What is the purpose of Thomson's atomic model

A Atomic model is a model that describes the structure of atoms on the basis of observable properties of matter and experimentally determined data.

The models of atomic physics could explain more and more observations over time, but they also became more complicated. Today we are able to describe atoms with the help of quantum mechanics.

When asked how one should imagine an atom, the physicist Werner Heisenberg answered: "Don't even try!"Today this is more relevant than ever, since newer atomic models can only be represented with mathematical formulas.


The prerequisite for the development of atomic models was the discovery of the atoms and the discovery that these, too, are made up of smaller particles.

  • Leukippus, Democritus (approx. 500 BC): All substances consist of defined smallest particles, the atoms (gr. Atomos, the indivisible).
  • Daniel Sennert (1618): Law of Conservation of the Elements. In a chemical reaction, elements are neither lost nor are new elements created.
  • Robert Boyle (1661): Elements are certain primitive and simple, completely unmixed bodies, they do not contain any other bodies, they are ingredients from which all perfectly mixed bodies are composed and into which they are ultimately broken down.
  • Antoine Laurent de Lavoisier (1785): Law of Conservation of Mass. The sum of the masses of the starting materials is always equal to the sum of the masses of the products.
  • Jeremias Benjamin Richter (1791/92): Law of Equivalent Proportions.

The first experimental evidence of atoms did not appear until the late 18th century, when John Dalton discovered his law of multiple proportions. Based on his atomic hypothesis, he predicts the law of constant proportions, which Joseph-Louis Proust formulated in 1799.

See also: atomic physics

Dalton's atomic hypothesis (1803)

  1. Matter consists of the smallest spherical particles or atoms.
  2. These atoms are indivisible and cannot be created or destroyed.
  3. All atoms of a chemical element are identical to each other, they only differ in mass from atoms of other elements.
  4. These atoms can enter into chemical bonds and can also be released from them again.
  5. The particle of a compound is formed from a certain, always the same number of atoms of the elements that make up the compound.

Counter arguments for Dalton's theory were, for example, the electrical charging of atoms, which presupposes the divisibility of the atoms.

Atoms were mostly assumed to be solid spheres. This only changed when Joseph John Thomson discovered the electron in 1897, which was first predicted by George Johnstone Stoney in 1874 and named by name in 1891.

Atomic models

Sphere model (approx. 400 BC)

Democritus explained his atom as a small indivisible particle that is solid and spherical. In order to come up with this, he came up with a thought: "It cannot be, if a board could be divided infinitely times, the board would ultimately consist of nothing!". So he put forward the thesis that it was a Indivisible must give. He also thought that atoms could be different in shape, such as crooked, smooth and irregular. Smooth objects should consist of round atoms and rough objects should consist of angular atoms. He also explained our matter, that plants, fire and humans consist of these small particles that collapse and form water, for example. He also assumed that our soul was made up Soul atoms that fall apart and fly to a new soul when a person dies. Parts of his theories are the basis of our modern thinking today.

Dynamid Model (1903)

After this Dynamid model Atoms consist for the most part of empty space between small, rotating electrical dipoles, the dynamids. The atomic mass number is equal to the number of dynamides in the atom. The model remained largely unknown.

Thomson's atomic model (1903)

After this Thomson's atomic model the atom consists of an evenly distributed positive charge and negatively charged electrons that move within it. This model is also known as the Plum pudding model or in German Raisin cake model designated.

Rutherford's atomic model (1911)

After this Rutherford's atomic model (according to Ernest Rutherford) the atom consists of a positively charged atomic nucleus, which contains almost the entire mass of the atom, and an atomic shell in which the electrons circle around the nucleus. This model was designed because charged particles can pass atoms largely without interference.

Bohr's atomic model (1913)

After this Bohr's atomic model (after Niels Bohr) the atom consists of a positively charged, mass-bearing nucleus and electrons that circle it on discrete orbits.

Bohr-Sommerfeld's atomic model (1916)

The Bohr-Sommerfeld model of the atom is an extension of Bohr's atomic model by Arnold Sommerfeld. In it certain elliptical orbits around the atomic nucleus are also permitted.

Orbital model (1928)

After this Orbital model the atom consists of a nucleus surrounded by orbitals. The shape of the orbitals is given by the spatial probability of the electrons being present. In the strict sense, an orbital is a solution to the Schrödinger equation.

Didactic models

Shell model

in the Shell model a positively charged atomic nucleus is surrounded by spherical shells in which the electrons are located. Only the outermost shell is responsible for the chemical properties of the element. No statement is made about the movement of the electrons. The shell model is

  • an extension of Bohr's model of the atom: electrons circle around the atomic nucleus like the planets around the sun and
  • a simplification of the orbital model: the location of the electrons can only be determined by a probability function - the so-called wave function as a solution to the Schrödinger equation. The wave function can be visualized by so-called probability clouds or shells.


Spherical cloud model

The Spherical cloud model (Kimball's atomic model, tetrahedron model) is an atomic model often used in schools, with which many phenomena (atomic bond, molecular structure) can be explained. It represents an extension of the shell model and is a simplification compared to the more precise orbital model.

Point particles and incompressible spheres

In some cases, atoms can be approximated as points without expansion (e.g. ideal gas), in others as spheres with a certain volume (e.g. van der Waals gas).

Core and shell

The radius of the atomic nucleus is around a factor of 10,000 smaller than the radius of the atomic shell.

So when one speaks of the radius of an atom, then the outer radius of the atomic shell is always meant (in the other case one speaks of the core radius). The atomic radius fluctuates between 0.3 · 10-10 m and 2.62 · 10-10 m.

The atomic nucleus is made up of protons and neutrons (these elementary particles are generally referred to as nucleons). It contains almost the entire mass of the atom (more than 99.9%) and is positively charged. The number of protons determines the affiliation to a certain element. Its radius is about 10-14 m

At atoms with the same number of protons, but different numbers of neutrons in the nucleus, one speaks of isotopes of the respective element.

The atomic shell is formed by the electrons. With its negative charge, it compensates for the charge of the positive atomic nucleus, so that the atom is mostly neutral to the outside world. The neutrality applies at a great distance based on the expansion of the atom or, on average over time, over sufficiently long periods of time based on the temporal changes in the charge distribution of the electrons in the shell.

If the shell contains more or fewer electrons than the nucleus protons, one speaks of an ion and electrical neutrality is no longer given, even at a great distance or on average over time. The ion is effectively charged.

Models of the atomic nucleus

There are also models that deal exclusively with the atomic nucleus.

Droplet model (1936)

The Droplet model describes the atomic nucleus as droplets of a charged liquid. Nuclear fission, for example, can be explained well with this classic model.

Shell model (1949)

The shell model of the atomic nucleus was postulated synchronously by Eugene Paul Wigner, Maria Goeppert-Mayer and J. Hans D. Jensen in 1949. It traces the structure of atomic nuclei back to quantum mechanical laws (Pauli principle). In contrast to the droplet model, the shell model is a model that allows nucleons to be relatively independent of movement.

  • History of atomic physics
  • History of the smallest "indivisible" particles Amazingly good introduction to the 12th grade advanced course at Georg-Forster-Gymnasium.
  • Very simple overview of the development of the atomic model in English
  • Animated representation of all atoms according to the Bohr model
  • Atomic Physics - an English introduction
  • Bohr's model of the atom - an introduction to schools (Ulm University)

Category: Atomic Physics