From Wikipedia, the miễn phí encyclopedia
Bạn đang xem: số proton
The atomic number or nuclear charge number (symbol Z) of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei composed of protons and neutrons, this is equal vĩ đại the proton number (np) or the number of protons found in the nucleus of every atom of that element. The atomic number can be used vĩ đại uniquely identify ordinary chemical elements. In an ordinary uncharged atom, the atomic number is also equal vĩ đại the number of electrons.
For an ordinary atom which contains protons, neutrons and electrons, the sum of the atomic number Z and the neutron number N gives the atom's atomic mass number A. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the mass defect of the nucleon binding is always small compared vĩ đại the nucleon mass, the atomic mass of any atom, when expressed in daltons (making a quantity called the "relative isotopic mass"), is within 1% of the whole number A.
Atoms with the same atomic number but different neutron numbers, and hence different mass numbers, are known as isotopes. A little more kêu ca three-quarters of naturally occurring elements exist as a mixture of isotopes (see monoisotopic elements), and the average isotopic mass of an isotopic mixture for an element (called the relative atomic mass) in a defined environment on Earth, determines the element's standard atomic weight. Historically, it was these atomic weights of elements (in comparison vĩ đại hydrogen) that were the quantities measurable by chemists in the 19th century.
The conventional symbol Z comes from the German word Zahl 'number', which, before the modern synthesis of ideas from chemistry and physics, merely denoted an element's numerical place in the periodic table, whose order was then approximately, but not completely, consistent with the order of the elements by atomic weights. Only after 1915, with the suggestion and evidence that this Z number was also the nuclear charge and a physical characteristic of atoms, did the word Atomzahl (and its English equivalent atomic number) come into common use in this context.
The rules above vì thế not always apply vĩ đại exotic atoms which contain short-lived elementary particles other kêu ca protons, neutrons and electrons.
The periodic table and a natural number for each element
Loosely speaking, the existence or construction of a periodic table of elements creates an ordering of the elements, and sánh they can be numbered in order.
Dmitri Mendeleev claimed that he arranged his first periodic tables (first published on March 6, 1869) in order of atomic weight ("Atomgewicht"). However, in consideration of the elements' observed chemical properties, he changed the order slightly and placed tellurium (atomic weight 127.6) ahead of iodine (atomic weight 126.9). This placement is consistent with the modern practice of ordering the elements by proton number, Z, but that number was not known or suspected at the time.
A simple numbering based on periodic table position was never entirely satisfactory, however. Besides the case of iodine and tellurium, later several other pairs of elements (such as argon and potassium, cobalt and nickel) were known vĩ đại have nearly identical or reversed atomic weights, thus requiring their placement in the periodic table vĩ đại be determined by their chemical properties. However the gradual identification of more and more chemically similar lanthanide elements, whose atomic number was not obvious, led vĩ đại inconsistency and uncertainty in the periodic numbering of elements at least from lutetium (element 71) onward (hafnium was not known at this time).
The Rutherford-Bohr model and cầu xin den Broek
In 1911, Ernest Rutherford gave a model of the atom in which a central nucleus held most of the atom's mass and a positive charge which, in units of the electron's charge, was vĩ đại be approximately equal vĩ đại half of the atom's atomic weight, expressed in numbers of hydrogen atoms. This central charge would thus be approximately half the atomic weight (though it was almost 25% different from the atomic number of gold (Z = 79, A = 197), the single element from which Rutherford made his guess). Nevertheless, in spite of Rutherford's estimation that gold had a central charge of about 100 (but was element Z = 79 on the periodic table), a month after Rutherford's paper appeared, Antonius cầu xin den Broek first formally suggested that the central charge and number of electrons in an atom was exactly equal vĩ đại its place in the periodic table (also known as element number, atomic number, and symbolized Z). This proved eventually vĩ đại be the case.
Moseley's 1913 experiment
The experimental position improved dramatically after research by Henry Moseley in 1913. Moseley, after discussions with Bohr who was at the same lab (and who had used Van den Broek's hypothesis in his Bohr model of the atom), decided vĩ đại test Van den Broek's and Bohr's hypothesis directly, by seeing if spectral lines emitted from excited atoms fitted the Bohr theory's postulation that the frequency of the spectral lines be proportional vĩ đại the square of Z.
To vì thế this, Moseley measured the wavelengths of the innermost photon transitions (K and L lines) produced by the elements from aluminium (Z = 13) vĩ đại gold (Z = 79) used as a series of movable anodic targets inside an x-ray tube. The square root of the frequency of these photons (x-rays) increased from one target vĩ đại the next in an arithmetic progression. This led vĩ đại the conclusion (Moseley's law) that the atomic number does closely correspond (with an offset of one unit for K-lines, in Moseley's work) vĩ đại the calculated electric charge of the nucleus, i.e. the element number Z. Among other things, Moseley demonstrated that the lanthanide series (from lanthanum vĩ đại lutetium inclusive) must have 15 members—no fewer and no more—which was far from obvious from known chemistry at that time.
After Moseley's death in 1915, the atomic numbers of all known elements from hydrogen vĩ đại uranium (Z = 92) were examined by his method. There were seven elements (with Z < 92) which were not found and therefore identified as still undiscovered, corresponding vĩ đại atomic numbers 43, 61, 72, 75, 85, 87 and 91. From 1918 vĩ đại 1947, all seven of these missing elements were discovered. By this time, the first four transuranium elements had also been discovered, sánh that the periodic table was complete with no gaps as far as curium (Z = 96).
The proton and the idea of nuclear electrons
In 1915, the reason for nuclear charge being quantized in units of Z, which were now recognized vĩ đại be the same as the element number, was not understood. An old idea called Prout's hypothesis had postulated that the elements were all made of residues (or "protyles") of the lightest element hydrogen, which in the Bohr-Rutherford model had a single electron and a nuclear charge of one. However, as early as 1907, Rutherford and Thomas Royds had shown that alpha particles, which had a charge of +2, were the nuclei of helium atoms, which had a mass four times that of hydrogen, not two times. If Prout's hypothesis were true, something had vĩ đại be neutralizing some of the charge of the hydrogen nuclei present in the nuclei of heavier atoms.
In 1917, Rutherford succeeded in generating hydrogen nuclei from a nuclear reaction between alpha particles and nitrogen gas, and believed he had proven Prout's law. He called the new heavy nuclear particles protons in 1920 (alternate names being proutons and protyles). It had been immediately apparent from the work of Moseley that the nuclei of heavy atoms have more kêu ca twice as much mass as would be expected from their being made of hydrogen nuclei, and thus there was required a hypothesis for the neutralization of the extra protons presumed present in all heavy nuclei. A helium nucleus was presumed vĩ đại be composed of four protons plus two "nuclear electrons" (electrons bound inside the nucleus) vĩ đại cancel two of the charges. At the other over of the periodic table, a nucleus of gold with a mass 197 times that of hydrogen was thought vĩ đại contain 118 nuclear electrons in the nucleus vĩ đại give it a residual charge of +79, consistent with its atomic number.
The discovery of the neutron makes Z the proton number
All consideration of nuclear electrons ended with James Chadwick's discovery of the neutron in 1932. An atom of gold now was seen as containing 118 neutrons rather kêu ca 118 nuclear electrons, and its positive nuclear charge now was realized vĩ đại come entirely from a nội dung of 79 protons. Since Moseley had previously shown that the atomic number Z of an element equals this positive charge, it was now clear that Z is identical vĩ đại the number of protons of its nuclei.
Each element has a specific phối of chemical properties as a consequence of the number of electrons present in the neutral atom, which is Z (the atomic number). The configuration of these electrons follows from the principles of quantum mechanics. The number of electrons in each element's electron shells, particularly the outermost valence shell, is the primary factor in determining its chemical bonding behavior. Hence, it is the atomic number alone that determines the chemical properties of an element; and it is for this reason that an element can be defined as consisting of any mixture of atoms with a given atomic number.
The quest for new elements is usually described using atomic numbers. As of 2023, all elements with atomic numbers 1 vĩ đại 118 have been observed. Synthesis of new elements is accomplished by bombarding target atoms of heavy elements with ions, such that the sum of the atomic numbers of the target and ion elements equals the atomic number of the element being created. In general, the half-life of a nuclide becomes shorter as atomic number increases, though undiscovered nuclides with certain "magic" numbers of protons and neutrons may have relatively longer half-lives and comprise an island of stability.
A hypothetical element composed only of neutrons has also been proposed and would have atomic number 0.