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Atomic structure

 Automatic translation  Automatic translation Updated June 01, 2013

Everything we see is made up of atoms, many atoms.
The word "atom" comes from the Greek word "atomos" (indivisible). Long ago already, in the 4th century BC, the Greek philosophers Leucippus and Democritus theorize that all matter is composed of tiny particles in constant motion, very solid and eternal. Today we have a somewhat more accurate because the atom is not indivisible. We know its approximate size since 1811, Amedeo Avogadro estimates the size of atoms, to 10-10 meters. In 1911, Ernest Rutherford specifies the structure of the atom and gives the atomic nucleus size of about 10−14 meters. Concerning the size of atoms, it is called atomic orbitals, i.e. the electron cloud surrounding the nucleus (see image opposite), this cloud has a theoretical diameter between 62 pm (picometers) for the Helium atom at 596 pm for the Cesium atom. But nothing is simple in the nature of matter and the tiny distance varies depending on the chemical nature of the surrounding atoms. While the nucleus concentrates essential of the mass of the atom (99.99%), we known as its mass for stable atoms, it is between 1.674 × 10-24 g for Hydrogen and 3.953 × 10-22 g for uranium. We also know its composition, inside we see a nucleus and electron cloud which occupies the spatial extent of the atom because it is more than 10,000 times larger than its nucleus. Even more amazing, we even know the number of atoms in the Universe, this number is extremely large, so we had to write should write a 1 followed by 72 zeros.
But what maintains the stability of atoms?
The stability of atom can not be explained by classical physics because in classical physics, the electron corpuscular negatively charged and the proton positively charged raise a paradox.


In classical physics, the matter should disappear, because an electron which radiates around nucleus loses energy (Maxwell's theory) and therefore should fall on the nucleus. This means that the stability of an atom is incomprehensible in the context of the classical theory. Scientific geniuses of the 20th century will solve this paradox by wave mechanics by 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 the exact value of a parameter without measure it.
Mathematical theory describes a state, not by a couple speed and position precisely, but by a wave function that calculates the probability of finding a particle at a point. Hence the probabilistic nature of quantum mechanics which predicts that particles are also waves and not only material points.
The electrons occupy atomic orbitals in interaction with the nucleus via the electromagnetic force, while the nucleons are held together in the nucleus by the nuclear binding, which is a manifestation of the strong nuclear interaction. The electron cloud is stratified into quantized energy levels around the nucleus defining layers and sub-layers electronics.
Nucleons are also divided into nuclear layers, although quite convenient model, popularized by the nuclear structure of the liquid drop model.
More atoms can establish chemical bonds between them through their electrons and generally, the chemical properties of atoms are determined by their electron configuration, which stems from the number of protons in their nucleus. This number, called the atomic number, defines a chemical element.

 image of the atom (electron cloud)

Image: Representation of the atomic structure of the atom of helium-4. For reasons of clarity, the above image is not to scale. The nucleus appears pink in the center, and all shades of gray around, the electron cloud or atomic orbital. The nucleus of helium-4 is a schematic enlarged, showing the two protons and two neutrons in red and purple, its size is 1 femtometer or 10-15 meter. In reality, the nucleus (and the wave function of each nucleon) is also spherical, as the electrons of the atom. Credit image: public domain.

Image of an atom


In 1911, Ernest Rutherford specifies the structure of the atom by bombarding gold foil with particles from the radioactive decay of uranium. It gives a size at the atomic nucleus of the order of 10-14 meters. Concerning the size of atoms, it is called atomic orbitals, i.e. the electron cloud surrounding the nucleus, this cloud has a theoretical diameter between 62 pm (picometers) for the Helium atom at 596 pm for the Cesium atom. Ernest Rutherford would like to see well the atoms but the wavelengths of visible light (400 to 800 nanometers) are greater than the dimensions of the atoms.
Today we can see atoms with the STM (Scanning Tunneling Microscope) invented in 1981, developed by IBM researchers, Gerd Binnig and Heinrich Rohrer (Nobel Prize in Physics for this invention in 1986).
The scanning tunneling microscope is a small microscope of a few centimeters, type scanning probe microscopes, with a tungsten tip (W) or platinum-iridium (Pt Ir) so fine the size of an atom, that it can scan in vacuum, the surface of a matter sample.
A computer adjusts and records in real time with high precision, the height of the probe to maintain a constant current. Then the computer measures and amplifies the resultant current, by quantum tunneling, of the passage of electrons between the tip and the sample surface.
This movement reflects the topography of the surface and thus the atoms themselves which allows to reconstruct the detailed picture of the surface covered at the atomic scale.


To see the atoms, scientists use a metal conductor of electricity that does not oxidize, such as gold or platinum iridized, because most of the matter surfaces are covered with a layer hyperfine oxide that prevents the passage tunnel current. Quantum tunneling is one of properties of a quantum particle, this property allows it to cross a potential barrier even if its energy is less than the minimum energy required.

NB: the spectrum of visible light is in the infrared to ultraviolet, it corresponds to wavelengths of 400 nanometers in the violet to 800 nanometers in the red, that is to say 4x10-7 to 8x10-7 meter. Between the wavelength (λ) and frequency (ν) is the following relationship: ν = c / λ where c is the speed of light is about 300,000 km/s.

Submultiples of the meterSymbol Name
10-1 meterdmdecimeter
10-2 metercmcentimeter
10-3 metermmmillimeter
10-6 meterµm micrometer
10-9 meternmnanometer
10-12 meterpmpicometer
10-15 meterfmfemtometer
10-18 meteramattometer
10-21 meterzmzeptometer
10-24 meterymyoctometer
 Image of pure gold atom seen by Scanning Tunneling Microscope

Image: This image, closer to matter, details the surface of a sheet of pure gold (Au (100), visualized by a scanning tunneling microscope. The gold atoms are visible in this image, they are evenly spaced them on the crystalline structure. This image atomic was made with STM Omicron low temperature by Erwin Rossen, Eindhoven University of Technology, 2006.

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