The list of the essential atomic models are as follows:
1.- Atomic Model of Democritus of Abdera (year 450 BC)
2. - Atomic Model of John Dalton (1808)
Dalton's atomic theory was the first scientifically-based model. Dalton defined the concept of the atom and concluded that in chemical reactions, atoms are neither created nor destroyed but rather changed their distribution.
Dalton defined atoms as the smallest particles of a substance. However, other models showed that an atom consists of an assembly of smaller particles ( electrons, protons, and neutrons). The number of protons is equal to the atomic number.
3.- Thomson's Atomic Model (1904)
JJ Thomson conceived the atom as a plum pudding, in which the atom would be made up of a positive mass to which the electrons would fix like raisins in the cake. Thus, it is also known as the plum pudding model.
4.- Modelo Atómico De Nagaoka (1904)
The Nagaoka model also called the atomic model of Saturn, is a hypothetical model of atomic structure devised in 1904 by Hantarō Nagaoka, as opposed to Thomson's plum pudding model. In this model, the existence of the atomic nucleus was postulated for the first time.
5. - Model of Rutherford (1911)
Ernest Rutherford said that atoms were quickly transposable by electrical charges. His model defined the atom as a heavy nucleus with a positive charge and electrons revolving around it.
Most of the atom was made up of empty space conducting experiments with alpha particles.
6.- Bohr's Atomic Model (1913)
The Niels Bohr model was a modification of the Rutherford model: the atom is like "a microscopic solar system" in which there are negatively charged electrons orbiting the nucleus. Bohr assumed that the electrons moved in circular orbits around the nucleus.
He introduced quantum theory and quantum mechanics in the atomic model.
Niels Bohr developed his model according to three fundamental postulates:
First, the electrons describe circular orbits around the nucleus of the atom without radiating energy.
The only orbits allowed for an electron are those for which the electron angular momentum is a certain integer multiple called quantum number.
The electron only emits or absorbs energy in the jumps from one allowed orbit to another. This change emits or absorbs a photon whose energy is the energy difference between both energy levels.
7.- Sommerfeld's Atomic Model
In 1916, Arnold Sommerfeld, with the help of Albert Einstein's theory of relativity, made the following modifications to the Bohr model:
Electrons move around the nucleus in circular or elliptical orbits.
From the second energy level, there are one or more sublevels at the same level.
The electron is a tiny electric current.
Although he could not demonstrate the emission shapes of elliptical orbits, he only ruled out their circular shape.
8.- Modern Atomic Model (1920)
The modern atomic model was developed during the 1920s, mainly by Erwin Schrödinger and Heisenberg.
It is a model of great mathematical complexity, so much so that only the hydrogen atom can be solved with precision. They used approximate methods to resolve other particles different from hydrogen.
In any case, the quantum-mechanical atomic model fits very well with the experimental observations.
From this atomic model, we do not speak of orbits, but of orbitals. An orbital is a region of space in which the probability of finding the electron is greatest. Atomic orbitals have different geometric shapes.