What's in an atom

The atom (from ancient Greek ἄτομος (ὕλη)átomos (hýle) “Indivisible (matter)”) is the smallest building block of matter that cannot be chemically further divided. Various atomic models have been proposed throughout the history of science.

Atoms consist of an electrically positively charged atomic nucleus and an atomshell from negatively charged electrons. Atoms are electrically neutral in their normal state; the number of protons and electrons is then always the same. If atoms carry an electrical charge, they are called ions. The process of converting a neutral atom into an ion (by removing or adding electrons) is called ionization.

The atomic nucleus is made up of positively charged protons and electrically neutral neutrons. Types of atoms or nuclides that have the same number of protons (atomic number) and thus the same atomic number belong to one and the same element and are called isotopes. Since the properties of the atomic shell determine the chemical behavior of an atom, isotopes of one and the same element are chemically indistinguishable.

Almost all of the inanimate and animate matter that we can perceive in our earthly environment consists of (neutral or ionized) atoms. There is also neutron matter from neutron stars and possibly a still hypothetical dark matter of a previously unknown nature.


The core is tiny compared to the shell. The shell has a diameter of about 10-10 m about ten thousand times larger in diameter than the core (10-14 m). To illustrate: If one were to inflate an atom to the size of a cathedral, the nucleus would correspond to the size of a fly (whereby almost the entire mass of the atom is accounted for by this tiny nucleus). The atomic nucleus takes up only about one quadrillionth of the total volume of an atom. The atom as the basic building block of matter therefore consists almost exclusively of empty space.

Atoms are spherical in a first approximation and have a size of 0.1 to 0.5 nm, so m to m. The atomic number determines the position of the element in the periodic table of the chemical elements, which therefore also Atomic number is called. Within the periodic table, the atomic radii decrease from left to right (the noble gases are an exception) and increase from top to bottom. However, there is no linear relationship between atomic number and atomic radius.

The nucleus makes up almost the entire mass of the atom. The total number of protons and neutrons in the nucleus is therefore called the mass number. A kind of atom or nuclide is described by specifying the element and the mass number, e.g. B. designated 16O the most common isotope of oxygen or 56Fe the most common iron isotope. The mass of an atom is between 10, depending on its mass number-24 and 10-22 G. The smallest and lightest atom is the hydrogen atom1H, the nucleus of which consists of a single proton. One of the heaviest naturally occurring nuclides is the uranium isotope238U with 92 protons (see periodic table). The heaviest nuclide that has been produced so far (as of Aug 2004) is Ununoctium294Uuo with 118 protons. However, it is extremely short-lived with a half-life of 0.89 ms.

Energy orders of magnitude

The different orders of magnitude of the diameter of the atomic shell and the atomic nucleus also correspond to different orders of magnitude of the binding energies, which determine the respective structure. The binding energies with which the electrons in the shell are bound to the core range from a few electron volts (eV) to hundreds of kiloelectron volts (keV), while the binding energies within the core are always many megaelectron volts (MeV). Processes that cause significant changes in a physical system always show energy conversions in the order of magnitude of the binding energy. Accordingly, there are many processes that are important for the shell and that leave the core unaffected. This results in a natural division of the research and application areas:

  • Chemistry is concerned with the connections between atoms and molecules. Typical energy sales here are in the eV range.
  • The Atomic physics in the narrower sense (physics of the atomic shell) deals with the structure and internal processes of the atomic shell.
  • Nuclear physics deals with the structure and internal processes of atomic nuclei.
  • Finally, elementary particle physics deals with the internal structure of the core components proton and neutron and other elementary particles through research into even higher-energy processes in the GeV range.

Energy levels

In contrast to macroscopic physical systems, the energy content of an atom (an atomic shell) cannot be arbitrary, but only certain discreet Assume ("quantized") values. In a certain approximation one can consider the binding of each electron to the nucleus individually; instead of the energy of "the atom" one often speaks of the energy of "an electron". So the electrons are on discrete energy levels. This critically important property was first assumed in 1913 by Niels Bohr (see below) in contradiction to the physical laws known up to that point in order to be able to explain the line spectra of the light emanating from atoms. The subsequent development of quantum mechanics led to today's theoretical understanding of this property. Quantum mechanics also explains the exact position of the energy levels and other properties of the atomic shell (see electron configuration).

The atomic nucleus also has discrete energy levels. The gamma radiation, the nuclear physical counterpart to the light emission of the atomic shell, therefore also shows line spectra.

Historical outline of atomic, nuclear and elementary particle physics

See also: atomism and atomic model

The story of the idea of ​​the atom begins in ancient Greece around 400 BC.


  • around 400 BC - Democritus and the Particle Model
Democritus, an ancient Greek scholar, was the first to suggest that the world is made up of indivisible particles - (Greek a-tomos = indivisible) atoms - stocks. Besides that, there would only be empty space. According to Democritus, all of the properties of the substances can be explained by the repulsion and attraction of these small particles. This idea was rejected by the contemporaries of Democritus, because at that time the world was viewed as something divine. Democrit's philosophical opponent was above all Empedocles, who founded the doctrine of the four elements fire, earth, air and water. Democrit's proposal went unnoticed for almost two millennia.
  • around 1400 - The Alchemists - Gold cannot be made
Even if the alchemists failed in their attempts to produce gold from lower substances (such as lead), they did preparatory work for the later experimental physics and chemistry.
  • 1803 - John Dalton - Atomic Theory of the Elements
The English chemist John Dalton was the first to take up the idea of ​​Democritus again. From constant proportions in chemical reactions, Dalton concludes that a certain number of atoms always react with one another.
  • 1896 - Becquerel - radioactivity
Henri Becquerel discovered that certain substances emit different types of radiation (radioactivity).
  • 1897 - Joseph John Thomson - electron
In experiments with electric current, the British physicist Thomson discovered that rays in vacuum tubes consist of small particles: the electrons. With this a first component of the atoms was found, although one was still not convinced of the existence of the atoms. The discovery was particularly special because it was thought that electricity was a liquid.
  • 1898 - Marie and Pierre Curie - radioactive conversion of chemical elements
More and more researchers are working on the smallest particles. The Curies examined, among other things, uranium, which they extracted from pitchblende. Marie Curie recognized that in some cases another element forms from one radioactive element.
  • 1900 - Ludwig Boltzmann - Atomic Theory
Boltzmann was a theoretical physicist who implemented the ideas of Democritus. He calculated some properties of gases and crystals from the idea of ​​the existence of atoms. However, since there was no experimental evidence, his ideas were controversial at the time.
  • 1900 - Max Planck - quanta
The Berlin physicist Planck investigated black body radiation. In the theoretical, thermodynamic justification of his formula, he introduced the so-called quanta and thus became the founder of quantum physics.
  • 1905 - Albert Einstein - Explanation of the Brownian Movement
In the third work of the "annus mirabilis“The physicist Albert Einstein explained Brownian motion with the help of the atomic hypothesis. This was the first time that an observable physical phenomenon was derived directly from Boltzmann's theory.
  • 1906 - Ernest Rutherford - experiments
The physicist Ernest Rutherford went on an experimental search for the atoms. In 1906, with the Rutherford experiment, he discovered that atoms are not massive, but are systems made up of a core and shell. (This means that the term "atom" is basically wrong. However, it was retained.) Up until 1911, Rutherford derived the size of an atom, i.e. the atomic shell, and the size of the atomic nucleus from the experiment. He was also able to determine that the atomic nucleus carries the positive charge, the atomic shell a corresponding negative charge. This is how he discovered the proton.
  • 1913 - Niels Bohr - shell model
The Danish physicist Niels Bohr developed a planet-like atomic model from the Rutherford atomic model. Then the electrons move around the nucleus in certain orbits, like planets orbiting the sun. The tracks are also known as bowls. The special thing about it was that the distances between the electron orbits are fixed according to strict mathematical principles, while according to classical physics, any distance should be possible. Each shell has a maximum capacity for electrons. According to Bohr, atoms strive for all orbits to be completely occupied. This explains both many chemical reactions and the spectral lines of hydrogen. Since the model proved inadequate for more complex atoms, it was improved in 1916 by Bohr and the German physicist Arnold Sommerfeld by assuming eccentric, elliptical orbits for certain electrons. The Bohr-Sommerfeld model of the atom explains many chemical and physical properties of atoms.
  • 1929 - Erwin Schrödinger, Werner Heisenberg and others - The orbital model
Based on Schrödinger's wave mechanics and Heisenberg's matrix mechanics, another atomic model, which is still modern today, was developed, which was able to remove further ambiguities.
  • 1929 - Ernest O. Lawrence - The first particle accelerator, the cyclotron
In order to get information about the structure of the atomic nuclei, the nuclei were bombarded with beams. In order not to be dependent on weak natural radiation, Lawrence developed the cyclotron, in which charged particles are accelerated on circular orbits.
  • 1932 - Paul Dirac and David Anderson - Antimatter
Theoretical physicist Paul Dirac found a formula with which certain observations of atomic physics can be described. However, this formula presupposed the existence of anti-particles. This idea met with fierce criticism until the American physicist Anderson was able to detect the positron in cosmic rays. This antiparticle to the electron carries a positive charge, but has the same mass as an electron. If a particle and its antiparticle come together, they can "annihilate" each other, or rather, transform them into another form of energy.
  • 1932 - James Chadwick - Neutron
The neutron was discovered by the English physicist James Chadwick.
  • 1933 - Irène and Frédéric Joliot-Curie - Energy becomes mass
Rather by chance, the married couple Irène and Frédéric Joliot-Curie observed that not only mass can be converted into energy, but also vice versa. In one experiment, a light beam turned into an electron and a positron (pairing).
  • 1938 - Otto Hahn and Lise Meitner - The first nuclear fission
The German chemist Hahn, a student of Rutherford, continued to investigate atomic nuclei. To do this, he bombarded uranium atoms with neutrons and received cesium and rubidium or strontium and xenon. He and his colleague Strassmann could not explain what actually happened. However, this was achieved by his colleague Lise Meitner, who had fled the Nazis to Sweden because of her Jewish religion. She found that the sum of the nuclear particles (protons and neutrons) in the products corresponds to that of uranium. Hahn received the Nobel Prize. To this day, only Hahn and Strassmann, not Meitner, are mentioned as discoverers of nuclear fission.
  • 1938 - Hans Bethe - Nuclear Fusion in the Sun
In addition to making numerous contributions to the structure of atoms, Bethe, who was born in Strasbourg, researched the production of energy in stars. He discovered that in our sun two hydrogen atomic nuclei fuse together, while in larger and brighter stars carbon nuclei are transformed into the heavier nitrogen nuclei. Bethe also worked on the development of the atomic bomb in Los Alamos, but became a committed opponent of weapons of mass destruction after the war.
  • 1942 - Enrico Fermi - The first nuclear reactor
The Italian physicist Fermi recognized the possibility of using nuclear fission for a chain reaction. The neutrons released during the splitting of uranium could be used to split other nuclei. In doing so, Fermi laid the foundations for both the warlike use of nuclear energy in atomic bombs and the peaceful use in nuclear reactors. Fermi built the first working experimental reactor.
  • 1942 - Werner Heisenberg - Uranium Project in National Socialist Germany
The National Socialists commissioned the physicist Heisenberg with his colleagues to investigate the possible uses of nuclear fission as an energy source ("uranium machine") or as a weapon. However, because of the probable duration of development, the weapons plan was never seriously pursued. The group carried out basic research on experimental reactor systems, most recently in 1945 in a secret laboratory in Haigerloch. However, criticality, i.e. a self-sustaining chain reaction, was not achieved due to a lack of uranium.
  • 1942 - Albert Einstein and Leo Szilard - Roosevelt is supposed to build the atomic bomb
Actually, Einstein himself did not contribute to the construction of the atomic bomb. But he supported a letter to the American President Roosevelt that the atomic bomb should definitely be developed before the Nazis. The Hungarian polymath Szilard also recognized the danger posed by a German atom bomb. Although he provided important ideas for building the atomic bomb, he was not involved in its development in Los Alamos. Even later, Szilard warned against the use of the atom bomb.
  • 1945 - J. Robert Oppenheimer - The first atomic bomb
Oppenheimer was the organizer who gathered the best physicists and engineers in Los Alamos. In a very short time, the development and construction of atomic bombs, the Manhattan Project, came about. After the atomic bombs were used in Hiroshima and Nagasaki, Oppenheimer became an opponent of atomic bombs.
  • 1951 - Erwin Müller - the field ion microscope
With the construction of a field ion microscope, Müller succeeds for the first time in direct imaging of atoms on a tungsten tip.
  • 1952 - Edward Teller - The Hydrogen Bomb
The Hungarian physicist Teller was an employee of Oppenheimer. However, he had a further idea. He wanted to build a nuclear fusion bomb, which Bethe found in the sun. Fearing communism, Teller became an armaments fanatic and developed the hydrogen bomb.
  • 1960 - Donald A. Glaser - The bubble chamber
After the end of the Second World War, research concentrated on the structure of elementary particles. With the development of the bubble chamber by Glaser, it was now possible to "see" the smallest particles that were created in particle accelerators.
  • 1964 - Murray Gell-Mann - The Quarks
With the help of the bubble chamber, a huge number of previously invisible particles could suddenly be made visible, which contradicted previous physics. To explain this, the physicist Gell-Mann postulated basic building blocks from which the core building blocks should be built. There are now a lot of indications for the existence of the quarks, even if they cannot be observed individually.
  • 1978 - The fusion reactor
For the technical use of the energy that is released in nuclear fusion reactions, one begins to develop nuclear fusion reactors. So far, however, it has only been possible to gain more power than has to be put into the process for a very short time.
  • 1995 - Eric Cornell, Wolfgang Ketterle and Carl Wieman - The Bose-Einstein Condensate
Cornell, Ketterle and Wieman are the first to produce a Bose-Einstein condensate in an ultracold gas made of rubidium atoms, a state of matter predicted by Einstein.
  • 2000 - CERN - The Higgs boson
The European Nuclear Research Center CERN in Geneva uses an accelerator to research the Higgs boson, which is known as the Redeemer Particle is designated because its existence would confirm existing theories of elementary particle physics. So far there is no clear experimental evidence for the existence of the Higgs boson.
  • 2002 - Brookhaven - strange matter
In the heavy ion accelerator ring RHIC (Relativistic Heavy Ion Collider) in Brookhaven, USA, gold ions of high energy collide. They should generate a quark-gluon plasma for an extremely short time and in a very small area.This is a state of matter that no longer occurs in nature today, but which presumably existed immediately after the Big Bang.


  • A thing only appears to have a color; it only appears to be sweet or bitter; in reality there are only atoms and empty space. - Democritus (5th century BC)
  • Richard Feynman once said that if he had to express the most important result of modern science in one sentence, he decided on: "The world consists of atoms." - Brian Greene (The stuff that the cosmos is made of, ISBN 3-88680-738-X, P. 255)
  • According to some estimates, every person has around a billion atoms in their body that were once part of Shakespeare, or of Genghis Khan, Buddha or Mozart. Due to the eternal cycle and the probably enormous longevity of atoms, a permanent new use as a dewdrop, leaf or new person is possible. From Paul Davis, "The Fifth Miracle".


  • Bernhard Brocker inter alia: dtv atlas atomic physics: tables and texts. 6th edition 1997. ISBN 3-423-03009-7.

See also