All we see is atoms , many atoms. The term “atom” comes from the Greek word “atomos” (indivisible). A long time ago, in the 4th century BC, the Greek philosophers Leucippus and Democritus hypothesized that all matter is composed of minute particles in perpetual motion, very solid and eternal. Today we have a slightly more precise idea of ​​the atom because it is not indivisible. We know its approximate size since 1811, Amedeo Avogadro estimates the size of the atoms, at 10 ^-10 meters. In 1911, Ernest Rutherfordspecifies the structure of the atom and gives a size to the atomic nucleus of the order of 10 ^-14 meters. Concerning the size of atoms, we speak of atomic orbitals , ie of the electronic cloud which surrounds the nucleus (see image opposite), this cloud has a theoretical diameter between 62 pm (picometers) for the atom Helium at 596 pm for the cesium atom. But nothing is simple in the nature of matter, and this minute distance varies according to the chemical nature of the surrounding atoms. Although the nucleus concentrates most of the mass of the atom (99.99%), we also know its mass, for stable atoms, it is between 1.674 × 10 ^-24 g for Hydrogen and 3.953 × 10 ^-22g for uranium. We also know its composition, inside we see a nucleus and an electronic cloud that occupies the entire spatial extent of the atom since it is more than 10,000 times larger than its nucleus. Even more surprising, we even know the number of atoms in the universe, this number is extraordinarily large, if we were to write it should write a 1 followed by 72 zeros.
But what keeps the stability of atoms?
The stability of the atom is not explained by classical physics because in classical physics, the negatively charged corpuscular electron and the positively charged proton raise a paradox .

In classical physics, the material should disappear, annihilate because an electron radiating around a nucleus, loses energy (Maxwell’s theory) and therefore should fall on the nucleus. Which means that the stability of an atom is incomprehensible in the framework of classical theory.
The scientific geniuses of the 20th century will solve this paradox thanks to the wave mechanics of Louis de Broglie in 1924 and generalized in 1926 by Erwin Schrödinger (Nobel Prize in physics in 1933 with Paul Dirac, for the wave equation called the Schrödinger equation In quantum mechanics , it is not possible to know exactly the value of a parameter without measuring it.The mathematical theory describes a state, not, by a couple speed and position precisely,wave function (state vector), which calculates the probability of finding the particle at a point. Hence the probabilistic nature of quantum mechanics, which predicts that particles are also waves and not just material points.
The electrons occupy atomic orbitals interacting with the nucleus via the electromagnetic force, while the nucleons are held together within the nucleus by the nuclear bond, which is a manifestation of the strong nuclear interaction. The electronic cloudis stratified into quantized energy levels around the core defining electronic layers and sublayers. Nucleons are also distributed in nuclear layers, although a fairly convenient model popularizes the nuclear structure from the liquid drop model .
Several atoms can establish chemical bonds with each other through their electrons and in general, the chemical properties of atoms are determined by their electronic configuration, which results from the number of protons in their nucleus. This number, called an atomic number, defines a chemical element.

In 1911, Ernest Rutherford specified the structure of the atom by bombarding a gold leaf with particles from the radioactive decay of uranium. It even gives a size, at the atomic nucleus of the order of 10 ^-14 meters. Regarding the size of the atoms, we speak of atomic orbitals, that is to say of the electronic cloud which surrounds the nucleus, this cloud has a theoretical diameter between 62 pm (picometers) for the Helium atom to 596 pm for the cesium atom. Ernest Rutherford would like to see the atoms but the wavelengths of visible light ( 400 to 800 nanometers) are larger than the dimensions of the atom ( ≈0.1 nm) . Today ‘
The scanning tunneling microscope (STM Scanning Tunneling Microscope ) in 1981, developed by IBM researchers Gerd Binnig and Heinrich Rohrer (Nobel Prize for Physics for this invention in 1986).
The tunneling microscope is a small microscope of a few centimeters, like near-field microscopesequipped with a tip of tungsten (W) or platinum iridium (Pt Ir) so fine the size of an atom, it allows to scan under vacuum, the surface of a sample of material. A computer adjusts and records, in real time with great precision, the height of the probe to maintain a constant current. Then the computer measures and amplifies the resulting tunneling current of the electrons passing between the tip and the surface of the sample. This movement reflects the relief of the surface and therefore that of the atoms themselves, which makes it possible to reconstruct the detailed image of the surface traveled on the atomic scale.
To see the atoms scientists use an electrically conductive metal that does not oxidize, like gold or iridium platinum, because most material surfaces overlap with a hyperfine layer of oxide that prevents the passage tunnel current.

The tunnel effect is one of the properties of a quantum particle, this property allows it to cross a potential barrier even if its energy is lower than the minimum energy required.

Table: theoretical atomic radius (calculated) of some atoms, the size is given in picometers (10 ^-12 meters). The atomic radius is half of the distance separating the nuclei from two contiguous atoms. The values ​​given in this table are only indicative.