomic radiation and used to clearly identify the elements. Moseley discovered in 1913 that all atoms lacking an electron in the innermost shell emit radiation that is characteristic of the respective element. The epoch-making elucidation of the basic form of all life, the double helix structure of DNA, was decisively prepared by methods of atomic physics: by diffraction of X-rays on crystallized DNA.

Modern developments and methods

Despite their pioneering role in the development of modern physics, atomic and molecular physics is anything but a closed, "classical" science. In the last few decades in particular, spectacular discoveries have been made, which is confirmed by the large number of Nobel Prizes during this period. Certainly the most momentous was the development of the laser, the stimulated, rectified emission of light, with the most important applications in countless areas such as technology, metrology, distance determination, frequency standards, data storage, reproduction, medicine ...

A current research area is the investigation of the course of chemical reactions in ultra-short time ranges of up to \ (10 ​​^ {- 15} \) seconds. With the help of ingenious methods ("laser cooling") it has already been possible to store atoms in a very small space and to cool them to almost zero Kelvin, creating a new coherent state of matter ("Bose-Einstein condensate"), which can be used in a wide variety of applications, for example "atomic lasers".

With the help of new microscopes ("scanning tunneling microscope", "atomic force microscope") it was possible to make individual atoms "visible" and manipulate them, with the future prospect of data storage on an atomic scale.

The fantastic properties of matter waves, which quantum mechanics used to predict largely in the form of thought experiments, have now been implemented experimentally on a broad front. So-called entangled atomic states are already being used in initial attempts as applied quantum cryptography for tap-proof data transmission.

This is only a small part of a largely unnoticed technical revolution at the atomic level that can and will lead to important practical results in the near future. Nevertheless, by far the most important point of atomic physics to date, its dual role as a pioneer and constant touchstone of all modern physics remains.