See also: Core

The atomic nucleus indicates the area located at the center of a Atome consisted of Proton S and Neutron S (the Nucléon S). Size of the core (10 -15 m) is considerably smaller than that of the atom (10 -10 m) and almost all its mass concentrates.

Physical characteristics

The atom has a lacunar structure , i.e. enters the electrons and the core there is only vacuum .

Composition

The core of an atom is composed of Nucléon S extremely dependant (except for the Hydrogène whose core simply consists of single a Proton). Its cohesion is ensured by the strong Interaction, force dominating on a core scale, which maintains the nucleons together and prevents them from moving away from/to each other.

To model this attraction between the nucleons, one can define a energy binding being able to be calculated starting from the Formule of Bethe-Weizsäcker.

Isotopes

The isotopes are atoms having identical numbers of protons (even atomic number Z) but of the numbers of different neutrons.

A element is characterized by the number of protons which compose its core, precisely called Atomic number and noted Z ; an atom having as many electron S that protons, Z is also the number of electrons of an element.

For the same element, it is possible to obtain different Nucléide S corresponding to numbers of different neutrons. The of the same nuclides atomic number Z are called Isotope S of the element having this atomic number. The mass number has of an atom is the full number of nucleons (protons and neutrons) which compose it. The number of neutrons NR is equal to has - Z .

A nuclide X is thus an atom characterized by its Mass number has and its atomic number Z ; it is noted ^A_ZX (to read X has , the atomic number being implicit).

For example, the Hydrogen ¹₁H, the Deuterium D or ² ₁H and the Tritium T or ³ ₁H are three isotopes of hydrogen.

Various isotopes of the same element have chemical properties similar, because they primarily depend on its number of electron S. Their stability and Demi-vie can however be different and them Atomic mass distinct makes it possible to separate them using a Centrifugeuse or from a Spectromètre.

Atomic mass

The isotopic atomic mass of an element is the mass corresponding to NR nuclides of this same isotope, NR being the Nombre of Avogadro (approximately 6,022 04).

The Atomic mass of a chemical element is the weighted average of the atomic masses of its natural isotopes; certain chemical elements very have radioactive isotopes at long period, and consequently their natural isotopic composition, as their atomic mass evolves/moves over long periods of time, such as the geological eras. It is in particular the case for the Uranium.

Stability

The stability of an atomic nucleus depends on the number of nucleons which compose it. Certain cores are stable, i.e. them energy binding is sufficient, making then their lifespan unlimited. Others are unstable and tend spontaneously to transform into a more stable core by emission of a radiation. Or more precisely, the transformation (spontaneous) of the radioactivity always occurs according to an increase in the average binding energy of the nucleons concerned.

Example: the Uranium 235 and 238 have half-lives higher than those of their " famille" respective before leading to the stable isotopes of the Lead.

• Radioactivité α if it emits one or more nucleons (proton, neutron or Particule α)

• Radioactivité β if it emits an electron or a Positron with a Neutrino.

These two types of radioactivity are accompanied most of the time by a gamma ray (emission of Photon S).

For example, the Azote -16 (16 nucleons, 7 protons, 9 neutrons) is transformed into Oxygène -16 (16 nucleons, 8 protons, 8 neutrons) a few seconds after its creation by radioactivity beta: the weak Interaction transforms one of the neutrons of the core into a proton and an electron, thus modifying the Atomic number of the atom.

The Demi-vie of an isotope is the period at the end of which, statistically, half of the atoms of an initial sample will have disintegrated. The cores can have very different half-lives: covering in fact all the beach of the durations!

Since the negligible fraction of a second, where the core is in its “normal” state, fundamental when it has more the trace of its last (trans) formation: ~10 15 S (?)

In fact are, the cores known as stable to it only insofar as they border that of the Proton, only Baryon (méta?)stable. The proton has a half-life (theoretical) of approximately 10³³ years!

But the experiments undertaken to measure this disintegration of the proton, genuine angular stone of the matter, did not check this prediction. The proton more stable than would be envisaged…. mmmh

Cut and forms

The ray of a nucleon is about 10^-15 m, that is to say 1 Fm (the term of ray gets along here within the meaning of having a probability sufficient to detect the nucleon in the volume of space considered). That is worth less than 0,01% of the total ray of the atom. The density of the core is thus considerably larger than that of the atom even. This density is about constant for all the cores in their fundamental state (nonexcited); approximately 200 million tons to cm ³: density of the fluid nuclear.

The real size and the shape of a specific core strongly depend on the number of nucleons which compose it, as well as theirs energy state. The most stable cores in general have a spherical form at rest and can take, for example, the form of a Ellipsoïde if they are excited.

In the case of the cores with halation, some nucleons would be located at a distance definitely larger others, thus surrounding of a halation the more compact core formed by the other nucleons. The Lithium 11 seems for example made up of a lithium 9 core (the most stable isotope) surrounded by a cloud of two neutrons; its size is then comparable with that of the Plomb 208 which has 20 times more nucleons.

The stable core heaviest (known as superheavy core) consists of 112 protons.

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