The atomic number ( Z ) is the term employed in Chimie and Physique to represent the number of Proton S of the core of a Atome. An atom can be schematized in first approach by a compact agglomeration (atomic nucleus) of protons (p+) and neutrons (N), around which circulate of the electrons (e-). In an atom of null electric charge, the number of electron is him also equal to the atomic number (Z protons of load +e, plus Z electrons of load - E ⇒ total load = 0 Coulomb).
In the beginning, this number determined the position of a chemical element in the periodic table. When Dmitri Mendeleïev ordered the known chemical compounds according to their properties, he realized that the classification in the strict order of Atomic mass could lead to illogical situations: for example, the Iodine and the Tellure must be reversed compared to their atomic mass. This classification according to the chemical properties determines the atomic number, roughly proportional to the mass of the atom, but as the few raised inversions prove it, representative of other properties that mass.
Relation with the mass and the load
These anomalies between the classifications by atomic number and atomic mass were finally explained by work of Henry Moseley in 1913, which highlighted a strict correlation between the spectrum of diffraction of the X-rays of the elements and their site in the periodic table. It was shown thereafter that the atomic number corresponds to the electric charge of the core (i.e. with the number of Proton S). It is well the load which determines the properties of an element, and not its mass.
The atomic number is correlated with the Atomic mass (even if they should not be confused), since the mass corresponds to the number of Proton S and Neutron S in the core of a Atome. The atomic mass is frequently mentioned after the name of the element: one speaks for example about “Carbon-14” (used for example for to date from the objects in archeology) to make the distinction with carbon “12” (the Isotope more running in nature).
Simple: Atomic number Zh-min-nan: Goân-chú-hoan
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