Size of atoms

From the Greek "atomos," indivisible, the atom is the smallest particle of a chemical element, it consists of a nucleus around which a number of moving electrons, one for hydrogen, 6 for carbon , 26 for iron, 92 for uranium, etc.
These are electron-electron interactions due to their amazing quantum properties that give rise to the wide variety of elements that we find in nature. The organization of the elements of nature is represented by the periodic table or table of Mendeleev, which classifies all natural and artificial chemical elements ordered by atomic number (number of protons) increasing and organized according to their electronic configuration.
The world of electrons, belongs to the quantum world of atoms, i.e. the microscopic world. Between the atom of the microscopic world and the macroscopic world, there is a very large number of magnitudes. In one gram of material such as carbon 12, there ≈10²² atoms.
We know the approximate size of atoms since 1811, Amedeo Avogadro (1776-1856) estimated its size to 1 angstrom, i.e. 10-10 meter and one century later, in 1911 Ernest Rutherford (1871-1937) specifies, the structure of atoms, and gives a size the atomic nucleus of the order of 10-14 meters. One can say that the atoms are separated from each other of a few angstroms.
But since the advent of quantum mechanics in the 1920s, we do not represent, the electron as a spinning object on a very regular orbit around the nucleus, as in a planetary model. Today we know that the motion of an electron is quite different from planetary motion. In quantum mechanics the electron does not follow a single trajectory, it is here and there, in a region around the nucleus called the electron cloud, or atomic orbital.
The orbitals of the electron may take different forms depending on the characteristics of the nature of atom, for example, the orbital of the hydrogen atom has a spherical shape, the orbital of the atom oxygen in the form of two drops of water, the orbital of the iron atom is in the form of four drops of water. This form of the atomic orbital sets the size of the atom.
Thus the diameter of the electron cloud around the nucleus i.e. the diameter of the whole atom is of the order of 0.1 nanometer or one ten billionth of a meter. An atom is so small that we could align 10 million atoms on a millimeter. However, the electron cloud of an atom does not have a well-defined dimension because it is a superposition of atomic orbitals of a probabilistic nature (they vary). There is therefore no single definition nor precise measure of the size of atoms because the shape of this region of atomic space depends on the energy of the electron and its angular momentum. But to get an idea of ​​the size of the atom, scientists have defined a theoretical atomic radius.

A theoretical atomic radius represents half of the average distance between the nuclei of linked atoms among themselves. Although this distance varies depending on the properties of the atom, it can be calculated for each atomic nucleus, the size of the atomic orbitals (see table). The size of the atoms increases with the number of electrons or rather according to the occupancy of the atomic orbital electrons of the outer layer, which is bonded to the nucleus far less than the inner layers. The more layers (quantum energy levels) in the atom and the more outer layer is extended, i.e. the superposition of the atomic orbitals increases the size of atoms as the outer layer is less and less related to the nuclei and therefore freer. However, the more electrons in the inner layers and the more the attraction of the atomic nucleus is growing because there is more and more protons and therefore more and more positive charges. This property (number of protons) limits the spatial extent of the atomic orbitals negatively charged (negative charges of electrons), bringing closer them to the nuclei.

Table: Theoretical Atomic radius (calculated) of certain atoms, the size is given in picometers (10-12 meters). The atomic radius is half the distance between the nuclei of two adjacent atoms. The values ​​in this table are only indicative.

Since the 1990's through tunneling microscope it is possible to see and manipulate individual atoms on the surface of a material.
This film was made by IBM, with iron atoms on a copper plate, this is the smallest movie in the world.
In this nano movie each light is an atom, which occupies 12 pixels on this screen. Small waves that create oscillations around bright points are not a fault of the film but the electron waves. These Friedel oscillations are not an oddity but betray the quantum nature of atoms, both particles and waves. The shape of these oscillations of electrons constitute a real ID.
This film at the nanometer scale, measuring 52 by 32 iron atoms, i.e. ≈8 nanometers on ≈5 nanometers. On a length of 1 mm we could align 10 000 films of this size.
Each image is produced with the tip of a scanning tunneling microscope, a few degrees of absolute zero, this is also the coldest film in the world.

NB: Jacques Friedel (1921 − 2014) was a French physicist, professor emeritus at the University Paris-Sud Orsay, specialist in theoretical solid state physics. He was president of the Academy of Sciences for the period 1993-1994.