First, let us say, modern atomic model was the model of Thomson, the
scientist that discovered the electron and measured its specific charge `e/m`
ratio. Around 1900, Thomson imaged a model of the atom having both positive and
negative charges, where the negative charges are located in the atoms like the
plums in a pudding, the pudding being the positive charge that occupied the
entire volume of the atom. Thus the name of this model was **the plum
pudding model of atom**.

Ten years later, Rutherford conducted its famous experiment in which alpha
particles were scattered on a gold thin foil. The results of this experiment
allowed Rutherford to demonstrate that the atom is more likely empty than
filled with positive charge, the positive charge being located in a very small
space in the center of the atom. The nucleus of the Rutherford atom contained
almost of the atom mass. In this model electrons occupied the remaining empty
space of the atom orbiting the nucleus. Thus the name of this model was
**the planetary model** of atom.

Only a few years later (1913), Bohr developed the model of the atom that has
his name. This model was the starting point in developing the entire quantum
mechanics in physics. He proposed that only certain orbits of electrons around
the nucleus are stable and thus are allowed. The energy of these orbits take
quantified values (in other words the energy is an integer multiple of a
fundamental energy). The Bohr model of atom did explained well the behavior of
the Hydrogen atom (one proton and one electron) and of a few simple atoms near
hydrogen in the periodic system, but revealed some errors in computation of
emission (or absorption) energy spectra of more complicated atoms. As said
above this model was named **the Bohr model or the shell model of
atom**.

As said above, the Bohr model of atom started the quantum theory
development. Another famous scientist Heisenberg stated the incertitude
principle for quantum particles. Following this and the de Broglie discovery
(any moving mass act also like a wave) a wave function (of probability of being
in a certain space element `dr` ) was attached to the electron. This wave wave
function `Psi(r)` need to obey a certain mathematical equation that was
developed by Schrodinger (1926). Thus **the quantum model of
atom** was finally developed. In this model the electron is described
not by a single quantum number `n` (like in the Bohr model) but by 4 quantum
numbers attached to it (`n`, `l`, `m_l` and `m_s`) . A certain set of
4 quantum numbers can be taken by only a single electron in the atom. This
model still stands today.

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