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300px, The Rutherford–Bohr model of the hydrogen atom () or a hydrogen-like ion (). In this model it is an essential feature that the photon energy (or frequency) of the electromagnetic radiation emitted (shown) when an electron jumps from one orbital to another be proportional to the mathematical square of atomic charge (). Experimental measurement by
Henry Moseley Henry Gwyn Jeffreys Moseley (; 23 November 1887 – 10 August 1915) was an English physicist, whose contribution to the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific met ...
of this radiation for many elements (from ) showed the results as predicted by Bohr. Both the concept of atomic number and the Bohr model were thereby given scientific credence. The atomic number or proton number (symbol ''Z'') of a
chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbers of protons in their atomic nu ...
is the number of
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

proton
s found in the
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
of every
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

atom
of that element. The atomic number uniquely identifies a
chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbers of protons in their atomic nu ...
. It is identical to the
charge number Charge number (''z'') refers to a Charge quantization, quantized value of electric charge, with the quantum of electric charge being the elementary charge, so that the charge number equals the electric charge (''q'') in coulombs divided by the elem ...
of the nucleus. In an uncharged atom, the atomic number is also equal to the number of
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...
s. The sum of the atomic number ''Z'' and the number of neutrons ''N'' gives the
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 subatomic particle, symbol or , with a positive electric charge of ...
''A'' of an atom. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the
mass defect Mass is both a property Property (''latin: Res Privata'') in the Abstract and concrete, abstract is what belongs to or with something, whether as an attribute or as a component of said thing. In the context of this article, it is one or mo ...
of
nucleon In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they under ...
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 variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom. The term ...
s. A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes (see
monoisotopic element A monoisotopic element is an element which has only a single stable isotope (nuclide). There are only 26 elements that have this property. A list is given in a following section. Stability is experimentally defined for chemical elements, as there ...
s), 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'') or atomic weight is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical element in a given sample to the atomic mass constant. The atomic mass constant (symbol: ''m ...
. 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(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germany, see also German nationality law * German language The German la ...

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 tabular display of the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical element ...

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 tabular display of the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical element ...

periodic table
of elements creates an ordering of the elements, and so they can be numbered in order.
Dmitri Mendeleev Dmitry (russian: Дми́трий); Church Slavic form: Dimitry or Dimitri (); ancient Russian forms: D'mitriy or Dmitr ( or ) is a male given name common in Orthodoxy, Orthodox Christian culture, the Russian version of Greek language, Greek Deme ...
claimed that he arranged his first periodic tables (first published on March 6, 1869) in order of
atomic weight Relative atomic mass (symbol: ''A'') or atomic weight is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical element in a given sample to the atomic mass constant. The atomic mass constant (symbol: ''m ...
("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 Tellurium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same num ...

tellurium
(atomic weight 127.6) ahead of
iodine Iodine is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same n ...

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 Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same num ...
and
potassium Potassium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

potassium
,
cobalt Cobalt is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbe ...
and
nickel Nickel is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

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 metallic chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an elem ...
elements, whose atomic number was not obvious, led to inconsistency and uncertainty in the periodic numbering of elements at least from
lutetium Lutetium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbe ...

lutetium
(element 71) onward (
hafnium Hafnium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same num ...

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 The (Latin Latin (, or ...
gave a
model In general, a model is an informative representation of an object, person or system. The term originally denoted the plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin ''modulus'', a measure. ...
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 Antonius Johannes van den Broek (4 May 1870, Zoetermeer Zoetermeer () is a city in the Western Netherlands The Netherlands ( nl, Nederland ), informally referred to as Holland, is a country primarily located in Western Europe and partly in the ...
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 Henry Gwyn Jeffreys Moseley (; 23 November 1887 – 10 August 1915) was an English physicist, whose contribution to the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific met ...
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 to test Van den Broek's and Bohr's hypothesis directly, by seeing if
spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spect ...
s 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 tubeAn X-ray tube is a vacuum tube A vacuum tube, an electron tube, valve (British usage) or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference ...
. 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 Moseley's law is an empirical law concerning the characteristic X-ray#Physics, x-rays emitted by atoms. The law had been discovered and published by the English physicist Henry Moseley in 1913-1914. Until Moseley's work, "atomic number" was merely ...
) 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 it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respectively). Like ch ...
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 metallic chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an elem ...
series (from
lanthanum Lanthanum is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numb ...

lanthanum
to
lutetium Lutetium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbe ...

lutetium
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 Prout's hypothesis was an early 19th-century attempt to explain the existence of the various chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an el ...
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 and nuclear chemistry, a nuclear reaction is a process in which two atomic nucleus, nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a tr ...
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 Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

hydrogen
nuclei, and thus there was required a hypothesis for the neutralization of the extra
protons A proton is a subatomic particle, symbol or , with a positive electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric cha ...
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 a British physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, ...

James Chadwick
's
discovery of the neutron The discovery of the neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the atomic nucleus, nucle ...
in 1932. An atom of gold now was seen as containing 118 neutrons rather than 118 nuclear electrons, and its positive charge now was realized to come entirely from a content of 79 protons. After 1932, therefore, an element's atomic number ''Z'' was also realized to be identical to the
proton number 300px, The Rutherford–Bohr model of the hydrogen atom () or a hydrogen-like ion (). In this model it is an essential feature that the photon energy (or frequency) of the electromagnetic radiation emitted (shown) when an electron jumps from one ...
of its nuclei.


The symbol of ''Z''

The conventional symbol ''Z'' possibly comes from the
German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germany, see also German nationality law * German language The German la ...

German
word (atomic number).Origin of symbol Z
frostburg.edu
However, prior to 1915, the word ''Zahl'' (simply ''number'') was used for an element's assigned number in the periodic table.


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 quan ...
. The number of electrons in each element's
electron shell In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they un ...
s, particularly the outermost
valence shell In chemistry and physics, a valence electron is an outer shell electron that is 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, both atoms in the bon ...
, is the primary factor in determining its
chemical bonding A chemical bond is a lasting attraction between atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday o ...
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.
A substance composed only by neutrons
A substance composed only by neutrons
is sometimes said to have atomic number 0.


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 ''t''1⁄2) is the time required for a quantity 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 Radioactive decay (al ...
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 coi ...

nuclide
becomes shorter as atomic number increases, though undiscovered nuclides with certain "
magic Magic or Magick may refer to: * Ceremonial magic, encompasses a wide variety of rituals of magic * Chaos magic#REDIRECT Chaos magic {{Redirect category shell, 1= {{R from miscapitalization {{R unprintworthy ..., a contemporary magical practic ...
" numbers of protons and neutrons may have relatively longer half-lives and comprise an
island of stability In nuclear physics, the island of stability is a predicted set of isotope Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the s ...
.


See also

* Effective 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 subatomic particle, symbol or , with a positive electric charge of ...
*
Neutron number The neutron number, symbol ''N'', is the number of neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitu ...
*
Atomic theory Atomic theory is the scientific theory A scientific theory is an explanation of an aspect of the natural world and universe that has been repeatedly tested and verified in accordance with the scientific method The scientific method is a ...
*
Chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbers of protons in their atomic nu ...
* History of the periodic table *
List of elements by atomic number This is a list of the 118 chemical elements which have been Timeline of chemical element discoveries, identified as of 2021. A chemical element, often simply called an element, is a species of atoms which all have the same number of protons in thei ...
*
Prout's hypothesis Prout's hypothesis was an early 19th-century attempt to explain the existence of the various chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an el ...


References

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