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The atomic number or nuclear charge number (symbol ''Z'') of a
chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that species. Unlike chemical compounds, chemical elements canno ...
is the charge number of an
atomic nucleus The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden experiments, Geiger–Marsden gold foil experiment. After th ...
. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

proton
s found in the nucleus of every
atom Every atom is composed of a atomic nucleus, nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, l ...

atom
of that element. The atomic number can be used to uniquely identify ordinary
chemical elements A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that species. Unlike chemical compounds, chemical elements canno ...
. In an ordinary uncharged atom, the atomic number is also equal to the number of
electron The electron ( or ) is a subatomic particle with a negative one elementary charge, elementary electric charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are generally thought t ...

electron
s. For an ordinary atom, the sum of the atomic number ''Z'' and the
neutron number The neutron number, symbol ''N'', is the number of neutrons in a nuclide. Atomic number (proton number) plus neutron number equals mass number: . The difference between the neutron number and the atomic number is known as the neutron excess: . ...
''N'' gives the atom's
atomic mass number The mass number (symbol ''A'', from the German word ''Atomgewicht'' tomic weight, also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It is approxi ...
''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 In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number, mass number (nucleon number). Until the 1960s, n ...
binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in unified atomic mass units (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
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbe ...
s. A little more than 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 Relative atomic mass (symbol: ''A''; sometimes abbreviated RAM or r.a.m.), also known by the deprecation, deprecated synonym atomic weight, is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical elem ...
. Historically, it was these atomic weights of elements (in comparison to hydrogen) that were the quantities measurable by chemists in the 19th century. The conventional symbol ''Z'' comes from the
German
German
word ' 'number', which, before the modern synthesis of ideas from chemistry and physics, merely denoted an element's numerical place in the
periodic table The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an Cultural i ...

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 (and its English equivalent ''atomic number'') come into common use in this context.


History


The periodic table and a natural number for each element

Loosely speaking, the existence or construction of a
periodic table The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an Cultural i ...

periodic table
of elements creates an ordering of the elements, and so they can be numbered in order.
Dmitri Mendeleev Dmitri Ivanovich Mendeleev (sometimes transliterated as Mendeleyev or Mendeleef) ( ; russian: links=no, Дмитрий Иванович Менделеев, Romanization of Russian, tr. , ; 8 February [Old Style and New Style dates, O.S. 27 J ...

Dmitri Mendeleev
claimed that he arranged his first periodic tables (first published on March 6, 1869) in order of
atomic weight Relative atomic mass (symbol: ''A''; sometimes abbreviated RAM or r.a.m.), also known by the deprecation, deprecated synonym atomic weight, is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical elem ...
("Atomgewicht").The Periodic Table of Elements
American Institute of Physics
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 Argon is a chemical element with the Symbol (chemistry), symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 Parts-per notatio ...

argon
and
potassium Potassium is the chemical element with the Symbol (chemistry), symbol K (from New Latin, Neo-Latin ''wikt:kalium#Latin, kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little fo ...

potassium
,
cobalt Cobalt is a chemical element with the Symbol (chemistry), symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. T ...

cobalt
and
nickel Nickel is a chemical element with Chemical symbol, symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and Ductility, ductile transition metal. Pure nickel is chemically reactive bu ...

nickel
) were known to have nearly identical or reversed atomic weights, thus requiring their placement in the periodic table to be determined by their chemical properties. However the gradual identification of more and more chemically similar
lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises the 15 Metal, metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium a ...
elements, whose atomic number was not obvious, led to inconsistency and uncertainty in the periodic numbering of elements at least from (element 71) onward (
hafnium Hafnium is a chemical element with the Symbol (chemistry), symbol Hf and atomic number 72. A lustre (mineralogy), lustrous, silvery gray, tetravalence, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirco ...

hafnium
was not known at this time).


The Rutherford-Bohr model and van den Broek

In 1911,
Ernest Rutherford Ernest Rutherford, 1st Baron Rutherford of Nelson, (30 August 1871 – 19 October 1937) was a New Zealand physicist who came to be known as the father of nuclear physics. ''Encyclopædia Britannica'' considers him to be the greatest ...
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 to be approximately equal to 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 , ), 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 on the periodic table), a month after Rutherford's paper appeared, Antonius van den Broek first formally suggested that the central charge and number of electrons in an atom was ''exactly'' equal to its place in the periodic table (also known as element number, atomic number, and symbolized ''Z''). This proved eventually to 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 In atomic physics, the Bohr model or Rutherford–Bohr model, presented by Niels Bohr and Ernest Rutherford in 1913, is a system consisting of a small, dense nucleus surrounded by orbiting electrons—similar to the structure of the Solar Syste ...

Bohr model
of the atom), decided to 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 to the square of ''Z''. To do this, Moseley measured the wavelengths of the innermost photon transitions (K and L lines) produced by the elements from aluminum (''Z'' = 13) to gold (''Z'' = 79) used as a series of movable anodic targets inside an
x-ray tube An X-ray tube is a vacuum tube that converts electrical input power into X-rays. The availability of this controllable source of X-rays created the field of radiography, the imaging of partly opaque objects with penetrating radiation. In contrast ...
. The square root of the frequency of these photons increased from one target to the next in an arithmetic progression. This led to 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) to the calculated
electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electron ...
of the nucleus, i.e. the element number ''Z''. Among other things, Moseley demonstrated that the
lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises the 15 Metal, metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium a ...
series (from to inclusive) must have 15 members—no fewer and no more—which was far from obvious from known chemistry at that time.


Missing elements

After Moseley's death in 1915, the atomic numbers of all known elements from hydrogen to 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 to atomic numbers 43, 61, 72, 75, 85, 87 and 91. From 1918 to 1947, all seven of these missing elements were discovered. By this time, the first four transuranium elements had also been discovered, so 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 to 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 to 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 In nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter. Nuclear physics should not be confused with atomic phy ...
between alpha particles and nitrogen gas,Ernest Rutherford , NZHistory.net.nz, New Zealand history online
Nzhistory.net.nz (19 October 1937). Retrieved on 2011-01-26.
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 than twice as much mass as would be expected from their being made of
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the chemical ...

hydrogen
nuclei, and thus there was required a hypothesis for the neutralization of the extra
protons A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Ele ...
presumed present in all heavy nuclei. A helium nucleus was presumed to be composed of four protons plus two "nuclear electrons" (electrons bound inside the nucleus) to cancel two of the charges. At the other end of the periodic table, a nucleus of gold with a mass 197 times that of hydrogen was thought to contain 118 nuclear electrons in the nucleus to 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 Sir James Chadwick, (20 October 1891 – 24 July 1974) was an English physicist who was awarded the 1935 Nobel Prize in Physics for his discovery of the neutron in 1932. In 1941, he wrote the final draft of the MAUD Committee, MAUD Repo ...

James Chadwick
's discovery of the neutron in 1932. An atom of gold now was seen as containing 118 neutrons rather than 118 nuclear electrons, and its positive nuclear charge now was realized to come entirely from a content 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 to the number of protons of its nuclei.


Chemical properties

Each element has a specific set of chemical properties as a consequence of the number of electrons present in the neutral atom, which is ''Z'' (the atomic number). The
configuration Configuration or configurations may refer to: Computing * Computer configuration or system configuration * Configuration file, a software file used to configure the initial settings for a computer program * Configurator, also known as choice board ...
of these electrons follows from the principles of
quantum mechanics Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including qua ...
. The number of electrons in each element's
electron shell In chemistry and atomic physics, an electron shell may be thought of as an orbit followed by electrons around an atom's Atomic nucleus, nucleus. The closest shell to the nucleus is called the "1 shell" (also called the "K shell"), followed by t ...
s, particularly the outermost
valence shell In chemistry and physics, a valence electron is an electron in the outer electron shell, shell associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed. In a single covalent bond, a sh ...
, is the primary factor in determining its
chemical bonding A chemical bond is a lasting attraction between atoms or ions that enables the formation of Molecule, molecules and crystals. The bond may result from the Coulomb's law, electrostatic force between oppositely charged ions as in Ionic 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.


New elements

The quest for new elements is usually described using atomic numbers. As of , all elements with atomic numbers 1 to 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 Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...
of a
nuclide A nuclide (or nucleide, from atomic nucleus, nucleus, also known as nuclear species) is a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy state. The word ''nuclide'' was co ...
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.


See also

*
Atomic theory Atomic theory is the scientific theory A scientific theory is an explanation of an aspect of the natural science, natural world and universe that has been reproducibility, repeatedly tested and corroborated in accordance with the scientifi ...
*
Chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that species. Unlike chemical compounds, chemical elements canno ...
* Effective atomic number (disambiguation) *
Even and odd atomic nuclei In nuclear physics, properties of a atomic nucleus, nucleus depend on even and odd numbers, evenness or oddness of its atomic number (proton number) ''Z'', neutron number ''N'' and, consequently, of their sum, the mass number ''A''. Most importan ...
*
Exotic atom An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same Electric charge, charge. For example, electrons may be replaced by other negatively charged particles such as muo ...
* History of the periodic table * List of elements by atomic number *
Mass number The mass number (symbol ''A'', from the German word ''Atomgewicht'' tomic weight, also called atomic mass number or nucleon number, is the total number of proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive e ...
*
Neutron number The neutron number, symbol ''N'', is the number of neutrons in a nuclide. Atomic number (proton number) plus neutron number equals mass number: . The difference between the neutron number and the atomic number is known as the neutron excess: . ...
* Neutron–proton ratio * Prout's hypothesis


References

{{DEFAULTSORT:Atomic Number Chemical properties Nuclear physics Atoms Dimensionless numbers of chemistry Numbers