The number atomic number or proton number (symbol Z ) of a chemical element is the number of protons found in the atomic nucleus. This is identical to the core charge number. The atomic number uniquely identifies the chemical element. In an uncharged atom, the atomic number is also equal to the number of electrons.
The number of atomic numbers Z and the number of neutrons, N , gives the mass number A of an atom. Because protons and neutrons have nearly the same mass (and the mass of electrons can be neglected for many purposes) and the defects in the nucleation binding mass are always small compared to the nucleons, atomic masses of any atom, when expressed in integrated atoms. the mass unit (making a quantity called "relative isotopic mass"), is within 1% of all the numbers A .
Atoms with the same atomic number Z but different neutron numbers N , and hence different atomic masses, known as isotopes. A little more than three-quarters of the naturally occurring element exists as a mixture of isotopes (see monoisotopic elements), and the average isotope mass of an isotope mixture for elements (called relative atomic mass) in an Earth-determined environment, determines the element's standard atomic weight. Historically, this atomic weight (compared to hydrogen) is a quantity measurable by chemists in the nineteenth century.
The conventional symbol Z is derived from the German Z ahl means numbers , which, prior to the synthesis of modern ideas of chemistry and physics, merely symbolizes the numerical place of elements in the periodic table, whose sequence is roughly, but not completely, consistent with the order of elements with weight atom. Only after 1915, with the suggestion and proof that this number Z is also a nuclear charge and the physical characteristics of the atom, does the word Atom < b> z ahl (and an atomic number equivalent to English ) is used generally in this context.
Video Atomic number
History
Periodic table and natural number for each element
Speaking loosely, the existence or construction of a periodic table of elements creates an order of elements, and thus they can be numbered sequentially.
Dmitri Mendeleev claims that he arranged his first periodic table (first published on March 6, 1869) in the atomic weight sequence ("Atomgewicht"). However, taking into account the chemical properties of the observed elements, it changes the order slightly and puts tellurium (atomic weight 127.6) in front of iodine (atomic weight 126.9). This placement is consistent with the modern practice of ordering elements with the proton number, Z , but the number is unknown or suspected at the time.
The simple numbering based on the position of the periodic table has never been entirely satisfactory. In addition to cases of iodine and tellurium, then several pairs of other elements (such as argon and potassium, cobalt and nickel) are known to have atomic weights that are almost identical or inverse, requiring their placement in the periodic table to be determined by their chemicals. property. But gradual identification is more and more like a lanthanide of a chemical element, whose atomic number is not clear, causing inconsistencies and uncertainties in the numbering of periodic elements of at least luteium (element 71) and so on (hafnium is unknown at this time).
Model Rutherford-Bohr and van den Broek
In 1911, Ernest Rutherford gave an atomic model in which the central nucleus holds most of the atomic mass and the positive charge which, in electron charge units, is approximately equal to half the weight of the atomic atoms, expressed in the number of hydrogen atoms. Thus this central charge is approximately half the weight of the atom (although it is almost 25% different from the gold atomic number ( Z = 79 , A = 197 ), the single element from which Rutherford makes his guess). However, despite Rutherford's estimation that gold has a central charge of about 100 (but an element of Z = 79 on the periodic table), a month after Rutherford's paper appears, Antonius van den Broek first formally suggest that the center charge and the number of electrons in the atom are exactly equal to the place in the periodic table (also known as the element number, atomic number, and denoted Z ). This eventually proved to be the case.
Moseley's 1913 experiment
The experimental position increased dramatically after research by Henry Moseley in 1913. Moseley, after discussing with Bohr who was in the same laboratory (and who had used the Van den Broek hypothesis in his atomic Bohr model), decided to test Van den Broek and Bohr Hypothesis directly, by seeing if the spectral lines emitted from the happy atoms correspond to the Bohr theory which states that the frequency of the spectral lines is proportional to the square Z .
To do this, Moseley measured the wavelengths of the deepest photon transitions (lines K and L) produced by elements from aluminum ( Z Ã, = 13) to gold ( Z Ã, = Ã, 79) is used as a series of anodic targets moving inside an x-ray tube. The square root of the frequency of this photon (x-rays) increases from one target to the next in the arithmetic progression. This leads to the conclusion (the Moseley law) that the atomic number is not closely related (with the offset of one unit for K-lines, in Moseley's work) to an electric charge calculated from the nucleus, the element number Z . Among other things, Moseley pointed out that the lanthanide series (from lanthanum to lutetium inclusive) should have 15 members - no less and no more - which was far from clear from the chemistry of the time.
Missing elements
After Moseley's death in 1915, the atomic number of all known elements of hydrogen to uranium ( Z = 92) was examined by his method. There are seven elements (with Z & lt; 92) that are not found and therefore identified yet to be found, corresponding to atomic numbers 43, 61, 72, 75, 85, 87 and 91. From 1918 to 1947, these seven missing elements are found. At this time the first four transuranium elements have also been found, resulting in a complete periodic table without gaps as far as the curium ( Z = 96).
Proton and nuclear electron idea
In 1915 the reason for the quantized nuclear charge in units Z , now recognized as element numbers, was not understood. An old idea called the Prout hypothesis has postulated that the elements are all made of residue (or "protyles") of the lightest hydrogen element, which in Bohr-Rutherford's model has a single electron and nuclear charge of one. However, since 1907 Rutherford and Thomas Royds have shown that the alpha particle, which has a charge 2, is a nucleus of helium atoms, which has a mass fourfold of hydrogen, not twice. If the Prout hypothesis is true, something must neutralize some of the hydrogen nuclear charge present in the heavier nuclei of the atom.
In 1917 Rutherford succeeded in generating a hydrogen nucleus from a nuclear reaction between an alpha particle and a nitrogen gas, and believed he had proved Prout's law. He called the proton a new heavy nuclear particle in 1920 (the alternative name being proutons and protyles). It is straightforward from Moseley's work that heavy atomic nuclei have twice as many masses as would be expected of those made of hydrogen nuclei, and thus a hypothesis is required for the extra proton neutralization to be expected. present in all heavy core. The helium core is thought to consist of four protons plus two "nuclear electrons" (electrons bound in the nucleus) to cancel two of the charges. At the other end of the periodic table, a gold nucleus with a mass 197 times of hydrogen, is thought to contain 118 nuclear electrons in the nucleus to provide a residual charge of 79, consistent with the atomic number.
The discovery of neutrons makes Z the proton number
All nuclear electron considerations ended with James Chadwick's discovery of neutrons in 1932. A gold atom now appears to contain 118 neutrons rather than 118 nuclear electrons, and its positive charge is now realized to be entirely derived from the 79 protons. After 1932, therefore, the atomic number of an element Z was also realized to be identical to the proton number of its nucleus.
Maps Atomic number
Symbols Z
Conventional symbols Z may be derived from German German Atom z ahl (atomic number). However, before 1915, the word Zahl (only number ) is used for the number defined elements in the periodic table.
Chemical properties
Each element has a set of certain chemical properties as a consequence of the number of electrons present in the neutral atom, ie Z (atomic number). The configuration of these electrons follows the principles of quantum mechanics. The number of electrons in each element of the electron shell, especially the outer shell of valence, is a major factor in determining its chemical bonding behavior. Therefore, the atomic number alone determines the chemical properties of an element; and for this reason an element can be defined as consisting of a mixture of atoms each with the given atomic number.
New element
The search for new elements is usually explained using atomic numbers. In 2010, all elements with atomic numbers 1 to 118 were observed. The synthesis of new elements is done by bombarding target atoms of heavy elements with ions, so that the number of atomic numbers of the target and the ionic element is equal to the atomic number of the created element. In general, the half-life becomes shorter when the atomic number increases, although "island stability" may exist for undiscovered isotopes with the number of protons and certain neutrons.
See also
- The history of the periodic table
- The effective atomic number
- The atomic theory
- Prout Hypothesis
References
Source of the article : Wikipedia
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